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Steroids as ␥-secretase modulators Joo In Jung,*,† Thomas B. Ladd,*,† Thomas Kukar,‡ Ashleigh R. Price,*,† Brenda D. Moore,*,† Edward H. Koo,§ Todd E. Golde,*,† and Kevin M. Felsenstein*,†,1 *Center for Translational Research in Neurodegenerative Disease and †Department of Neuroscience, College of Medicine, University of Florida, Gainesville, Florida, USA; ‡Department of Pharmacology and Neurology, Emory University School of Medicine, Atlanta, Georgia, USA; and §Department of Neuroscience, University of California, San Diego, La Jolla, California, USA Aggregation and accumulation of A42 play an initiating role in Alzheimer’s disease (AD); thus, selective lowering of A42 by ␥-secretase modulators (GSMs) remains a promising approach to AD therapy. Based on evidence suggesting that steroids may influence A production, we screened 170 steroids at 10 M for effects on A42 secreted from human APP-overexpressing Chinese hamster ovary cells. Many acidic steroids lowered A42, whereas many nonacidic steroids actually raised A42. Studies on the more potent compounds showed that A42-lowering steroids were bonafide GSMs and A42-raising steroids were inverse GSMs. The most potent steroid GSM identified was 5-cholanic acid (EC50ⴝ5.7 M; its endogenous analog lithocholic acid was virtually equipotent), and the most potent inverse GSM identified was 4-androsten-3-one17-carboxylic acid ethyl ester (EC50ⴝ6.25 M). In addition, we found that both estrogen and progesterone are weak inverse GSMs with further complex effects on APP processing. These data suggest that certain endogenous steroids may have the potential to act as GSMs and add to the evidence that cholesterol, cholesterol metabolites, and other steroids may play a role in modulating A production and thus risk for AD. They also indicate that acidic steroids might serve as potential therapeutic leads for drug optimization/development.—Jung, J. I., Ladd, T. B., Kukar, T., Price, A. R., Moore, B. D., Koo, E. H., Golde, T. E., Felsenstein, K. M. Steroids as ␥-secretase modulators. FASEB J. 27, 3775–3785 (2013). www.fasebj.org ABSTRACT
Abbreviations: A, amyloid ; AD, Alzheimer’s disease; AICD, amyloid precursor protein intracellular domain; APP, amyloid precursor protein; CHO, Chinese hamster ovary; Cmpd2, compound 2; CTF, carboxyl-terminal fragment ; DMSO, dimethyl sulfoxide; GSI, ␥-secretase inhibitor; GSM, ␥-secretase modulator; HRP, horseradish peroxidase; iGSM, inverse ␥-secretase modulator; IP/MS, immunoprecipitation and mass spectrometry; MALDI-TOF, matrix-assisted laser desorption/ionization time of flight; MI, modulation index; MTS, 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)2-(4-sulfophenyl)-2H-tetrazolium; NSAID, nonsteroidal antiinflammatory drug; PS1, presenilin 1, S15, 4-androsten-3-one17-carboxylic acid ethyl ester; S529, 5-cholanic acid; sAPP, soluble amyloid precursor protein; wt, wild type 0892-6638/13/0027-3775 © FASEB
Key Words: Alzheimer’s disease 䡠 amyloid 䡠 A 䡠 cholesterol metabolites Accumulation of amyloid  (A) in the brain is one of the hallmark pathological features of Alzheimer’s disease (AD). A is a normally secreted peptide proteolytically derived from the amyloid precursor protein (APP) through sequential cleavage by - and ␥-secretase, respectively (1). -Secretase cleaves the ectodomain of APP, producing soluble APP (sAPP) and carboxyl-terminal fragment  (CTF). CTF is subsequently cleaved by ␥-secretase, producing soluble A and the APP intracellular domain (AICD). A was originally described as an ⬃4-kDa protein, as it exhibits extensive heterogeneity at both its amino and carboxyl termini when isolated from the AD or trisomy 21 brain (1–3). Under normal conditions, a diverse collection of A peptides is also produced, but these peptides do not entirely account for the heterogeneity of A deposited in the brain (4 –7). A1– 40 is the major species normally produced, along with a number of minor species, including but not limited to A1–37, A38, A39, and A42 (1). In particular, Ax–42 has been implicated as the pathogenic form in AD (8). In vitro experiments have demonstrated that Ax– 42 is more prone to aggregation than Ax–40 and other shorter nonmutant A peptides, and Ax– 42 is typically the earliest detectable form of A deposited in the brain (3–5). A more direct causal role in AD comes from the study of genetic mutations in presenilin 1 (PS1), presenilin 2 (PS2), and APP that lead to either relative or absolute increases in A42 (1). Transgenic modeling studies also show that A42 is required for A deposition in the mouse brain and that A40 may actually protect from amyloid deposition (13, 14). In consideration of this evidence, selective lowering of A42 appears to be a promising approach to prevent A aggregation and ultimately A deposition in the brain (9 –14). 1 Correspondence: Center for Translational Research in Neurodegenerative Disease and Department of Neuroscience, College of Medicine, University of Florida, Gainesville, FL 32610, USA. E-mail:
[email protected] doi: 10.1096/fj.12-225649 This article includes supplemental data. Please visit http:// www.fasebj.org to obtain this information.
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Small molecules that lower A42 by shifting ␥-secretase cleavage are generally referred to as ␥-secretase modulators (GSMs). Since the initial finding that a subset of nonsteroidal anti-inflammatory drugs (NSAIDs) possess GSM activity, numerous compounds with GSM activity have been identified (15–21). In general, there are 2 recognized classes of GSMs: NSAID-like GSMs, which have a hydrophobic scaffold and have an absolute requirement for a carboxylic acid group for A42 lowering; and nonacidic GSMs, including the diarylaminothiazoles and diarylureas (15, 16, 22, 23). Many nonacidic compounds structurally related to the NSAID-like GSMs raise A42 and are referred to as inverse GSMs (iGSMs; ref. 20). GSMs from both classes have shown therapeutic efficacy in preclinical mouse models after chronic administration, with significant reduction in brain amyloid histopathology and behavioral deficit readouts having been demonstrated (15, 16, 19). As opposed to ␥-secretase inhibitors (GSIs), which have inherent mechanism-based toxicity, GSMs are postulated to represent an intrinsically safe approach for AD therapy because they do not inhibit normal APP processing or processing of other ␥-secretase substrates, e.g., Notch 1 (15–19). Instead, they alter ␥-secretase cleavage and appear to have some degree of specificity, as they have stronger modulatory effects on APP and APLP2 then other ␥-secretase substrates evaluated to date (21). Nevertheless, while the preclinical potential of GSMs has been clearly demonstrated, it has proven challenging to identify a compound from the existing chemical leads that combines high potency with a high therapeutic safety window required for long-term chronic administration to treat AD (24). Therefore, there is a considerable need for the elaboration of the biology of novel chemical lead GSM structures with potential for improved safety as described herein. In this study, we have discovered a novel class of GSMs from the screening of a small library of ⬃170 commercially available steroids for their potential to lower A42. A subset of both naturally occurring and synthetic derivatives of cholesterol, which included all readily available compounds that contained a carboxylic acid as well as structurally related nonacidic steroids, was identified. This screen identified 40 steroids that raised A42 and 41 steroids that lowered A42. Focused studies on the most potent A42-modulating steroids, 5-cholanic acid (S529; ursocholanic acid) and 4-androsten-3-one-17carboxylic acid ethyl ester (S15), demonstrated that they were indeed bonafide GSM or inverse GSMs, respectively. Notably, the GSM identified in this study, S529, and its endogenous counterpart, 5-cholanic acid-3-ol (lithocholic acid), have the potential to interact with and regulate the EphA2 and FXR receptors (25–27). Finally, we further explored whether the steroid hormones estrogen and progesterone were also GSMs. Although estrogen has complex effects on APP processing, as previously reported (28), both estrogen and progesterone also appear to possess weak iGSM activity. Our results suggest that an endogenous A42 3776
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modulatory system may exist and that its regulation or direct use of the steroids may represent a novel therapeutic approach to the treatment of AD.
MATERIALS AND METHODS Compounds and cell culture A collection of 170 diverse steroids, both naturally occurring and synthetic, was obtained from Steraloids (Newport, RI, USA) without preference to inherent biological properties; GSM-1 and Cmpd2 were synthesized by A. Fauq (Mayo Clinic Chemical Core, Jacksonville, FL, USA); and compound E was purchased from EMD Millipore (Billerica, MA, USA). All compounds were dissolved in dimethyl sulfoxide (DMSO) as a 30 – 60 mM stock. We attempted using a 45% solution of hydroxypropyl--cyclodextrin (HP--CD) as the vehicle since the compound(s) may precipitate from solution when DMSO is the vehicle and added to aqueous medium. Although HP--CD is broadly used for challenging pharmaceutical formulations, it was not able to enhance drug availability and in all likelihood lowered the available concentration, which resulted in no significant effects on the A measurements when compared with vehicle (data not shown). Compounds were screened either in Chinese hamster ovary (CHO)-2B7 cells or in human neuroglioma (H4) cells stably expressing the APP695 wild-type (wt) protein, maintained as described previously (29). For compound testing, the cells were incubated for 16 h in the presence of the compound appropriately diluted into OptiMEM-reduced serum medium (Life Technologies, Carlsbad, CA, USA) containing 1% fetal bovine serum. The CellTiter 96 aqueous nonradioactive cell proliferation assay [3-(4,5-dimethylthiazol-2-yl)5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay; Promega, Madison, WI, USA] was used to determine toxicity according to the manufacturer’s protocol (30). Unless otherwise noted, 1% DMSO was used as the vehicle control. Antibodies and ELISAs Monoclonal antibodies to A were generated by the Mayo Clinic Immunology Core facilities (Jacksonville, FL, USA). Ab5 recognizes an epitope in the amino terminus of A (A1–16), recognizes both monomeric and aggregated A, and is human specific. Ab13.1.1. was raised against A35–40, is specific for Ax–40, and exhibits minimal cross-reactivity with other A peptides. A2.1.3. was raised against A35–42 and is specific to Ax–42. The A38 antibody (Ab38), supplied by P. Mehta (Institute of Basic Research, Staten Island, NY, USA), specifically recognizes Ax–38 and shows no cross-reactivity with other A peptides (data not shown). For cell-based screens, A was captured from conditioned medium with either Ab5, Ab38, Ab13.1.1, or Ab2.1.3 (coated at 10 –50 g/ml in EC buffer: 5 mM NaH2PO4·H2O, 20 mM Na2HPO4, 400 mM NaCl, 2.5 mM EDTA, 151.5 M BSA, 813 M CHAPS, and 7.7 mM NaN3) on Immulon 4HBX Flat-Bottom Microfilter 96-well plates (Thermo Scientific, Waltham, MA, USA). Total A level was determined by capture with Ab5 and detected with horseradish peroxidase (HRP)-conjugated 4G8 (a monoclonal antibody against A17–24; Covance, Waltham, MA, USA) with the other A peptides detected with HRPconjugated Ab5. For the cell-free assay, HRP-conjugated 4G8 was used as the secondary detection antibody for all testing. A standards (Bachem, King of Prussia, PA, USA) were prepared by dissolving in hexafluoroisopropanol (HFIP) at 1
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mg/ml with sonication, dried under nitrogen, resuspended at 2 mg/ml HFIP, sonicated again, and dried under nitrogen. The resulting A was resuspended in 0.01% ammonium hydroxide, portioned into aliquots in EC buffer, and frozen at ⫺80°C. Following these steps, the A is monomeric, as determined by size-exclusion chromatography (data not shown). Mass spectrometry of A For matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry analyses of A peptides, CHO 2B7 cells were treated with the various compounds, as described above. Secreted A peptides were analyzed by essentially as described previously (5, 29). Briefly, A1–x was immunoprecipitated using Ab5, with the peptide being eluted with trifluoroacetic acid:acetonitrile:water (1:20:20) saturated with ␣-cyano-4-hydroxycinnamic acid (Sigma, St. Louis, MO, USA). Samples were analyzed on a Voyager DE STR MALDI-TOF mass spectrometer (PE Biosystems, Foster City, CA, USA). For identification of A species, the expected mass (m/z) is compared with the observed mass (m/z), italicized in parentheses: A37. 4071.5 (4075.93); A38, 4131.5 (4133.32); A39, 4230.7 (4323.04); A40, 4323.8 (4331.38); A42, 4514.0 (4515.01).
Measurement of sAPP␣ CHO-2B7 cells were incubated with 17-estradiol or progesterone at the indicated concentrations (10, 100, or 200 M) for 16 h. The conditioned medium was collected, and the cells were harvested/lysed in RIPA buffer for immunoblotting for sAPP␣ and CTF␣ detection, respectively (28). Ab5 (1:500) was used for sAPP␣ detection, and anti-APP-CT-20 (1:1000) was used for CTF␣ detection. Both CTF␣ and sAPP␣ were normalized to actin following quantitation using Odyssey 3.0 software (LiCor Biosciences). Statistical analysis In vitro data were expressed and graphed as means ⫾ se using GraphPad Prism 5 software (GraphPad, San Diego, CA, USA). Unless otherwise noted, analysis was either by Student’s t test or by 1-way ANOVA followed by Dunnett’s post hoc testing for group differences. MALDI-TOF MI calculations were analyzed using 2-way ANOVA with repeated measures. The level of significance was set at P ⬍ 0.05 in all tests.
RESULTS Notch cleavage assay DNA encoding AD1-Q15-mNotch1V1711-E1809 fusion with a FLAG tag was synthesized and cloned into pET-21b (⫹) vector (Life Technologies). The protein was overexpressed in Escherichia coli BL21 and purified using a HiTrap Q-column (GE Life Sciences, Little Chalfont, UK) with NaCl elution gradient. Gel filtration and anti-FLAG M2 affinity column can be used if further purification is needed. CHAPSO solubilized ␥-secretase was prepared as described from CHO S-1 cell line (31, 32). For in vitro assay, 25 M substrate was incubated with CHAPSO solubilized ␥-secretase (100 g/ml protein concentration) in sodium citrate buffer (150 mM, with 1⫻ complete protease inhibitor, pH 6.8; Roche, Indianapolis, IN, USA) for 2 h at 37°C in the presence or absence of test compounds. The final total volume was 120 l. To capture the N-terminal product, 50 l magnetic sheep-anti-mouse IgG beads (Life Technologies) was incubated with 4.5 g Ab5 antibody for 30 min at room temperature with constant shaking. The beads were then washed with PBS and incubated with 60 l in vitro assay mixture for 30 min. The C-terminal product was captured with anti-FLAG M2 magnetic beads (Sigma). Bound beads were washed 3 times with water. Samples were eluted with 10 l 0.1% trifluoroacetic acid (Thermo Scientific) in water. Eluate (2 l) was mixed with an equal volume of saturated ␣-cyano-4-hydroxycinnamic acid (ACCA) solution (Sigma) in 60% acetonitrile, 40% methanol. The sample mixture (1 l) was loaded onto an ACCA-pretreated MSP 96 target plate and analyzed with a Bruker Microflex mass spectrometer (Bruker Daltonics, Billerica, MA, USA) for the cleavage product. In vitro ␥-secretase assays Cell-free ␥-secretase assays were performed as described previously (31, 32). The carbonate-extracted membranes derived from the H4 neuroglioma cells overexpressing APP695wt were incubated at 37°C for 2 h; A levels were quantified by a panel of sandwich ELISAs; and AICD by immunoblotting, as described, with rabbit anti-APP-CT20; 1:1000 (Calbiochem, La Jolla, CA, USA), using an Odyssey scanner (LiCor Biosciences, Lincoln, NE, USA). CHOLESTEROL METABOLITES AND ALZHEIMER’S DISEASE
Identification of oxysterol-type GSMs The effects of steroid treatment on A42 levels were examined using a cell-based screen in CHO cells overexpressing the WT 695-aa isoform of APP (wtAPP695; CHO-2B7 cells) at an initial screening concentration of 10 M (29). After 16 h treatment, secreted A42 levels from the conditioned medium were measured by an A42-specific sandwich ELISA (Supplemental Table S1). The 20 most effective A42-lowering steroids and the 20 most effective inducers of A42 were selected for additional analysis. To differentiate the compounds from general A inhibitors or inducers and to categorize them as GSMs or iGSMs, the effect of the compounds on A38 production, as well as total A production, by specific ELISA assays was examined (data not shown). Aside from those compounds having no effect on generalized inhibition or induction, the results indicated that many of the steroids tested showed either GSM or iGSM activity with either increases or decreases in A38, respectively, and little, if any, effect on total A. As additional confirmation, GSM and iGSM activities were examined in a second cell line, H4 human neuroblastoma cells stably transfected with wtAPP695. The magnitude of the effect between the cells varied on a compound-by-compound basis, but it is difficult to make absolute comparisons due to differing pharmacokinetic variables; however, with few exceptions the overall rank of order of the compounds was essentially unchanged (data not shown). Interestingly, a number of the A42-lowering steroids exist endogenously, including but not limited to 5-cholanic acid-3-ol (lithocholic acid) and 5-cholenic acid-3-ol. The best representative A42-lowering and A42-raising steroids were selected for additional testing, including EC50 determination, in comparison to known GSMs (Fig. 1). 3777
Figure 1. ␥-Secretase modulatory activity of select compounds in CHO-2B7 cells. A) Chemical structures of the various GSM compounds tested, along with their calculated EC50 for A42 modulation determined in triplicate. GSM-1 (piperidine acetic acid–type), EC50 ⫽ 92.17 nM; Cmpd2 (piperazinyl pyrimidine type), EC50 ⫽ 44.81 nM; S529, EC50 ⫽ 5.68 M; and 4-androsten-3-one-17-carboxylic acid ethyl ester (S15), 50% increase at 6.25 M. B–E) Concentration-response curves for A42 (B), A38 (C), A40 (D) and total A (E). A levels from DMSO-treated cells (n⫽6) served as the control.
Representative GSM compounds include S529 and other various steroids, which comprise various hydroxy and keto derivatives, including various endogenous bile acids. Examples of the structure-activity relationship
(SAR) are presented in Fig. 2. S529 acid is an acidic steroid with GSM activity, with a similar pharmacophore structure as the NSAID carboxylates, containing a lipophilic domain with a carboxylic acid group. Other
Figure 2. Markush structure of acidic steroid/sterol and associated structure-activity relationships. Effects of the various S529 derivatives, some of which are known endogenous steroidal metabolites (indicated by a checkmark) on A42 production are summarized as compared to the DMSO vehicle-treated cells. 3778
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representative compounds, showing iGSM activity, are the androstenedione-based compounds, including S15, which has a lipophilic domain and a modified carboxylic acid group containing an ethyl ester (Fig. 1A), were chosen for further study based on the magnitude and potency of their effects on A42. For comparison, GSM-1, a piperidine acetic acid chemotype GSM, and compound 2 (Cmpd2), a piperazinyl pyrimidine nonacidic type GSM, were also utilized (22, 23). Doseresponse curves are shown in Fig. 1B–E. S529 decreased A42 up to 75% and raised A38 ⬎300% in a dosedependent manner. At these concentrations, S529 was not toxic when evaluated by MTS assays (30). The EC50 of S529 for lowering A42 is ⬃5.7 M in CHO-2B7 cells. The identified inverse modulator S15 selectively raised A42 by 200% and decreased A38 up to 20% at 25 M in a dose-dependent manner. Cell toxicity was indicated for S15 at concentrations ⬎50 M. This pattern is similar to that of fenofibrate, a drug utilized to reduce cholesterol levels; however, it is a classic iGSM (20). The steroid modulators did not appreciably alter A40 or total A levels. For comparison, GSM-1 showed the same pattern of A modulation by raising A38 and lowering A42, with no changes in A40 or total A. This phenotype is different from that of Cmpd2, which lowers both A40 and A42, with concomitant increases in A37 and A38. The EC50 for lowering A42 for GSM-1 is 92.2 nM and for Cmpd2 is 44.8 nM. Cell toxicity was observed at concentrations ⬎10 M for
GSM-1 and 2 M for Cmpd2, respectively. Inhibition of the proteolytic processing of Notch and other ␥-secretase substrates by the classical GSIs has severely limited their clinical development (15, 16). GSMs, by definition, do not show effects on Notch proteolytic processing. To further characterize the identified steroid class for their effects on ␥-secretase-mediated Notch processing, the compounds were examined by proteolytic analysis of an A-Notch fusion protein. Consistent with their profile as GSMs, no apparent alteration of Notch processing was detected as compared with the GSI LY-411,575 (Supplemental Fig. S1). A-specific immunoprecipitation and mass spectrometry (IP/MS) demonstrates that S529 shows a similar A profile to known acidic-type GSMs IP/MS peptide analysis was performed from conditioned media from cells treated, at the maximal effective concentration where no toxicity was observed, either with GSM-1, Cmpd2, S529, S15, or DMSO vehicle alone, respectively (Fig. 3A–E). From the vehicletreated cells, A40 was the major species detected, with minor amounts of A37, A38, A39, and A42 also detected. GSM-1 treatment altered the A profile showing marked increases in A38 and decreases in A42 without changing A40; Cmpd2 lowered both A40 and A42 peaks, with A37 and A38 increasing after treatment. S529 treatment showed a similar pattern to
Figure 3. Pattern of A isoforms after MALDI-TOF analysis from conditioned medium from GSM-treated cells. A–E) Effect of various GSM treatments on A isoforms with representative A spectra from 2–3 experiments with 3 replicates/experiment: DMSO (A), GSM-1 (B), Cmpd2 (C), S529 (D), and S15 (E). Identity of the A peptide is noted above the appropriate peak and stacked bar graph, demonstrating the ratio of each peak height to the sum of all the peaks. F) Modulation index (MI) reflecting overall shifting in each A profile induced by the treatments was calculated. As an example, the MI is determined by comparison of the DMSO-treated group to the drug-treated group such that the peak height for the longer A species, A42, is 0.08, and the sum of the peaks for A37, A38, and A39 is 0.17, with the difference being 0.09. With the use of the same calculation, the difference between the longer and shorter A peptides for GSM-1 is 0.35. This value is normalized with the value from DMSO treatment, resulting in MI ⫽ ⫺0.26. Negative value indicates GSM activity; positive value indicates iGSM activity. **P ⬍ 0.01, ***P ⬍ 0.001. CHOLESTEROL METABOLITES AND ALZHEIMER’S DISEASE
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GSM-1, while S15 showed an increase in A42 and modestly reduced A38, consistent with iGSM. We performed quantification of the A peptide spectra (see Supplemental Fig. S2A) using the standardized method by measuring the height of each A peak (i.e., A37, A38, A39, A40, and A42) between 3500 (m/z) and 5000 (m/z) where A shifting occurs. The analyses show the ratio between each A peak height to the total A peak heights. Moreover, we have established a quantification method to selectively illustrate the magnitude of overall shifting in the A profiles after compound treatment (Fig. 3F), by comparing the ratio of each peak to the sum of the total peaks and then calculating the difference between sum of the longer A peptides (i.e., A42) and that of the shorter A peptides (i.e., A37, A38, and A39). The result was subsequently normalized to the vehicle control. As this value represents the modulation induced by either a GSM or an iGSM, it is referred to as the modulation index (MI). The MI following each treatment is shown in Fig. 3F. All of the GSMs show a negative MI, whereas an iGSM shows a positive MI. Cmpd2, which alters both A42 and A40, demonstrated a more significant MI (⫺0.79) than GSM-1 (⫺0.26) or S529 (⫺0.06), which alter only A42. In vitro ␥-secretase activity study shows direct modulatory effects of select steroids To ascertain whether the apparent shift in ␥-secretase cleavage on APP is due to direct modulation by the select steroids, broken cell in vitro ␥-secretase activity assays were performed. It has been previously demonstrated that direct ␥-secretase activity can be measured
using a cell-free membrane preparation with APP or -secretase-derived C-terminal fragment C99 as substrate (32). The carbonate-extracted membranes from H4 APP695wt cells are incubated in the presence of the GSM compounds and the select steroids. Figure 4A shows the results from A42, A40, and total A ELISAs. S529, as well as GSM-1 and Cmpd2, lowered A42 levels, while S15 raised A42 levels. As in the cell-based assay, Cmpd2 lowered A40, as well as A42. Furthermore, overall ␥-secretase activity was assessed by examination of the production of the AICD by immunoblot. AICD fragments, the intracellular fragments remaining after ␥-secretase cleavage, were detected by an APP C-terminal antibody (anti-CT-20; Fig. 4B). Unlike the ␥-secretase inhibitor (compound E)-treated membranes, the steroidal compounds and the known GSMs had no discernible effects on the production of AICD (Fig. 4B) nor the precise position ε-cleavage site that generates an AICD fragment of 49 or 50 aa in length (Fig. 4C). These observations are consistent with those described elsewhere (33). Steroid hormones demonstrate complex effects on A production Since the steroid hormones estrogen and progesterone have been implicated as protective molecules for AD, we explored the possibility of whether they may play a role as GSMs. Utilizing the cell-based A assay, we performed end-specific ELISA analysis from the conditioned medium after each treatment. 17-Estradiol, the most common form of estrogen, and progesterone raised A42 levels and lowered A38 levels, characteristic of an iGSM (Fig. 5A, B). IP/MS analysis of the
Figure 4. In vitro ␥-secretase activity assay shows a direct association of each GSM and iGSM in APP processing. A) Graph shows that each treatment was capable of modulating ␥-secretase cleavage activity in a cell-free assay using carbonate-extracted membranes from H4-APP695wt-overexpressing cells. Control (Ctrl) refers to the net activity of membranes incubated for 2 h with DMSO (T2) after the subtraction of the basal ␥-secretase activity (T0). To measure basal ␥-secretase activity, samples were treated with compound E, an irreversible pan-␥-secretase inhibitor, at the starting point T0 of membrane incubation (61). Each sample is then normalized to the control. Each group is normalized to percentage of control (n⫽2); graph is representative of 4 assays. *P⬍0.05, ***P⬍0.001; unpaired, 2-tailed Student’s t test. B) AICD immunoblot, along with its quantitation is shown after each drug treatment. AICD is markedly reduced or not detected after compound E treatment. GSM-1, Cmpd2, S529, and S15 treatments generated AICD with no significant quantitative changes. *P ⬍ 0.05; 1-way ANOVA with Dunnett’s multiple comparison test. C) With the use of bicine urea gels, AICD fragments can be separated into C49 and C50 after each drug treatment (62). No significant changes are detected in either the level or position of ε-cleavage or after GSM-1, Cmpd 2, S15, or S529 treatment consistent with modulatory activity (33). MW, molecular weight. 3780
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Figure 5. Estrogen and progesterone show inverse GSM activity. A, B) Dose-response curves of 17-estradiol (A) and progesterone (B) in CHO2B7 cells show varying effects on A42, A38, and total A levels. iGSM response of 17estradiol is weak but notable at 200 M (note that the total A increase at 50 M is not typical for these studies). Effects of progesterone ⬎50 M (grayed area) are rejected due to cellular toxicity. C–F) IP/MS analyses of 17estradiol (D) and progesterone (E, F) in CHO-2B7 APPoverexpressing cells. DMSO (C) is used as the control to compare A spectra of estrogen and progesterone-treated group (n⫽3). Species of A peptides are marked above the each peak. G) MI for 17-estradiol and progesterone. ***P ⬍ 0.001; 1-way ANOVA with Dunnett’s multiple comparison test.
treated conditioned medium was consistent with their signature as iGSMs (Fig. 5C–F and Supplemental Fig. S2B). In vitro ␥-secretase assay confirms that their effects for raising A42 are also through direct modulation of ␥-secretase (Supplemental Fig. S3). They possess a positive MI value, consistent with inverse modulation (Fig. 5G). It has been suggested that 17estradiol has a role as an ␣-secretase activator (28); therefore, we measured sAPP␣ levels from the medium, and CTF␣ level from the cell lysates, which were treated with 17-estradiol or progesterone (Fig. 6). Modest increases with respect to sAPP␣ levels were detected, whereas CTF␣ fragments were significantly increased at 100 and 200 M with 17-estradiol treatment (Fig. 6A, C). No statistically significant changes were detected for sAPP␣ and CTF␣ levels after progesterone treatment (Fig. 6B, D). MTS assay demonstrated that at the highest concentration (200 M) 17-estradiol did not appear detrimental to the cells, while progesterone appeared to be toxic at these levels (data not shown). The physiological relevance of the A42 increases that were detected is questionable, but this observation raises the potential of these types of molecules to act as iGSMs. CHOLESTEROL METABOLITES AND ALZHEIMER’S DISEASE
DISCUSSION These studies have identified the acidic steroid as a novel class of GSM, demonstrating that endogenous GSMs, resulting from the normal metabolic oxidation of cholesterol, may exist, and show that select steroid hormones may alter A production in a highly complex fashion. Although many acidic steroids selectively act as GSMs, closely related nonacidic analogs, including ester derivatives, either have no effect or manifest iGSM activity. Of the steroids tested, S529 was the most potent GSM in the collection, while S15 was the most potent iGSM among the nonacidic steroids. Analysis of the activities of these GSMs using cell-free ␥-secretase assays shows that S529 and S15 were able to directly interact with the ␥-secretase/substrate complex and influence A42 levels. S529 demonstrates an A-altering pattern similar to the carboxylate containing compound GSM-1, which selectively lowers A42 while concomitantly raising A38 levels without affecting total A. This pattern is distinct from the nonacidic Cmpd2, which lowers both A40 and A42 and raises A37 and A38 without affecting total A. 3781
Figure 6. ␣-Secretase assay was performed for 17-estradiol and progesterone. Immunoblots show minimal change in sAPP␣ and increase in CTF␣ in 17-estradiol-treated cells (A) while sAPP␣ and CTF␣ levels did not change in progesterone-treated cells (C). Each blot was quantified (B, D). DMSO served as the control; results are representative of 2–3 repeats of 3 individual experiments. *P ⬍ 0.05, **P ⬍ 0.01; 1-way ANOVA with Dunnett’s multiple comparison test.
It is not fully understood what contributes to the differential activities of acidic and nonacidic GSMs. There are most certainly differences in binding interactions either with the APP substrate, the PS1 ␥-secretase enzyme, or a ternary complex of both (34, 35). Given the evidence for a pore- or channel-like active site for ␥-secretase substrate cleavage and the requirement of high lipophilicity for high potency, it is likely that all GSMs modulate processivity of ␥-secretase cleavage through complex interactions with substrate, presenilin, and also membrane lipids. The binding sites and binding modes may vary with each class and each compound, respectively (36 – 41). The original low-tomoderate potency GSMs, derived from carboxylate containing NSAIDs, are suggested to bind directly to APP (36, 37). However; the optimized, high-potency carboxylic acid GSM-1 is reported to bind the PS1 N-terminal fragment (NTF) in the ␥-secretase complex (38). Nonacidic GSMs have been shown to bind to ␥-secretase components rather than the substrate (38 – 41). Overall, the evidence suggests a primary GSM binding interaction with PS1 and a weaker binding interaction with substrate, which is nonetheless highly significant for overall potency due to the chelate effect. One may postulate that the essential carboxylate group of acidic GSMs forms a strong ionic bond at a lysine residue at a binding site that is not occupied by nonacids. Cholesterol has been shown to bind APP CTF at a site that overlaps with the site affected as the acidic GSM binding site on APP; we and others have found that lysine residue 624, which is presumed to delineate the APP ectodomain from its transmembrane domain, dramatically alters ␥-secretase processivity without altering the initial ε-cleavage (42). Thus, one might speculate that acidic steroids can bind to APP CTF in a manner similar to cholesterol but also form an ionic bond to the positively charged lysine, resulting in enhanced processivity and thus ␥-secretase modulation. Additional studies will be needed to assess this possible mechanism of action. Our study supports the general concept that a lipophilic anchor, represented by the steroid-backbone, with a carboxylate side chain, is required for A42-lowering activity (18). The mechanism 3782
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for iGSM activity has not been defined; however, the shift in the activity profile can be attributed to the elimination of the aforementioned lysine binding interaction. It is predicted that they act on the same site as GSMs (20, 21). This study adds to the growing body of evidence that cholesterol and cholesterol metabolites, including bile acids, may play a role in modulating A production. Although the exact mechanisms showing their effects on A production are not fully defined, it is apparent that many of them have effects on APP secretases. For example, there is a positive correlation between cholesterol and A production. Cholesterol has been reported to negatively regulate ␣-secretase, whereas - and ␥-secretase activities are positively regulated by cholesterol (43– 46). Considering that cholesterol directly influences ␥-secretase activity and cholesterol colocalizes within the lipid rafts with APP and ␥-secretase, cholesterol could alter physical conformation of ␥-secretase to change the site or rate of cleavage of APP (46 – 49). Likewise, since many cholesterol metabolites found in the brain (including some bile acids) also show a correlation with A production (49 –51), perhaps they function as endogenous A regulators through modulation of ␥-, -, and ␣- secretase activity, as well as trafficking of APP and its associated secretases. It is conceivable that, like cholesterol, a bile acid may be able to bind the same site proposed by Beel and colleagues (52, 53) but is further capable of influencing the processivity of the ␥-secretase activity. Therefore, potentially increasing levels of these acidic steroids or analog compounds therapeutically may serve as a novel approach to modulating A42 production with respect to AD prevention and treatment. Interestingly, acidic steroids, such as the closely related chenodeoxycholic acid, are already used therapeutically for the treatment of gallstones. Beyond the investigation of the select steroidal and cholesterol metabolites, we also explored steroid hormones as potential A-altering molecules. Although they are frequently reported to play a protective role in AD, it remains unknown whether estrogen or progesterone have direct effects on A production (54 – 60). Our results show that 17-estradiol and progesterone can potentially
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raise A42 levels; a characteristic of iGSMs. 17-Estradiol appears to have additional effects on APP processing by increasing ␣-secretase activity. We confirmed that 17estradiol lowered total A levels in concentration-dependent manner and increased CTF␣, the cleavage product of ␣-secretase. This is a finding consistent with previous reports (28, 57). Although progesterone did not increase ␣-secretase cleavage products, it is implicated as a neuroprotective molecule and is associated with a A clearance mechanism by activating A-degrading enzymes (60). These observations suggest that the roles of 17-estradiol and progesterone in altering A metabolism are complex and may include iGSM activities. The biological relevance of these observations is questionable considering absolute values for these steroid hormones under normal conditions. However, it does imply that similar molecules with greater intrinsic iGSM activity may exist endogenously that may have relevance to the disease state. These studies have identified many acidic steroids as GSMs and nonacidic steroids as iGSMS, further linking steroid metabolism to APP processing. Although unproven, these data suggest that the naturally occurring steroids may have the potential to act as endogenous GSMs and add to the evidence that cholesterol, cholesterol metabolites, and other steroids may play a role in modulating A production and thus risk for AD (63). They also indicate that S529 and other steroids might serve as a potential lead for drug optimization and development. Along a similar line, a triterpene derivative of Actaea racemosa (black cohosh) has been characterized as a GSM (64, 65). (It is of note that the black cohosh GSM profile is distinct from those previously defined GSMs, suggesting that it potentially has complicated effects on the ␥-secretase processivity.) However, triterpenes are precursors to steroids in both plants and animals, suggesting that this entire steroidal class of molecules may represent a largely untapped potential for drug development and may serve as a viable alternative for AD therapy. These studies were supported by funds from U.S. National Institutes of Health grant AG-20206 and the Coins for Alzheimer’s Research Trust, supported by Rotarians in the southeastern United States. The authors are extremely grateful to Dr. Rong Wang (Department of Genetics and Genomic Sciences, Mount Sinai School of Medicine, New York, NY, USA) for the use of the PE Biosystems Voyager DE STR MALDI-TOF mass spectrometer. The authors also thank Dr. Yong Ran for technical support in performing the notch inhibition assay. The authors are also grateful to Gideon Shapiro and Kristi Ayers for critical reading of the manuscript.
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