Breaking rules, cytoplasmic IB is a cofactor of glucocorticoid hormone receptor transcription.
Vassilis Doucas, Dr.
The Molecular ChoroBiology Laboratory - VMCBL & BIOCHOROS Biotechnologies 1 rue du Cdt Jean-Duhail, 94120 FRANCE,
Running Title: Pro-inflammatory IB Controls NRs Signalling
Correspondance: V. Doucas, Tel: (+33) 6 66 79 49 85; E-mail:
[email protected]
A related manuscript has been submitted.
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ABSTRACT
Nuclear hormone receptors (NRs) such as the glucocorticoid receptor (GR) belong to a large family of transcription factors that control many aspects of human physiology and diseases. Lipophilic ligands, such as the glucocorticoids (GCs) are critical in mediating a coordinate control on specific and unrelated aspects of human disease, including cell growth, differentiation and cancer. At the functional level, GCs have been tightly linked to the glucocorticoid receptor transcription properties; this has given rise to the general assumption that no second message is required for transmitting the signals (Evans, 2005). Recent data provided direct evidence for post-translational modifications (PTMs) on NRs that control the pathophysiological progression of many diseases including cancers. We have previously shown that both GR and other NRs such the RAR/RXR express a PML-NBs spatial association revealing PML as the major co-factor of GCs/NRs signalling (Doucas et al., 1999; Doucas & Evans, 1999 & 2001; Doucas, 2006). It is also known that GCs, NRs and co-activators PML as also the oncogenic PMR-RAR are critical in controlling many aspects of cancer development including breast cancer metastasis and acute promyelocytic leukemia (APL). These data permit to suggest that the PML-NRs cellular partition activates a cross-communication with the chromatin structure and NRs-PTMs biochemistry (Doucas, 2000, 2006). To address this hypothesis, I have developed an in vitro drug screening analysis and cancer related proteomics. I establish that cytoplasmic IB ternary complex represses GR-transcription, which is prevented by specific PMTs on IB. Drug screening analysis reveals that IB per se switches from being GR-activator to being GR-repressor. This suggests that certain pro-inflammatory forms of IB differentially control NR-dependent signalling. Surprisingly, IB, similarly to cytoplasmic RelA, abolishes the function of co-activator PML, which is restored by RelAS276A point-mutation (Doucas et al. 2000). These data identify a cytoplasmic IB-dependent mechanism that controls NRs signalling and suggest that new cellular targets identified in the GCs/IB-differential proteomics play a catalytic role in GR hormone-dependent transcription. I propose the presence of a second message in controlling NRs signalling in human physiology and disease.
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FIGURE Model depicting a mechanism contributing to IB-mediated control of GCs signalling. The model introduces the IB-mediated cytoplasmic control in hormone signalling, which unveils a functional association between IB PMTs, PKAc and NRs co-activator PML. Lipophilic ligands, such as the glucocorticoids (GC) are critical in mediating a coordinate control on specific and unrelated aspects of human physiology and disease, such as various aspects of metabolism, cell growth and differentiation, apoptosis, inflammation and cancer. At the functional level, GCs have been tightly linked to the glucocorticoid receptor transcription properties; this has given rise to the general assumption that no second message is required for transmitting the signals (Evans, R. M. 2005). In a hypothetical bipolar model signalization, i.e. the GCs and the nuclear factor kappa B (NF-B), the function of GCs lies in the establishment of a minimal tripartite equilibrium between GCs, NF-B and chromatin. We have previously shown that negative transcriptional regulation or cross-coupling between NF-B (RelA) and the GR requires a cytoplasmic step and is regulated by a protein kinase A (PKA)-associated signalling (Doucas et al, 2000). To elaborate the model it was considered important to suggest that the cytoplasmic partition of this cross-coupling implies a cross-communication with the chromatin structure (Doucas, 2000, 2006).
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FOOTNOTES The abbreviations used are: GR, glucocorticoid receptor; GCs, glucocorticoids; NR, nuclear hormone receptor; PKAc, catalytic subunit of protein kinase A; NF-B, nuclear factor kappa B; Dex, dexamethasone; 8-Br-cAMP, 8-Bromoadenosine 3',5'-cyclic monophosphate; PML, promyelocytic protein; IB, inhibitor of the transcription factor nuclear factor kappa B (NF-kappaB); IKK, IB kinase RXRa, retinoid X receptor alpha.
ACKNOLEDGEMENTS V.D. is Founder and Research Director of the Molecular ChoroBiology Laboratory - VMCBL. Services: Proteomics & Microscopic Platforms. I thank the Microscopic platform of University Paris 6. Funding: BpiFrance (French National Innovation Price to V. Doucas), Biochoros Biotech.
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Evans, R. M. (2005) The nuclear receptor superfamily: a rosetta stone for physiology. Mol Endocrinol. 19, 1429-1438.
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Doucas, V., Shi, Y., Miyamoto, S., West, A., Verma, I., and Evans, R. M. (2000) Cytoplasmic catalytic subunit of protein kinase A mediates cross-repression by NF-kappa B and the glucocorticoid receptor. Proc Natl Acad Sci U S A 97, 11893-11898.
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Doucas V. (2000) The promyelocytic (PML) nuclear compartment and transcription control. Biochem Pharmacol. 60(8):1197-201.
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Doucas, V. and Evans, R. M. US 6,319,663 B1 (2001), Method for the identification and use of substances that modulate POD function and/or structure, Filed Nov 21, 1997.
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