MECHANISMS OF PARATHYROID HYPERPLASIA IN RENAL FAILURE: ROLE OF PHOSPHATE
by
Mario Cozzolino, MD, PhD
Corresponding Author: Mario Cozzolino, MD, PhD Renal Division, San Paolo Hospital, University of Milan Via A. di Rudinì, 8 – 20142 – Milano Tel: 0039-02-81844381 FAX: 0039-02-89129989 E-mail:
[email protected]
INTRODUCTION In chronic kidney disease (CKD) patients, secondary hyperparathyroidism (SHPT) is characterized by parathyroid (PT) hyperplasia and enhanced synthesis and secretion of PT hormone (PTH). Elevated PTH levels cause renal osteodystrophy and cardiovascular complications, with significant increased morbidity and mortality in renal failure. The three main direct causes of SHPT are hypocalcemia, hyperphosphatemia, and vitamin D deficiency (Figure 1). A link between the mechanisms controlling proliferation and hormonal production also exists in normal parathyroid cells, which respond to the stimulus of chronic hypocalcemia, not only by an increase in PTH release but also with a consequent PT cell proliferation. Which are the mechanisms responsible for this link? Recently, new insights into the molecular mechanisms of PT hyperplasia and PTH secretion in CKD have been investigated. PT glands are generally characterized by a low turnover and rarely undergo mitoses. However, PT cells respond to the growth stimuli triggered by renal failure, hypocalcemia, hyperphosphatemia, and vitamin D deficiency. In these conditions PT glands leave quiescence and divide by increasing the activity of regulatory cell cycle enzymes and/or their inhibitors. Conversely, the antimitogenic signals elicited by high calcium (Ca), low phosphate (P) or calcitriol administration may arrest PT cell growth. In SHPT, PT glands initially growth diffusely and policlonally. Cells in the nodules then transform monoclonally and proliferate aggressively. Neither the mechanisms triggering the initial increase in proliferative activity nor those resulting in changes in growth patterns are clearly understood. The rapid de-differentiation of hyperplastic PT cells in culture has precluded further assessment of the relative contribution of changes in Ca, P, and vitamin D to the expression of components of the cell-cycle critical for growth control. The experimental approach in identifying molecular mechanisms is therefore limited to the in vivo 5/6 nephrectomized rat model.
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PHOSPHATE AND PARATHYROID HYPERPLASIA IN UREMIA In contrast to the slow growth rate of human PT adenomas, PT cells proliferate in 2 weeks after 5/6 nephrectomy in rats. In an identical time frame, uremia-induced mitotic activity was further enhanced by high dietary P, but prevented by P restriction. In contrast to the mitogenic effects of hyperphosphatemia, dietary P restriction appears to counteract the proliferative signals induced by uremia, thus preventing PT cell replication and consequently, the increase in PT hyperplasia. Interestingly, low P-induction of p21 (an inhibitor of kinases of the cell-cycle) mRNA and protein content in PT glands contributes to the antiproliferative effects of P restriction on uremiainduced PT cell growth. In the uremic rat model, a role of low-P induction of p21 expression in the arrest of PT cell growth is supported by the demonstration that temporal increases in p21 protein expression correlate inversely with PT levels of the marker of mitotic activity proliferating nuclear cell antigen (PCNA). (Figure 2) In the search for mitogenic stimuli triggered by high dietary P, our research next focused on transforming growth factor-α (TGFα). TGFα, known to promote growth not only in malignant transformation but also in normal tissues (10), is enhanced in hyperplastic and adenomatous human PT glands. The opposing effects of high and low dietary P on uremia-induced PT hyperplasia were demonstrated: high P diet worsens uremia-induced PT hyperplasia by increasing PT expression of TGFα. The rapid return of PT TGFα content to normal levels by P restriction also suggests that low P may counteract uremia-induced PT cell growth not only through induction of p21 expression, but also by preventing the enhancement of PT TGFα. Increases in PT TGFα may induce cell growth through autocrine and paracrine mechanisms upon activation of its receptor, the epidermal growth factor receptor (EGFR). In human PT glands, it was demonstrated the presence of EGFR protein in 4 out 5 adenomas, in 13 of 15 tissue samples of hyperplasia secondary to renal failure, and in most samples of normal PT tissue. No differences in the expression patterns were observed among groups. However, studies in 104 human 3
hyperplastic PT glands, which failed to detect EGFR protein, showed higher EGFR mRNA expression in carcinoma and primary hyperplasia compared to adenomas and hyperplasia secondary to renal failure. Similar to the changes in TGFα expression, high dietary P enhances PT EGFR content to levels above normal, while P restriction prevented the increases in EGFR levels. (Figure 3) These findings indicate that the induction of PT co-expression of TGFα and its receptor, EGFR, by uremia, acts as a mitogenic signal which is further enhanced by high dietary phosphorus, can be prevented by P restriction and counteracted through induction of p21. These new insights into the molecular mechanisms of PT hyperplasia suggest that, in addition to P restriction or the use of P binders, therapeutic manoeuvres oriented to further induce p21 and inactivate TGFα/EGFR growth-promoting signals may be more effective in slowing the progression of SHPT.
CONCLUSIONS Figure 4 schematically summaries the novel insights into the molecular mechanisms of the role of P in the pathogenesis of PT hyperplasia in renal failure. The levels of co-expression of TGFα and EGFR, induced by uremia and further enhanced by either high dietary P correlate directly with increased proliferating activity and the enlargement of the PT glands. Conversely, P restriction, by preventing the increase in PT TGFα and EGFR and simultaneous induction of p21, counteract the mitogenic signals and arrested gland growth. The demonstration that phosphate as one of the main regulator of PT growth, modulates TGFα /EGFR signalling and p21 expression indicate the importance of these pathways in the pathogenesis of PT hyperplasia and mark them as appropriate targets for more effective therapy.
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Figure 1. Pathogenesis of Secondary Hyperparathyroidism in Chronic Kidney Disease
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Pathophysiology in CKD: New Insights into sHPT Development Kidney function
Calcitriol
Intest. Caresorption
Ca
Phosphate
PTH
sHPT
Figure 2. Pathogenesis of PT Hyperplasia in CKD: Phosphate Control Increases PT p21 Content Low P Intake Induces PT p٢١ Protein Content High P
Low P
Uremia- ٧ days
Figure 3. PT TGFalpha and EGFR Expression: Role of Phosphate on PT Hyperplasia
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High dietary P Induces Enhanced TGFα TGFα /EGFR CoCo-Expression in Rat Parathyroid Glands Uremia = ٧ days PCNA
TGFα
LP
HP
EGFR
Cozzolino et al., Am J Physiol Renal Physiol, ٢٠٠٥.
Figure 4. Role of Phosphate on Uremia-induced Parathyroid Hyperplasia.
Renal Failure
U+LP
U+HP p٢١ TGFα TGFα TGFα EGFR
EGFR
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