p21(WAF1) and transforming growth factor-alpha mediate dietary phosphate regulation of parathyroid cell growth

Vitamin D receptor and analogs

In chronic kidney disease (CKD), high circulating levels of parathyroid hormone (PTH) cause osteitis fibrosa, bone loss, and cardiovascular complications that increase morbidity and mortality. Impaired production of 1,25-dihydroxyvitamin D (calcitriol), the hormonal form of vitamin D, is a major contributor to the generation and maintenance of parathyroid hyperplasia and increased synthesis and secretion of PTH. Calcitriol inhibits PTH gene transcription and ameliorates parathyroid hyperplasia by suppressing the expression of and growth signals from the autocrine transforming growth factor alpha (TGFalpha)/epidermal growth factor receptor (EGFR)-growth loop, a main determinant of parathyroid cell proliferation. Calcitriol reduction of parathyroid hyperplasia and serum PTH levels demands a functional vitamin D receptor (VDR). Although VDR is normal in CKD, parathyroid VDR content is reduced markedly. Furthermore, VDR function, as a transcriptional regulator of vitamin D responsive genes, is impaired by several factors including hypocalcemia, hyperphosphatemia, accumulation of uremic toxins, and reduction in cellular levels of the VDR partner, retinoid X receptor. Therapy with calcitriol analogs can overcome the antagonism on calcitriol-VDR actions induced by CKD. Although not all analog formulations are equally effective, they offer a wider therapeutic window in counteracting vitamin D resistance and survival advantage over exclusive calcitriol therapy.

Pathogenesis and treatment of renal osteodystrophy

Renal osteodystrophy is the term used to describe the many different patterns of the skeletal abnormalities that occur in patients with chronic kidney disease. The main two conditions are osteitis fibrosa, characterized by high bone turnover, increased osteoclastic and osteoblastic activity, and high levels of circulating parathyroid hormone (PTH) and adynamic bone disease characterized by low bone turnover and low levels of circulating PTH. Retention of phosphorus, decreased levels of calcitriol in blood, decreased levels of serum ionized calcium, reduced numbers of vitamin D receptors and calcium sensors in the parathyroid gland, and skeletal resistance to the calcemic action of PTH play a major role in the development of renal osteodystrophy. This review will describe the current approach for the treatment of renal osteodystrophy.

Phosphophloretin sensitivity of rabbit renal NaPi-IIa and NaPi-Ia

The effect of phosphorylated phloretins on Na+-dependent phosphate uptake into rabbit renal brush-border membrane vesicles (BBMV) was examined. Na+-dependent phosphate uptake into isolated rabbit cortex BBMV was sensitive to 2′-phosphophloretin (2′-PP) and 2′-phospho-4′,4,6′-trimethoxy phloretin (PTMP) in a dose-dependent and pH-dependent manner. PTMP inhibition of Na+-dependent phosphate uptake was maximum at alkali pH, and 2′-PP inhibition of Na+-dependent phosphate uptake was maximum at acidic pH. Increasing Na+concentrations did not increase PTMP inhibition of renal cortex BBMV Na+-dependent phosphate uptake at pH 6. The effect of phosphophloretins on Na+-dependent phosphate uptake was examined in BBMV isolated from purified proximal tubules and distal tubules. 2′-PP and PTMP inhibition of Na+-dependent phosphate uptake into BBMV isolated from purified proximal tubules was similar to the inhibition seen with BBMV from renal cortex. 2′-PP, but not PTMP, inhibited Na+-dependent phosphate uptake into BBMV isolated from purified distal tubules. The pH dependence of inhibition, the absence of PTMP inhibition of Na+-dependent phosphate uptake into distal tubule BBMV, and the inhibition of Na+-dependent phosphate uptake into distal tubule BBMV suggest that NaPi-Ia is 2′-PP sensitive and NaPi-IIa is PTMP sensitive.

1,25-Dihydroxyvitamin D downregulation of TGFalpha/EGFR expression and growth signaling: a mechanism for the antiproliferative actions of the sterol in parathyroid hyperplasia of renal failure

Elevated serum levels of parathyroid hormone (PTH) contribute to the increased morbidity and mortality in renal failure patients. Parathyroid gland hyperplasia is a major cause of high serum PTH. The present studies used the rat model of renal failure to address the mechanisms underlying uremia-induced parathyroid hyperplasia and the antiproliferative properties of vitamin D therapy (1,25-dihydroxyvitamin D (1,25(OH)(2)D(3)) or its less calcemic analogs). Enhanced TGFalpha/EGFR co-expression is the major mitogenic signal in uremic parathyroid glands. At early stages of renal failure, vitamin D therapy efficiently counteracts uremia- and high phosphorus-induced hyperplasia by inhibiting the increases in parathyroid-TGFalpha/EGFR co-expression. In established hyperparathyroidism, characterized by highly enhanced-TGFalpha/EGFR co-expression, vitamin D therapy arrests growth by suppressing EGFR-growth signals from the plasma membrane and nuclear EGFR actions as a transactivator of the cyclin D1 gene, an important contributor to parathyroid hyperplasia in humans. In advanced renal failure, reduced-parathyroid vitamin D receptor levels limits the antiproliferative efficacy of vitamin D therapy. However, non-antiproliferative doses of 1,25-dihydroxyvitamin D enhance the anti-EGFR actions of EGFR-tyrosine kinase inhibitors (TKI). In fact, combined 1,25-dihydroxyvitamin D/TKI therapy inhibits parathyroid hyperplasia more efficiently than phosphorus restriction, the most powerful promoter of parathyroid growth arrest available at present.

Management of secondary hyperparathyroidism of dialysis patients

Hyperphosphatemia, vitamin D deficiency, and resulted hypocalcemia have been regarded as classical pathogeneses of secondary hyperparathyroidism. These factors have been treated by the administration of phosphorus binder and vitamin D derivatives. However, these therapies have not brought about a successful result for the prevention and treatment of secondary hyperparathyroidism. The reason could be mainly attributed to the hypercalcemia that results from the administration of calcium salts as a phosphorus binder and the calcemic action of vitamin D. To prevent hypercalcemia, non-calcium containing phosphorus binder (sevelamer hydrochloride) and vitamin D analogues, which suppress PTH secretion with minimum calcemic action, have been developed. These new vitamin D analogues include 19-nor-1-alpha, 25-dihydroxyvitamin D2 (paricalcitol), 1-alpha-hydroxyvitamin D2 (doxercalciferol), 22oxa-calcitriol (maxacalcitol) and F6-calcitriol (falecalcitriol). Furthermore, calcimimetics that stimulate calcium-sensing receptor of parathyroid cells as calcium and suppress PTH secretion are now under clinical trial. Percutaneous direct injection therapy of vitamin D, vitamin D analogue or calcimimetics into parathyroid gland has also been reported. The combination of these new strategies is expected to effectively and safely suppresses secondary hyperparathyroidism that has been resistant to conventional medical treatments.

Sensing phosphate across the kingdoms

PURPOSE OF REVIEW

The present review summarizes recent findings that may help in understanding how the cell senses changes in serum phosphate.

RECENT FINDINGS

The sensing of phosphate determines the organism's response to change in supply of this essential nutrient. Phosphate depletion or surfeit results in homeostatic responses that involve changes in transcription, transcript stability, transporter recruitment or breakdown, and cell replication. These responses are shared across the biological kingdoms, and lessons from unicellular organisms may be relevant to multicellular mammals. An understanding of nutrient sensing in general may help in determining how the cell senses changes in phosphate concentration.

SUMMARY

Research has yielded important advances in unravelling phosphate sensing and the response to nutrient phosphate supply. However, the actual sensing event for phosphate and most other nutrients must still be defined. Lessons may be learned from those examples in which the sensing event is known, and these are summarized here.

Comparative study of the expression of proteins involved in the cell cycle in renal secondary hyperparathyroidism

BACKGROUND

In renal hyperparathyroidism, parathyroid cell proliferation seems to play a key role in the progression of the disease. Therefore, G1/S transition, a main cell cycle regulatory step, could be deregulated in these patients.

METHODS

One hundred and one parathyroid glands, taken from parathyroidectomies performed on 41 patients on hemodialysis (HD), and 15 glands, taken from 7 patients with post-transplantation persistent hyperparathyroidism (HPT), were studied. Twelve normal parathyroid (PT) glands were used as the control. Biochemical data, immunohistochemical (IHC) profiles of G1/S transition regulators belonging to the two main pathways (cyclin D1/p16INK4A/pRb and p14ARF/p53/MDM2), and proliferation rate (Ki67) were correlated.

RESULTS

All of the other proteins differed from normal IHC profiles in both groups that showed significant higher proliferating rates, decreases in p27KIP1, pRb, and cyclin D1, as well as increases in p16INK4A, p53, MDM2, and p21WAF1 levels, in comparison with normal PT glands, with the exception of cyclin D3. Contrary to patients with HPT who were on hemodialysis, in post-transplantation HPT, consistent correlations between biochemical data and IHC profiles were obtained.

CONCLUSION

In both groups IHC profiles of proteins involved in G1/S transition regulation significantly differed from normal PT glands. The results support partial reversion to normal IHC profile in post-transplantation HPT.

New strategies for the treatment of secondary hyperparathyroidism

Classic pathogeneses of secondary hyperparathyroidism (2HPT), hyperphosphatemia, vitamin D deficiency, and hypocalcemia, have been treated by the administration of phosphorus binders and vitamin D derivatives. However, these therapies have not brought about a successful result. The main reason could be attributed to hypercalcemia resulting from the administration of calcium salts as a phosphorus binder and the calcemic action of vitamin D. To prevent hypercalcemia, non-calcium-containing phosphorus binders and vitamin D analogues, which suppress parathyroid hormone (PTH) secretion with minimum calcemic action, have been developed. Furthermore, calcimimetics that stimulate the calcium-sensing receptor of parathyroid cells and suppress PTH secretion are now under clinical trial. Direct injection therapy of vitamin D analogues or calcimimetics into the parathyroid gland also has been reported. These new strategies are expected to effectively and safely suppress 2HPT, which has been resistant to conventional medical treatments.

1,25-Dihydroxyvitamin D down-regulates cell membrane growth- and nuclear growth-promoting signals by the epidermal growth factor receptor

1,25(OH)(2)D(3) antiproliferative properties are widely known. However, the molecular bases of these properties are only partially elucidated. Since 1,25(OH)(2)D(3) effectively arrests growth in many tumors and hyperplastic tissues whose growth is driven by co-expression of EGFR and its ligand TGF-alpha, it was hypothesized that 1,25(OH)(2)D(3) could affect the TGF-alpha/EGFR-autocrine growth loop. This study examined 1,25(OH)(2)D(3) regulation of EGFR-growth signals, using human epidermoid A431 cells, in which the overexpression of EGFR and TGF-alpha constitute the major autocrine mitogenic signal. 1,25(OH)(2)D(3) inhibited autocrine and EGF-induced A431 cell proliferation. Furthermore, 1,25(OH)(2)D(3) changed the cellular localization of both TGF-alpha and EGFR and inhibited ligand-dependent phosphorylation of EGFR and ERK1/2. In addition, 1,25(OH)(2)D(3) impaired autocrine and EGF-induced nuclear translocation of activated EGFR and, consequently, its binding to AT-rich DNA sequences and transcriptional activation of the cyclin D1 promoter. These results demonstrate that 1,25(OH)(2)D(3) alters EGFR membrane trafficking and down-regulates EGFR growth signaling.

Reduced p21, p27 and vitamin D receptor in the nodular hyperplasia in patients with advanced secondary hyperparathyroidism

BACKGROUND

In uremic patients with secondary hyperparathyroidism (2HPT), nodular hyperplasia of parathyroid gland shows a monoclonal pattern of cell proliferation, in which a decreased density of vitamin D receptor (VDR) also is demonstrated. The present study aimed at elucidating the mechanism of parathyroid cell proliferation in relation to cell cycle determinants in patients with advanced 2HPT.

METHODS

The expression of cyclin-dependent kinase inhibitors, p21 and p27, and VDR were examined and compared among four groups of nodular (Nd; N = 23) or diffuse (Df; N = 6) hyperplastic parathyroid glands resected due to 2HPT, primary adenomas (Ad; N = 15), and histologically-normal parathyroid glands (C; N = 20) removed during thyroidectomy. Immunohistochemical analyses for VDR, p21, p27 and Ki67 antigen were performed in formalin-fixed paraffin-embedded tissues by using specific polyclonal antibody. The distribution and the intensity of immunoreactivity was quantified by using NIH imaging, and was expressed as the labeling index (LI) of positive nuclear staining in a random set of 1000 cells.

RESULTS

p21 LI was significantly diminished in both Nd (85 +/- 110; mean +/- SD) and Ad (136 +/- 122) as compared to that in Df (360 +/- 191) or C (359 +/- 228; P < 0.01). p27 LI was also significantly diminished in both Nd (97 +/- 156) and Ad (187 +/- 196) as compared to that in Df (532 +/- 146) or C (631 +/- 170; P < 0.01). VDR LI in Nd (162 +/- 194) was also significantly lower than that in Df (495 +/- 337), Ad (383 +/- 262), or C (659 +/- 234), respectively (P < 0.01). Parathyroid sections with high nuclear VDR expression elicited high p21 and p27 expression. Both p21 and p27 LI in Nd correlated significantly with nuclear VDR LI (r = 0.92; P < 0.01, r = 0.76; P < 0.01), but not with p53 LI, and inversely correlated with the glandular weight (r = 0.44; P < 0.05, r = 0.41; P < 0.05).

CONCLUSIONS

The reduced expression of p21 and p27, in a VDR-dependent manner, is a major pathogenic factor for a nodular parathyroid gland growth.

Mechanisms of secondary hyperparathyroidism

Small decreases in serum Ca(2+) and more prolonged increases in serum phosphate (P(i)) stimulate the parathyroid (PT) to secrete parathyroid hormone (PTH), and 1,25(OH)(2)D(3) decreases PTH synthesis and secretion. A prolonged decrease in serum Ca(2+) and 1,25(OH)(2)D(3), or increase in serum P(i), such as in patients with chronic renal failure, leads to the appropriate secondary increase in serum PTH. This secondary hyperparathyroidism involves increases in PTH gene expression, synthesis, and secretion, and if chronic, to proliferation of the PT cells. Low serum Ca(2+) leads to an increase in PTH secretion, PTH mRNA stability, and PT cell proliferation. P(i) also regulates the PT in a similar manner. The effect of Ca(2+) on the PT is mediated by a membrane Ca(2+) receptor. 1,25(OH)(2)D(3) decreases PTH gene transcription. Ca(2+) and P(i) regulate the PTH gene posttranscriptionally by regulating the binding of PT cytosolic proteins, trans factors, to a defined cis sequence in the PTH mRNA 3'-untranslated region, thereby determining the stability of the transcript. PT trans factors and cis elements have been defined.

Pathogenesis of parathyroid dysfunction in end-stage renal disease

The parathyroid functions to maintain normal calcium and phosphate homeostasis and is central to normal bone physiology. In end-stage renal disease (ESRD), there is a failure of these normal homeostatic mechanisms with the frequent development of secondary hyperparathyroidism, which contributes to the pathogenesis of renal bone disease. The phosphate retention of ESRD, together with the reduced serum calcium and 1,25-dihydroxycholecalciferol vitamin D(3) (1,25[OH](2)D(3)) concentrations are the known factors that determine the progression to secondary hyperparathyroidism. 1,25(OH)(2)D(3) markedly decreases parathyroid hormone (PTH) gene transcription, whereas the effects of calcium and phosphate are on PTH mRNA stability, PTH secretion, and parathyroid cell proliferation. The mechanisms of these effects are discussed in this review.

Pathogenesis of refractory secondary hyperparathyroidism

Calcitriol is currently used to reduce parathyroid hormone (PTH) levels in uremic patients. However, a significant number of patients fail to respond to calcitriol therapy. The data suggest that a poor response to calcitriol can be anticipated in patients with severe hyperparathyroidism (with a high basal PTH levels) and uncontrolled serum phosphate. The abnormal parathyroid response to calcitriol in uremic patients with severe parathyroid hyperplasia may be attributed, to a large extent, to the development of nodular hyperplasia as a result of clonal transformation from a diffuse polyclonal hyperplasia. The factors involved in the development of polyclonal parathyroid hyperplasia, at earlier stages of secondary hyperparathyroidism, appear to be the same factors that stimulate PTH secretion and synthesis: hypocalcemia, hyperphosphatemia and low serum calcitriol levels. Studies performed in vitro using parathyroid tissue from uremic patients who required parathyroidectomy demonstrate that in nodular hyperplasia there is an abnormal response to calcium and calcitriol, which suggests that there are factors intrinsic to the hyperplastic cell (such as decrease in calcium sensor receptors and vitamin D receptors) responsible for an abnormal regulation of parathyroid function. Accumulation of phosphate is a key factor in the pathogenesis of secondary hyperparathyroidism and a poor response to calcitriol treatment is associated with the failure to control the serum phosphorus. High phosphate stimulates PTH secretion as demonstrated by in vivo and in vitro studies. In addition, animal studies strongly suggest that phosphate increases parathyroid cell proliferation. There are growth-related genes potentially involved in uremic hyperparathyroidism; however, changes in the expression of these genes may be the consequence rather than the cause of parathyroid hyperplasia.

p21WAF1 and TGF-alpha mediate parathyroid growth arrest by vitamin D and high calcium

BACKGROUND

High dietary phosphorus (P) worsens uremia-induced parathyroid (PT) hyperplasia through increases in the growth promoter transforming growth factor-alpha (TGF-alpha). In contrast, P restriction prevents PT hyperplasia by inducing the cell cycle inhibitor p21. Since 1,25(OH)2D3-antiproliferative action in various cell types involve increases in p21, we studied whether induction of p21 by 1,25(OH)2D3 or the vitamin D analog, 19-Nor-1,25(OH)2D2, could counteract the PT hyperplasia induced by high dietary P in early uremia.

METHODS

Normal (N) and uremic (U; 5/6 nephrectomized) female Sprague-Dawley rats were fed high P (HP), low P (LP) or high Ca (HCa) diets and administered intraperitoneally (IP) either vehicle or vitamin D metabolites for seven days, as follows: N-HP; U-HP + vehicle; U-HP + 1,25(OH)2D3 (4 ng/day); U-HP + 19-Nor-1,25(OH)2D2 (30 ng/day); U-LP; U-HCa. Serum PTH and PT gland weight assessed secondary hyperparathyroidism. Immunohistochemical quantitation of two markers of mitotic activity, Ki67 and PCNA measured PT hyperplasia. Immunohistochemical expression of PT p21 and TGF-alpha addressed potential mechanisms regulating PT cell growth.

RESULTS

1,25(OH)2D3 and 19-Nor-1,25(OH)2D2 were effective in suppressing both PTH secretion and PT hyperplasia induced by uremia and high dietary P independent of increases in ionized Ca. Both vitamin D compounds enhanced PT p21 expression and prevented high P-induced increases in PT TGF-alpha content. Induction of PT p21 and reduction of TGF-alpha content also occurred when uremia-induced PT hyperplasia was suppressed by high dietary Ca.

CONCLUSIONS

In early uremia, vitamin D suppression of high P-induced PT hyperplasia and high dietary Ca arrest of PT growth involve induction of PT p21 and prevention of increases in TGF-alpha.

Parathyroid hyperplasia in uremic rats precedes down-regulation of the calcium receptor

BACKGROUND

Recent evidence points to a relationship between the down-regulation of the calcium-sensing receptor (CaR) and parathyroid cell hyperplasia that is associated with chronic renal failure. It is not known, however, if down-regulation of the CaR precedes, and perhaps initiates, parathyroid cell proliferation, or if a decrease in the expression of the CaR occurs subsequently to hyperplasia or the conditions promoting it. The current study examined the temporal relationship of these two events.

METHODS

Rats were made uremic by subtotal nephrectomy and were (1) placed immediately on a high phosphate (HP) diet that promotes parathyroid gland hyperplasia, or (2) maintained on a low phosphate (LP) diet that inhibits development of secondary hyperparathyroidism before being switched to the HP diet. Serum chemistries and parathyroid gland (PTG) weights were examined; CaR content and parathyroid cell proliferation (PCNA/Ki-67) were analyzed by immunohistochemistry.

RESULTS

When rats were nephrectomized and placed immediately on a HP diet, parathyroid cell proliferation was significantly increased by day 2 and continued to increase at day 4. CaR content was unchanged at 1 and 2 days post-nephrectomy, but fell by day 4. When nephrectomized rats were maintained for 1 week on a LP diet, then switched to a HP diet, an increase in parathyroid cell proliferation was again seen at day 2; down-regulation of the CaR did not occur until after 7 days of uremia and the HP diet.

CONCLUSION

These data indicate that parathyroid cell hyperplasia precedes down-regulation of CaR expression in the uremic rat model.