Hyperparathyroidism, in the setting of renal failure, is an important clinical problem, the pathophysiology of which is not completely understood. Decreased biosynthesis of l ,25(OH)2D3 and resistance to the biological effects of this calcium regulating hormone are thought to contribute to the development of secondary hyperparathyroidism by their effects on serum calcium levels as well as on parathyroid cellular proliferation and parathyroid hormone gene transcription. Autonomous parathyroid function developing secondary to renal failure has, thus far, only been observed in humans. Since the abnormalities in l,25(OH)2D3 responsiveness in the parathyroid glands of uremic patients undergoing parathyroidectomy are likely to reflect the most severe end of the spectrum, studies will be performed using pathological specimens obtained from these patients. Abnormalities of l,25(OH)2D3 receptor biosynthesis will be addressed by analyzing 1,25(OH)2D3 receptor protein and mRNA levels in these glands. This data will be carefully correlated with serum l,25(OH)2D3 levels, to determine whether the abnormalities observed are associated with decreased circulating hormone levels. [3H]l,25(OH)2D3 binding will also be examined to address the effects of uremia on the accessory factor thought to be required for high affinity ligand binding of the receptor. Nuclear extracts will be prepared from the surgically removed glands and used as a source of 1,25(OH)2D3 receptor to characterize 1,25(OH)2D3 receptor interactions with DNA sequences from the human parathyroid hormone gene which have been shown to mediate transcriptional repression in response to 1,23(OH)2D3 as well as with sequences which mediate transcriptional activation in response to l,25(OH)2D3. Complementation studies will be performed to address whether the abnormalities observed are due to decreased availability or binding of the receptor or of the accessory factors thought to interact with the receptor to mediate transcriptional repression or transcriptional induction. The same experiments will be performed in peripheral blood monocytes isolated from patients with uremic hyperparathyroidism. These studies will clarify whether the abnormalities observed above are parathyroid specific. Similar findings in the monocytes would provide a useful tool for analyzing l ,25(OH)2D3 receptor abnormalities at earlier and multiple stages of renal failure. By further clarifying the pathophysiology of uremic hyperparathyroidism, these studies may lead to the design of more effective therapeutic measures for the prevention and treatment of hyperparathyroidism in the setting of chronic renal failure.
