A parathyroid hormone (PTH) like protein implicated in the pathogenesis of hypercalcemia in malignancy has recently been identified. It has isoforms of 139 and 141 amino acids and is strongly homologous with PTH only at its amino-terminus. The PTH-like protein (PLP) is expressed by squamous and renal carcinomas, binds with high affinity to the PTH receptor in kidney and bone, and is a potent hypercalcemic substance. Although it thus has all the properties predicted of a humoral mediator of hypercalcemia, PLP has yet to be identified as a circulating factor in malignancy-associated hypercalcemia. Besides solid tumors, HTLV-1 infected malignant lymphocytes express PLP, and clinical dta suggest a humoral role in adult T-cell leukemia, which if frequently complicated by hypercalcemia. We have developed a radioimmunoassay (RIA) to measure PLP is serum, and preliminary results indicate that PLP is detectable in hypercalcemic patients with squamous carcinoma, but not in normal subjects. The further RIA studies proposed are intended to forge the final link in a chain of evidence that implicates PLP in humoral hypercalcemia. PLP may also have a second role in the pathogenesis of hypercalcemia, inducing local osteolysis when secreted at low levels by betastatic tumors in bone. In view of expression of PLP by lactating mammary tissue and its isolation from a single breast carcinoma, we will screen for expression of PLP in breast carcinoma, a tumor in which hypercalcemia is thought to result from local osteolysis. PLP is also expressed in normal embryonic and adult tissues, including skin and lactating breast. Its potency and its remarkable evolutionary conservation (human and rat PLP are identical through 110 amino acids) suggest a significant physiological role for PLP. That it is not detectable in normal serum raises the possibility that these physiological effects are local. Expression in differentiating keratinocytes and embryonal carcinoma cells suggests that PLLP may be a differentiation factor, a possibility that we will explore in both systems. At the same time, we will attempt uncover paracrine targets for PLP by localizing its receptors autoradiographically. We have begun to study how the secretion of PLP is controlled by calcium, cytokines and hormones, where the control of its synthesis is exerted at the cellular level, and how PLP is processed for secretion. Besides their implications for the physiological function of PLP, these studies will cast light on the reasons for its excessive secretion in malignancy.
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