PARATHYROID HORMONE-RELATED PROTEIN IS AN AUTOCRINE MODULATOR OF RABBIT PROXIMAL TUBULE CELL-GROWTH

Parathyroid hormone-related protein (PTHrP), a likely mediator for hum oral hypercalcemia of malignancy, is also synthesized in various norma l tissues. In the kidney, PTHrP, mainly detected in proximal and dista l tubules, has been shown to stimulate proliferation of rat mesangial cells in culture. Experiments were carried out to investigate the poss ible mitogenic effect of PTHrP in cultures of rabbit proximal tubule c ells (PTC). Immunocytochemical analysis, using antihuman (h)PTHrP anti bodies to (38-64) and (107-111) epitopes in the PTHrP molecule, showed strong cytoplasmic staining in PTC and in proximal tubule-like LLC-PK 1 cells. PTC secreted immunereactive PTHrP (54.8 +/- 7.0 fmol/10(6) ce lls) into the culture medium. Human PTHrP(1-141) stimulated proliferat ion in subconfluent cultures of these cells dose-dependently. This eff ect was similar to that induced by [Tyr(34)] hPTHrP(1-34) amide (hPTHr P[1-34]), hPTHrP(1-86), and bovine (b)PTH(1-34), while hPTHrP(38-64) a mide, hPTHrP(107-111) amide, and hPTHrP(107-139) amide were ineffectiv e. Addition of anti-hPTHrP neutralizing antibodies to (1-34), (38-64), and (107-111) epitopes of PTHrP decreased PTC growth. The mitogenic e ffect of these agonists was abolished in confluent PTC. In contrast, [ Nle(8,18) Tyr(34)]bPTH(3-34) amide (PTH[3-34]) increased DNA synthesis in either subconfluent or confluent PTC. In LLC-PK1 cells, which also secreted PTHrP and are devoid of PTH receptors, none of these peptide s affected proliferation. Forskolin (10 mu M) or H-8 (2 mu M), a prote in kinase A inhibitor, did not affect basal or hPTHrP(1-34)-stimulated DNA synthesis, respectively, in subconfluent PTC. On the other hand, 10 nM staurosporine and 100 nM calphostin C, protein kinase C (PKC) in hibitors, blunted the effects of hPTHrP(1-34) or bPTH(3-34) on DNA syn thesis in these cells. These studies suggest that PTHrP may function a s an autocrine factor in the regulation of proximal tubule cell growth by a PKC-mediated mechanism.