Protein kinase C is involved in PTH-induced homologous desensitization by directly affecting PTH receptor in the osteoblastic osteosarcoma cells.
We have investigated mechanisms of PTH-induced homologous desensitization reflected in the refractoriness of cAMP response to the second exposure to PTH in the clonal rat osteosarcoma cell line, UMR-106. Preincubation with 10(-7) M rat (r) PTH-(1-34) for 6 h caused the desensitization, resulting in a 65% decrease in cAMP accumulation in response to further exposure to rPTH. This desensitization was apparent at 10(-10) M rPTH and maximal at 10(-7) M rPTH. UMR-106 cells treated with protein kinase C (PK-C) activating phorbol ester, phorbol 12-myristate 13-acetate (PMA, 10(-6) M) for 6 h also induced desensitization manifested by a loss of rPTH-stimulated cAMP accumulation to 50% of that in the control cells. On the other hand, 4 alpha-phorbol 12,13-didecanoate, incapable of activating PK-C, failed to induce desensitization. Fifty micromolar H-7 (PK-C inhibitor) significantly blocked both rPTH- and PMA-induced desensitization. Thus, PK-C seemed to play a major role in rPTH-induced desensitization. Pretreatment with neither rPTH nor PMA changed the cAMP responsiveness to 10 micrograms/ml cholera toxin or 100 microM forskolin. Islet activating protein failed to influence the desensitization in this cell line. PTH receptor binding, assessed by using 125I-labeled [Nle8,Nle18,Tyr34]PTH-(1-34) as a radioligand, was decreased along with PTH receptor numbers by pretreatment with rPTH or PMA. These data indicate that rPTH-induced homologous desensitization occurs at least in part through the activation of PK-C and that PK-C directly affects PTH receptor in UMR-106 cells.
Ikeda
K
K
Department of Medicine, Kobe University School of Medicine, Japan.
Sugimoto
T
T
Fukase
M
M
Fujita
T
T
D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D000818:Animals; D000242:Cyclic AMP / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism; D007546:Isoquinolines / Q000494:pharmacology; D010006:Osteoblasts / Q000378:metabolism*; D012516:Osteosarcoma / Q000378:metabolism* / Q000473:pathology; D010281:Parathyroid Hormone / Q000494:pharmacology*; D010879:Piperazines / Q000494:pharmacology; D011493:Protein Kinase C / Q000502:physiology*; D047428:Protein Kinase Inhibitors; D011956:Receptors, Cell Surface / Q000187:drug effects* / Q000378:metabolism; D018016:Receptors, Parathyroid Hormone; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D007546:Isoquinolines; D010281:Parathyroid Hormone; D010879:Piperazines; D047428:Protein Kinase Inhibitors; D011956:Receptors, Cell Surface; D018016:Receptors, Parathyroid Hormone; D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D000242:Cyclic AMP; D011493:Protein Kinase C; D019204:GTP-Binding Proteins; D013755:Tetradecanoylphorbol Acetate