CROSS-TALK OF PARATHYROID HORMONE-RESPONSIVE DUAL SIGNAL-TRANSDUCTIONSYSTEMS IN OSTEOBLASTIC OSTEOSARCOMA CELLS - ITS ROLE IN PTH-INDUCED HOMOLOGOUS DESENSITIZATION OF INTRACELLULAR CALCIUM RESPONSE

The present study was designed to characterize the cross-talk of parat hyroid hormone (PTH)-responsive dual signal transduction systems (cAMP -dependent protein kinase (PKA) and calcium/protein kinase C [PKC]) an d its participation in PTH-induced homologous desensitization of intra cellular calcium ([Ca2+]i) in osteoblastic UMR-106 cells. Although our recent study revealed that prolonged (more than 2 h) pretreatment wit h PKC-activating phorbol ester, phorbol 12-myristate 13-acetate (PMA) significantly decreased the PTH-stimulated cAMP production, pretreatme nt with PMA (10(-7) and 10(-6) M) but not 10(-6) M 4alphaphorbol 12,13 -didecanoate (PDD), incapable of activating PKC for 30 min significant ly augmented 10(-7) M hPTH-(1-34)-stimulated cAMP production. H-7 (50 uM), a PKC inhibitor, significantly antagonized this PMA-induced effec t. Pretreatment with 10(-6) M PMA for 30 min did not affect PTH recept or binding but significantly augmented a cAMP responsiveness to 10(-5) M forskolin and 1 ug/ml cholera toxin. Pertussis toxin (0.5 ug/ml) di d not affect the PMA-induced augmentation of the PTH-stimulated cAMP p roduction. PTH caused a complete homologous desensitization of [Ca2+]i response within 30 min. Pretreatment with 10(-9) M dibutyryl cAMP for 30 min and 6 h significantly reduced and completely blocked the PTH-i nduced increase in [Ca2+]i, respectively. Pretreatment with 10(-4) M S p-cAMPS, a direct PKA activator, for 30 min completely blocked the PTH -induced increase in [Ca2+]i. Rp-cAMPS (10(-4) M), an antagonist of PK A, slightly but significantly antagonized the PTH-induced homologous d esensitization of [Ca2+]i response. The present study indicates that t he time of exposure to PKC activation is a critical determinant in mod ulating the cAMP system, while PKA activation counterregulatorily acts on the [Ca2+]i system, and that PKA activation is linked to the PTH-i nduced homologous desensitization of [Ca2+]i response.