Cross‐talk of parathyroid hormone‐responsive dual signal transduction systems 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 parathyroid hormone (PTH)‐responsive dual signal transduction systems (cAMP‐dependent protein kinase (PKA) and calcium/protein kinase C [PKC]) and its participation in PTH‐induced homologous desensitization of intracellular calcium ([Ca 2+ ]i) in osteoblastic UMR‐106 cells. Although our recent study revealed that prolonged (more than 2 h) pretreatment with PKC‐activating phorbol ester, phorbol 12‐myristate 13‐acetate (PMA) significantly decreased the PTH‐stimulated cAMP production, pretreatment 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 significantly augmented 10 −7 M hPTH‐(1‐34)‐stimulated cAMP production. H‐7 (50 uM), a PKC inhibitor, significantly antagonized this PMA‐induced effect. Pretreatment with 10 −6 M PMA for 30 min did not affect PTH receptor binding but significantly augmented a cAMP responsiveness to 10 −5 M forskolin and 1 ug/ml cholera toxin. Pertussis toxin (0.5 ug/ml) did not affect the PMA‐induced augmentation of the PTH‐stimulated cAMP production. PTH caused a complete homologous desensitization of [Ca 2+ ]i response within 30 min. Pretreatment with 10 −4 M dibutyryl cAMP for 30 min and 6 h significantly reduced and completely blocked the PTH‐induced increase in [Ca 2+ ]i, respectively. Pretreatment with 10 −4 M Sp‐cAMPS, a direct PKA activator, for 30 min completely blocked the PTH‐induced increase in [Ca 2+ ]i. Rp‐cAMPS (10 −4 M), an antagonist of PKA, slightly but significantly antagonized the PTH‐induced homologous desensitization of [Ca 2+ ]i response. The present study indicates that the time of exposure to PKC activation is a critical determinant in modulating the cAMP system, while PKA activation counterregulatorily acts on the [Ca 2+ ]i system, and that PKA activation is linked to the PTH‐induced homologous desensitization of [Ca 2+ ]i response. © 1994 Wiley‐Liss, Inc.