Cross talk of dual-signal transduction systems in the regulation of DNA synthesis by parathyroid hormone in osteoblastic osteosarcoma cells

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Abstract
There has been recent evidence that calcium/protein kinase C (Ca/PKC) messenger system as well as adenylate cyclase are involved in the signal transduction stimulated by PTH. We therefore examined the role of these dual-signal transduction systems and the interaction of these systems in the regulation of DNA synthesis by PTH in the osteoblastic osteosarcoma cells, UMR-106. As recently reported, 10−4 M Sp-cAMPS, a direct activator of cAMP-dependent protein kinase (PKA), and 10−4 M dibutyryl-cAMP, as well as hPTH-(1–34), caused the significant inhibition of [3H]thymidine incorporation (TdR). Both A23187 and ionomycin (10−8-10−6 M) inhibited TdR in a dose-dependent manner, with a minimal effective dose at 10−7 M. Although 10−6 M phorbol 12-myristate 13-acetate (PMA) caused slight but significant stimulation of TdR by itself, it augmented not only dibutyryl-cAMP- but also Sp-cAMPS-induced inhibition of TdR. On the other hand, 4α-phorbol 12,13-didecanoate, incapable of activating PKC, failed to augment these cAMP analogs-induced effects. Pretreatment with 50 μM H-7, an inhibitor of PKC, not only abolished the PMA-induced augmentation of effect by cAMP analogs but also significantly blocked the PTH-induced inhibitory effect on TdR. Pretreatment with 10−6 M PMA, which downregulates PKC, significantly inhibited the PTH-induced suppression of TdR. Combined treatment with cAMP analog (dibutyryl-cAMP or Sp-cAMPS) and calcium ionophore (A23187 or ionomycin) caused additive effects on TdR, and PMA used in combination with both cAMP analog and calcium ionophore induced the further inhibition of TdR. The present study indicated that in addition to the PKA system, the Ca/PKC system also played a contributory role in the full expression of PTH response in the regulation of DNA synthesis in osteoblasts. The present data provided additional evidence for a dual pathway of target cell activation by PTH and the existence of interaction between the adenylate cyclase system and the Ca/PKC system.

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