Down-regulation and differential restoration of cAMP responses upon transient phorbol ester treatment of primary osteoblastic cells.

Abstract We studied cAMP responses induced by parathyroid hormone (PTH), prostaglandin E 2 (PGE 2 ) and forskolin in foetal rat calvariae-derived osteoblastic cells after 24 h treatment with a protein kinase C (PKC) activating phorbol ester. After this treatment, meant to down-regulate PKC activity, all tested cAMP responses were attenuated and were indeed accompanied by a decline in PKC activity. PTH receptor affinity was not altered and PTH receptor number was only slightly lowered after 24 h phorbol ester treatment. These results indicate that modulation of the CAMP responses by 24 h PMA treatment was mainly caused by a general impairment of adenylyl cyclase activity. Removal of the phorbol ester and subsequent culture for 2 days rendered the cells hyper-responsive to PTH: the PTH-induced cAMP response was 2 to 3 times higher than in control cells. Again no change in binding affinity of the PTH receptor was observed and receptor number was just 10% lower than in control cells. The PGEZ- and forskolin-induced CAMP responses were not higher than normal. So, transient phorbol ester treatment leads to a differential, agonist-dependent restoration of the cAMP signalling system.