Transduction Mechanisms Involved in Thrombin Receptor‐Induced Nerve Growth Factor Secretion and Cell Division in Primary Cultures of Astrocytes

In astrocytes, thrombin and thrombin receptor-activating peptide (TRAP-14), a 14-amino-acid agonist of the proteolytic activating receptor for thrombin (PART), significantly increased cell division as assessed by [3H]-thymidine incorporation into DNA (EC50 = 1 nM and +650% at 100 nM for thrombin; EC50 = 3 microM and +600% at 100 microM for TRAP-14) and nerve growth factor (NGF) secretion (approximately twofold at 100 nM thrombin or 100 microM TRAP-14). The [3H] thymidine incorporation was prevented by protein kinase C inhibitors (staurosporine and H7) or by down-regulation of this enzyme by chronic exposure of astrocytes to phorbol 12-myristate 13-acetate (PMA). Thrombin-induced NGF secretion was completely inhibited by protein kinase C inhibitors. Treatment with PMA stimulated NGF secretion 19-fold, and this effect was not further enhanced by thrombin. These data suggest an absolute requirement of protein kinase C activity for thrombin-induced NGF secretion and cell division. Pretreatment of astrocytes with pertussis toxin (PTX) reduced thrombin- and TRAP-14-induced DNA synthesis. PART activation caused a decrease in forskolin-stimulated cyclic AMP accumulation. PTX treatment prevented the inhibitory effect of PART activation on cyclic AMP accumulation, suggesting that a PTX-sensitive G protein, such as Gi or G(o), is involved in thrombin-induced cell division. In contrast, thrombin-induced NGF secretion was not inhibited by PTX. Finally, the protein tyrosine kinase inhibitor herbimycin A partially but significantly prevented thrombin- and TRAP-14-induced cell division but was without effect on NGF secretion. Taken together, these results demonstrate that, in astrocytes, PART(s)-triggered cell division or NGF secretion is mediated by distinct transduction mechanisms.