Inhibition of TGF-beta-stimulated CTGF gene expression and anchorage-independent growth by cAMP identifies a CTGF-dependent restriction point in the cell cycle.

CTGF is a 38 kDa cysteine-rich peptide whose synthesis and secretion are selectively induced by transforming growth factor beta (TGF-beta) in connective tissue cells. We have investigated the signaling pathways controlling the TGF-beta induction of connective tissue growth factor (CTGF) gene expression. Our studies indicate that inhibitors of tyrosine kinases and protein kinase C do not block the signaling pathway used by TGF-beta to induce CTGF gene expression. In contrast, elevation of cAMP levels within the target cells by a variety of methods blocked the induction of CTGF by TGF-beta. Furthermore, agents that elevate cAMP blocked the induction of anchorage-independent growth (AIG) by TGF-beta. Inhibition of AIG could be overcome by the addition of CTGF, indicating that it was not a general inhibition of growth but a selective inhibition of CTGF synthesis that is responsible for the inhibition of TGF-beta-induced AIG by cAMP. Kinetic studies of the induction of DNA synthesis by CTGF in cells arrested by cAMP indicate that the block occurs in very late G1. These and other studies in monolayer cultures suggest that the CTGF restriction point in the cell cycle is distinct from the adhesion-dependent arrest point.