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 a re selectively induced by transforming growth factor beta (TGF-beta) i n connective tissue cells. We have investigated the sig naling pathway s controlling the TGF-beta induction of connective tissue growth facto r (CTGF) gene expression. Our studies indicate that inhibitors of tyro sine kinases and protein kinase C do not block the signaling pathway u sed by TGF-beta to induce CTGF gene expression. In contrast, elevation of cAMP levels within the target cells by a variety of methods blocke d the induction of CTGF by TGF-beta, Furthermore, agents that elevate cAMP blocked the induction of anchorage-independent growth (AIG) by TG F-beta, Inhibition of AIG could be overcome by the addition of CTGF, i ndicating that it was not a general inhibition of growth but a selecti ve 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 bloc k occurs in very late G(1), These and other studies in monolayer cultu res suggest that the CTGF restriction point in the cell cycle is disti nct from the adhesion-dependent arrest point.