INHIBITION OF COLLAGEN-SYNTHESIS BY PROSTAGLANDINS IN THE IMMORTALIZED RAT OSTEOBLASTIC CELL-LINE PY1A - STRUCTURE-ACTIVITY RELATIONS AND SIGNAL-TRANSDUCTION MECHANISMS

We previously showed that prostaglandin E(2) (PGE(2)) can selectively inhibit collagen synthesis and gene transcription in the immortalized rat osteoblastic clonal cell line Py1a, particularly in the presence o f insulin-like growth factor I (IGF-I). In the present study, we exami ned the structure-activity relations for this effect. PGF(2 alpha) was approximately 100 times more potent than PGE(2). The prostaglandin F receptor (FP) selective agonist, fluprostenol, was the most potent ago nist tested, significantly inhibiting incorporation of [H-3]proline in to both collagen and noncollagen protein at 10(-11) M, with more than 90% inhibition of collagen synthesis at 10(-8) M. The PGE(2) analog, s ulprostone, and PGD(2) showed activity similar to that of PGE(2). PGI( 2), and its stable analog, carbacyclin, were the least effective. Para thyroid hormone (PTH), forskolin, and isobutylmethylxanthine (IBMX) we re ineffective. Phorbol myristate acetate (PMA) inhibited collagen syn thesis in a manner similar to that of the prostanoids. The inhibitory effects of PGF(2 alpha), fluprostenol, and PMA show a similar time cou rse on alpha(1)(I) procollagen mRNA levels. The inhibition appeared to be caused by a decrease in collagen gene transcription as measured by nuclear run-on analysis. Further evidence for a transcriptional effec t was obtained with COL1A1 promoter-CAT reporter constructs, although these showed somewhat smaller effects of prostanoids on CAT activity t han on mRNA levels or labeling. Based on these results, we conclude th at in the Py1a cell line prostanoids inhibit collagen synthesis by a p athway involving activation of protein kinase C that is not dependent on adenylate cyclase. The structure-activity relations for this respon se suggest that it is mediated by an FP receptor and is distinctly dif ferent from the stimulatory effects on bone resorption and formation.