Induction of differentiation in v-Ha-ras-transformated MDCK cells by prostaglandin E 2 and 8-bromo-cyclic AMP is associated with a decrease in steady-state level of inositol 1,4,5-trisphosphate

Abstract

We used Ha-ras-transformated Madin-Darby canine kidney (MDCK) cells as a model to study possible signal transduction mechanisms underlying the induction of glucagon responsiveness by the differentiation inducers prostaglandin E 2 (PGE 2) and 8-bromo-cyclic (8-Br-cAMP) AMP and the inhibition of induction by phorbol ester or a serum factor. The steady-state level of inositol 1,4,5-trisphosphate (IP 3) was higher in Ha-ras-transformed MDCK cells than in parental MDCK cells. In contrast, the steady-state level of intracellular cAMP of transformed cells was similar to that of normal cells. PGE 2 and 8-Br-cAMP increased cAMP content but decreased IP 3 levels in a concentration-dependent fashion after 5 days of treatment. We examined the time course for effects of PGE 2 and 8-Br-cAMP and found that there was a lag period of 8 to 16 h between elevation of cAMP after the addition of 8-Br-cAMP or PGE 2 and the decrease of IP 3 levels. Another lag period of 2 days existed before the induction of differentiation. Both the reduction of IP 3 levels and the induction of glucagon responsiveness were blocked by phorbol-12-myristate-13-acetate or serum, suggesting that a decrease in the IP 3 level might be causally involved in induction of differentiation in transformed MDCK cells. However, induction of differentiation was not due to changes in the expression or guanine nucleotide-binding properties of p21 protein. It is likely that cAMP has a direct regulatory effect on the phospholipid signalling pathway. We conclude that perturbation of the inositol phosphate signalling pathway may be responsible for the induction of differentiation by PGE 2 and 8-Br-cAMP in transformed MDCK cells.