CALCIUM MEDIATES EXPRESSION OF STRESS-RESPONSE GENES IN PROSTAGLANDIN A(2)-INDUCED GROWTH ARREST

We have explored the mechanisms involved in the induction of five stress-response genes (heme oxygenase [HO], c-fos, Egr-1; gadd153, and HSP70) in human diploid fibroblasts growth-arrested by treatment with the antiproliferative prostaglandin A(2) (PGA(2)). The kinetics of c-fos and Egr-1 induction were found to be rapid with maximum expression occurring within 60 min of treatment, whereas maximum expression of HO, gadd153, and HSP70 occurred between 4 and 8 h of treatment. Nuclear run-on assays and measurements of mRNA clearance in the presence of actinomycin D demonstrated that increases in both the rates of gene transcription and/or mRNA stability contribute to the genetic response to PGA(2). Although the mechanisms responsible for increasing the mRNA levels differ for the individual genes, additional experiments provided evidence that alterations in intracellular calcium ([Ca2+](i)) levels were important in initiating the genetic response to PGA(2). PGA(2) treatment resulted in a rapid increase in [Ca2+](i) with the dose-response relationship for Ca2+ mobilization consistent with that seen for the induction of all five genes. [Ca2+](i) chelators that attenuate Ca2+ mobilization by PGA(2) also blocked the mRNA induction by PGA(2) treatment. Density-inhibited confluent cells were less responsive than proliferating subconfluent cells with respect to Ca2+ mobilization after PGA(2) treatment. This was correlated with a lower level of gene induction. These studies support the hypothesis that increased Ca2+ mobilization is an early and central event in the signal transduction pathway (or pathways) mediating the activation of genes in response to PGA(2) treatment.