Existence of endogenous inhibitor(s) of prostaglandin endoperoxide H synthase activities in murine NIH3T3 fibroblasts.

The production of prostaglandins (PGs) is regulated by several processes, i.e. arachidonic acid release, prostaglandin endoperoxide H synthase (PGHS) activity and its induction. In the present study, we investigated the possibility that inhibitor(s) of PGHS activities exist endogenously and regulate PG production. NIH3T3 cell, a cloned murine fibroblast, expressed PGHS-1 under unstimulated conditions and induced PGHS-2 protein under stimulation by serum. When PGHS activity was measured by individual intact cell assay, there was heterogeneity in PGHS activities in individual cells after serum treatment. We isolated and cultured many cloned cells which showed different PGHS activities after serum treatment. In these cells, however, there was no difference in the expression of PGHS-1 and PGHS-2 mRNA. The protein level of PGHS-1 and PGHS-2 was also not different in these cloned cells. The lysate obtained from the cells that showed lower PGHS activity after the serum treatment suppressed PGHS-1 activities of sheep seminal vesicle and PGHS-2 activity in purified ovine placenta. The suppression by this lysate was stronger than that by the lysate obtained from the cells which showed higher PGHS activity after the serum treatment. The production of the inhibitor(s) was up-regulated by the serum treatment and abolished by actinomycin D and cycloheximide during the serum treatment. From these results, we suggest that there is some endogenous inhibitor(s) of PGHS-1 and -2 activities in NIH3T3 cells, and that the inhibitor(s) is induced by serum.