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 possi bility that inhibitor(s) of PGHS activities exist endogenously and regulate PG production. NIH3T3 cell, a cloned murine fibroblast, expressed PGHS-1 u nder unstimulated conditions and induced PGHS-2 protein under stimulation b y serum. When PGHS activity was measured by individual intact cell assay, t here was heterogeneity in PGHS activities in individual cells after serum t reatment. 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 leve l of PGHS-1 and PGHS-2 was also not different in these cloned cells. The ly sate obtained from the cells that showed lower PGHS activity after the seru m treatment suppressed PGHS-1 activities of sheep seminal vesicle and PGHS- 2 activity in purified ovine placenta. The suppression by this lysate was s tronger 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 an d cycloheximide during the serum treatment. From these results, we suggest that there is some endogenous inhibitor(s) of PGHS-1 and -2 activities in N IH3T3 cells, and that the inhibitor(s) is induced by serum.