Human endothelial cells contain two isoforms of prostaglandin H synthase (PGHS). PGHS-1 is constitutively expressed, whereas PGHS-2 is inducible. To determine whether expression of PGHS-1 is regulated, we treated cultured human umbilical vein endothelial cells (HUVEC) with phorbol 12-myristate 13-acetate (PMA) or its inactive analogue and measured PGHS-1 mRNA levels by Northern analysis and competitive polymerase chain reaction. PMA increased PGHS-1 mRNA levels determined by both techniques in a time- and concentration-dependent manner. The mRNA level was increased about twofold over the basal level after 4-6 h of PMA (10-50 nM) treatment. The level of PGHS-1 protein was similarly increased by PMA. Stimulation of PGHS-1 mRNA levels was abrogated by cycloheximide, actinomycin D, staurosporine, or calphostin C. The 5'-promoter activity of human PGHS-1 gene was increased twofold over the basal level by PMA in NS-20 cells. These results indicate that the constitutive PGHS-1 in HUVEC is transcriptionally stimulated by PMA in a protein kinase C-dependent manner.