Microsomal postaglandin (PG) E synthase (mPGES)-1 is an inducible enzyme that acts downstream of cyclooxygenase (COX) and specifically catalyzes the conversion of prostaglandin (PG)H₂ to PGE₂, most prominently in inflammatory conditions. Specific inhibitors of mPGES-1 are not yet available, however, mice with genetic deletion of mPGES-1 have been generated that have given insight into the specific role of mPGES-1 in eicosanoid biosynthesis in vivo and in peritoneal macrophages. We created mouse embryo fibroblast (MEF) cell lines that would facilitate investigation of the effect of mPGES-1 genetic deletion on prostanoid biosynthesis in fibroblast lineage cells and its subsequent effect on the expression of inducible NOS (iNOS) and nitrite biosynthesis using cells derived from mPGES-1 wild-type (WT), heterozygous (Het), and null mice. The results show that genetic deletion of mPGES-1 results in a dramatic decrease in PGE₂ production in Het and null MEFs under basal conditions and after stimulation with interleukin (IL)-1, suggesting that mPGES-1 is critically important for PGE₂ production. Furthermore, we show that mPGES-1 gene deletion results in diversion of prostanoid production from PGE₂ to 6-keto PGF₁ (the stable metabolic product of PGI₂; prostacyclin) in a gene dose-dependent manner in Het and null MEFs compared with their WT counterparts, suggesting a shunting phenomenon within the arachidonic acid (AA) metabolic pathway. In addition, we show that mPGES-1 gene deletion and subsequent decrease in PGE₂ levels results in a differential induction profile of iNOS and nitrite levels (the stable breakdown product of nitric oxide (NO) in mPGES-1 WT MEFs compared with null MEFs. These results provide important information regarding the therapeutic potential for pharmacologic inhibition of mPGES-1 in inflammatory conditions.--Kapoor, M., Kojima, F., Qian, M., Yang, L., and Crofford, L. J. Shunting of prostanoid biosynthesis in microsomal prostaglandin E synthase-1 null embryo fibroblasts: regulatory effects on inducible nitric oxide synthase expression and nitrite synthesis.