Proliferative effects of insulin and epidermal growth factor on mouse mammary epithelial cells in primary culture. Enhancement by hydroxyeicosatetraenoic acids and synergism with prostaglandin E2.

Linoleate metabolism via the cyclooxygenase pathway enhances the proliferation of mammary epithelial cells in serum-free culture in the presence of epidermal growth factor and insulin (Bandyopadhyay, G.K., Imagawa, W., Wallace, D., and Nandi, S. (1987) J. Biol. Chem. 262, 2750-2756). Prostaglandin E2 (PGE2) can fully substitute for linoleic acid provided endogenous hydroxyeicosatetraenoic acids (HETEs, lipoxygenase metabolites) are available. The PGE2 effect is partial if lipoxygenase activity is inhibited by nordihydroguaiaretic acid. Any combination of two HETEs out of three tested (5-, 12-, and 15-HETEs) stimulates growth synergistically with PGE2; and together (i.e. PGE2 + HETEs), they completely substitute for linoleate. In the absence of PGE2, maximal stimulation cannot be attained with HETEs. Exogenous 5-HETE, compared with 12- or 15-HETE, is preferentially incorporated by the mammary epithelial cells, and about 25-30% of it is retained esterified in phospholipids. The cellular level of nonesterified, free HETE is low. Radioimmunoassay revealed that the concentrations of 12- and 15-HETEs in the culture media (with or without added linoleate) were always higher than that of 5-HETE. Both intra- and extracellular free HETEs are rapidly metabolized by the cells. Since these cells are capable of producing eicosanoids from linoleate, periodic supplementation of the cultures with linoleate allows maintenance of higher HETE and PGE2 levels. Thus, it appears that not only are HETEs short-lived in the cell cultures, but cells handle 5-HETE differently than 12- and 15-HETEs. Whatever may be the pathways of interaction, synergism between HETEs and PGE2 seems to explain how linoleate stimulates the growth of mammary epithelial cells in the presence of epidermal growth factor and insulin.