ARACHIDONIC ACID AND LINOLEIC ACID METABOLISM IN CELL PROLIFERATION

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We have studied the response to growth factors (EGF, FGF and PGDF) and lipid metabolism in Syrian hamster embryo cell FGF was observed to stimulate the expression of PGHS-2 and mitogenesis. FGF did not enhance the metabolism of linoleic acid to 13-HODE. Some data suggest that prostaglandins were involved in FGF dependent mitogenesis but complete evidence is lacking. EGF-dependent mitogenesis was inhibited by lipoxygenase inhibitors but not attenuated by PGHS inhibitors. EGF did not stimulate prostaglandin formation but did stimulate the metabolism of linoleic acid to 13-HODE. The formation of 13-HODE is regulated by the tyrosine kinase activity of the EGF receptor but the mechanism is poorly understood. Since these metabolism studies used exogenous substrates we need to determine if 13-HODE was endogenously formed. We used GG-MS techniques to measure the formation of endogenous 13-HODE. 13-HODE was detected in sufficient concentrations to enhance EGF- dependent mitogenesis. A biphasic formation of 13-HODE in response to EGF was observed which was dependent on the EGF receptor tyrosine kinase activity. The linoleic acid metabolites potentiated EGF-dependent mitogenesis in the supB+ SHE cells but not in the supB-. We have also examined the structural characteristics necessary for enhancing EGF mitogenesis. We found that 13-S-HpODE and 13-S-HODE are the most potent and enhanced mitogenesis was selective for the metabolites which suggest a very specific biological target such as an uncharacterized receptor. EGF stimulated the expression of c-myc, c-jun and junB. The addition of PGE2 stimulated the formation of cAMP. Other agents that changed cAMP levels correspondingly altered mitogenesis. Recent reports in the literature indicate that cAMP can block or stimulate growth factor signaling pathways at the RAF-1 kinase. The data indicate that lipid metabolism appears to play an important role in the EGF-signaling pathway in SHE cells.