Conjugated linoleic acid modulation of phorbol ester-induced events in murine keratinocytes

Recent work in our lab has shown that the chemoprotective fatty acid, conjugated linoleic acid (CLA), inhibits phorbol ester skin tumor promotion in mice. Because little is known about the deposition of CLA into tissues as well as its biological activity, this study compared the incorporation and biological activity of CLA to linoleic acid (LA; 18:2, c9,c12) and arachidonic acid (AA; 20:4 c5,c8,c11,c14) in cultured keratinocytes. When keratinocytes (HEL-30) were grown in media containing C-14-CLA for various periods, more than 50% of the C-14-CLA was incorporated into cellular lipids by 9 h. The distribution of CLA in phospholipid classes was similar to LA. Approximately 50% of C-14-LA and C-14-CLA were incorporated into phosphatidylcholine (PC), while the remainder was taken up by phosphatidylethanolamine (PE) and phosphatidyl-serine/phosphatidylinositol (PS/PI). In contrast, C-14-AA was more equitably distributed into PC, PE, or PS/PI (27, 30, or 38%, respectively). When keratinocytes were prelabeled with radio-labeled fatty acids, phorbol ester-induced release of C-14-CLA was 1.5 times higher than C-14-LA and C-14-AA. However, C-14-prostaglandin E (PGE) release in C-14-CLA prelabeled cultures was 6 and 13 times lower than cultures treated with C-14-LA and C-14-AA, respectively. Moreover, the ability of non-radiolabeled CLA to support ornithine decarboxylase activity, a hallmark event of tumor promotion, was significantly lower than in LA- and AA-treated cultures. These studies suggest that CLA inhibits skin tumor promotion, in part, through a PGE-dependent mechanism.