Metabolites of arachidonic acid (AA), particularly, those of the cyclooxygenase pathway: prostaglandin E (PGE2), PGF2alpha and PGD2 as well as those of the 5-lipoxygenase pathway: leukotriene B4 (LTB4) LTC4 and LTD4 are recognized as pivotal in modulating cutaneous inflammatory and proliferative/differentiation processes. Despite this recognition, their underlying mechanisms of action are unclear. Even less clear are the functional roles of another group of lipoxygenase products: The hydroxy polyunsaturated fatty acids derived from the various polyunsaturated fatty acids (PUFAs). Although minimally known the hydroxy fatty acids have recently excited great interest because they can be re-incorporated into membrane lipids, modulate cell proliferation, induce intracellular calcium formation, thus, representing a novel concept regarding the mechanism of eicosanoid action. For example, the HETEs (hydroxyeicosatetraenoic acids generated from 20-carbon AA) are reported to modulate cell proliferation and intracellular calcium. In contrast, little is known about hydroxy acids generated from 18-carbon PUFAs. Linoleic acid (LA) a major PUFA in the epidermis is transformed by epidermal 15-lipoxygenase into 13- hydroxyoctadecadienoic acid (13-HODE). This metabolite inhibits and reverses epidermal hyperproliferative lesion to normal in guinea pig epidermis. Since eicosanoid/signal transduction/protein kinase C interactions are now recognized as modulators of proliferation and carcinogenesis we propose to test the hypothesis that: the antiproliferative function of 13HODE involves its reincorporation into epidermal phospholipids, particularly, the inositol phospholipids and release of novel 13HODE-substituted diacylglycerol (13HODE-DAG). Specifically, we will: (i) double label phosphatidylinositol 4, 5- bisphosphate (PtdIns 4, 5-P2) with radioactive substrates: [14C]13-HODE and [3H]inositol; (ii) induce hydrolysis of [14C/3H]PtdIns 4,5-P2 by incubating with epidermal homogenate (containing phospholipase C) to release a novel 13HODE-substituted diacylglycerol (13HODE-DAG) and (iii) test the effects of the novel 13HODE-DAG on selective protein kinase C (PKC) isozyme activities (associated with epidermal hyperproliferation) in in vitro and in vivo systems. Findings from these studies should reveal for the first time that hydroxy fatty acids once regarded as nonfunctional breakdown products of PUFAs can be re-incorporated into epidermal lipids. That released putative 13HODE-DAG could be the in vivo modulator of epidermal cellular PKC-associated proliferation/differentiation.
