Temporal and regional production of 12(S)hydroxyeicosatetraenoic acid [12(S)-HETE] in rat lens

Synergy of epidermal growth factor and 12(S)-hydroxyeicosatetraenoate on protein kinase C activation in lens epithelial cells

12(S)-hydroxyeicosatetraenoic acid (12(S)HETE) is a bioactive metabolite of arachidonic acid synthesized by 12-lipoxygenase. The 12-lipoxygenase blocker, baicalein, prevents epidermal growth factor (EGF)-induced activation of protein kinase C (PKC) alpha and beta in lens epithelial cells, whereas supplementation with 12(S)HETE reverses this effect, suggesting that EGF and 12(S)HETE may work together to activate PKC. This study investigates the mechanism of PKCbeta activation by EGF and 12(S)HETE. 12(S)HETE alone directed translocation of PKCbeta through the C1 rather than the C2 domain, without activating phosphoinositide 3-kinase (PI3K) or MAPK signaling or increasing intracellular calcium concentration. In the presence of baicalein, EGF triggered an asymmetric phosphorylation of the EGF receptor initiating signaling through PI3K and MAPK, but not PLCgamma. Together, 12(S)HETE and EGF synergistically increased phosphorylation of PKCbeta in the activation loop and C terminus as well as PKCbeta-specific activity. PI3K inhibitors blocked phosphorylation, but MEK1 inhibitors did not. Microvesicles containing phosphatidylinositol 3,4,5-trisphosphate mimicked the action of EGF on PKCbeta activity in the presence of 12(S)HETE. Kinase-inactive PKCbeta mutations in either activation loop or C terminus were effectively translocated by 12(S)HETE, as was PKCbeta in the presence of chelerythrine or Gö-6983. These findings indicate that unphosphorylated PKCbeta is translocated to the membrane by 12(S)HETE and phosphorylated by EGF-dependent PI3K signaling, to generate catalytically competent PKCbeta.

Synthesis of 12(R)- and 12(S)-hydroxyeicosatetraenoic acid by porcine ocular tissues

Microsomal fractions from porcine ocular tissues synthesized 12(S)-5,8,10,14-hydroxyeicosatetraenoic acid [12(S)-HETE] from arachidonic acid by a membrane-bound lipoxygenase and 12(R)-HETE by the cytochrome P450-dependent monooxygenase system. Both activities were the highest in corneal microsomes. The 12(R)-HETE synthesizing activity of corneal microsomes was dependent on NADPH and inhibited by 0.1 mM SKF-525A, an inhibitor of P450 enzymes. The activity to form 12(R)-enantiomer was significantly enhanced by treatment of corneal epithelium with 3-methylcholanthrene or clofibrate. The induced activity was suppressed by cycloheximide, indicating that the induction of enzyme activities involved a translational process. The effect of these inducers on 12(R)-HETE synthesizing activity appeared to be additive. The activity to form 12(S)-enantiomer was markedly stimulated by 3 mM CaCl2. The 12-lipoxygenase of corneal microsomes was capable of oxygenating linoleic acid in addition to arachidonic acid, a characteristic of 12-lipoxygenases of the leukocyte type. 12(R)-HETE at 10(-6) M inhibited almost completely the Na,K-ATPase of corneal epithelium but had little or no effect on ciliary epithelial enzymic activity. 12(S)-HETE at 10(-6) M also inhibited corneal enzymic activity but to a lesser extent, and had no significant effect on ciliary epithelial Na,K-ATPase activity.