Mas 7

Light-induced expression of the Gsa gene encoding the heme and chlorophyll biosynthetic enzyme glutamate 1-semialdehyde aminotransferase in Chlamydomonas reinhardtii was previously shown to involve Ca2+ and calmodulin (CaM) (C. lm et al. 1996, Plant Cell 8: 2245-2253). To further analyze the signal transduction pathway for light-induced Gsa expression, the effects of several pharmacological agents were examined. Treatment of light-dark synchronized cells with the heterotrimeric G-protein agonist Mas-7 caused partial induction of Gsa in the dark. The phospholipase C inhibitor U73122 inhibited light induction of Gsa. Exposure of cells to light caused a sustained 3-fold increase in cellular D-inositol 1,4,5-trisphosphate (InsP3) concentration. KN-93, a specific inhibitor of Ca2+/CaM-dependent protein kinase II, inhibited light induction of Gsa. In contrast, cyclosporin A, a specific inhibitor of the Ca2+/CaM-dependent phosphoprotein phosphatase calcineurin, did not affect light induction of Gsa. These results, together with the earlier results, suggest the involvement of a canonical signal transduction pathway for light-regulated Gsa expression that involves a heterotrimeric G-protein activation, phospholipase C-catalyzed InsP3 formation, InsP3-dependent Ca2+ release, and activation of a downstream signaling pathway through a Ca2+/CaM-dependent protein kinase.

Mastoparan 7 (Mas-7), an amphiphilic peptide possessing membrane perturbing activity, has been known to selectively stimulate some lipases. To examine changes in the lipid composition induced by Mas-7, we carried out systemic lipid analysis of L1210 cells after Mas-7 treatment. The total lipid was determined by HPLC, gas-liquid chromatography, and electrospray ionization mass spectrometry in conjunction with differential radiolabelling with [(32)P]orthophosphate, [(3)H]myristic acid, and [(3)H]arachidonic acid. The lipid analysis revealed multiple changes in more than 10 lipid classes. Free fatty acids (FFAs) and phosphatidylethanol (PEt), the phospholipase D product in the presence of ethanol, were increased significantly and phosphatidylcholine (PC) was decreased. Digitonin, a membrane permeabilizing reagent, similarly affected the lipid composition of L1210. The FFA released showed a very broad distribution of saturated, monounsaturated, and polyunsaturated fatty acids, implying that phospholipase A(2) alone could not account for all of the FFAs released. By comparing the molecular species of PEt with those of endogenous PC, we showed that phospholipase D in L1210 cells appeared to act selectively on diacyl-PC.

The presence of functional endothelin receptors and their signal transduction mechanism has not been determined so far in the pineal gland. We examined the effect of endothelin-1 (ET-1) on phosphoinositide turnover in whole pineal gland. Endothelin-1 increased monophosphate accumulation in a dose-dependent manner. The phosphoinositide (PI) response elicited by ET-1 was dependent on the presence of extracellular Ca (++) since its chelation resulted in a marked decrease in ET-1-stimulated InsP(1) accumulation. On the contrary, phosphoinositide hydrolysis was not changed by the calcium blocker amlodipine. ET-1 induced PI breakdown was inhibited by neomycin, an inhibitor of phospholipase C. However, mastoparan 7, a G protein activator via Gi/Go s timulation, did not alter ET-1-induced InsP(1) accumulation. Our data indicate that stimulation of PI turnover constitutes one of the signaling pathways of ET in rat pineal gland through the stimulation of a receptor-coupled phospholipase C. And they demonstrate, for the first time, the presence of functional binding sites for endothelin in the pineal gland.