Evidence for the presence of phosphoinositide cycle and its involvement in cellular signal transduction in the rabbit lens

Abstract

In the lens, free inositol is present at high concentrations. The lens transports inositol from the extracellular source but can also synthesize inositol from glucose via inositol-1-phosphate. The inositol containing phospholipid (phosphoinositides) constitutes only 10% of the total phospholipid in the membrane and was suggested to play some key role in the cellular differentiation. Recently, one of the phosphoinositides, PIP2, was located in the epithelial cells but not in fiber cells. Prostaglandin, which uses one of the phosphoinositide metabolites, diacylglycerol, as a precursor in its biosynthesis was also found in the lens. The evidence, although scanty, do provide some clues to the possibility that lens may contain a phosphoinositide cycle similar to retina and cornea. In this study we demonstrated that rabbit lens epithelial cells could incorporate 3H-inositol into the membrane and the label accumulated in all three phosphoinositides, PI, PIP and PIP2 with PI as the predominant form. Both PI Kinase and PIP Kinase were found in the lens epithelial homogenate which incorporated (gamma-32P) ATP into PI and PIP to form their respective product, PIP and PIP2. The membrane bound PI Synthase was also demonstrated by using a cell free system. The lens cells showed distinctive response to some agonists such as Ca2+, EGF, glucagon, serotonin but not the others such as insulin, FGF. It is therefore concluded that lens epithelium cells, like other cell types has a complete and functional phosphoinositide cycle.