Effects of UV-C irradiation on phosphoinositide turnover in plant cells: similarities with those occurring via the formation of reactive oxygen intermediates in animal cells

With the aim of examining the response of plant cells to UV-C irradiation, we investigated the behaviour of the phosphatidylinositol 4,5 bisphosphate (PtdIns 4,5-P2) molecule (the precursor of the phosphoinositide signal transduction cascade) by exposing callus cells from Peucedanum verticillare to UV-C (130 J m-2) and by examining the level and the fatty acid composition of PtdIns 4,5-P2 at different times after irradiation. We show that a pathway for the UV-C response includes transient PtdIns 4,5-P2 breakdown. The effect of ultraviolet rays is mimicked by H2O2 suggesting that in this plant it may be brought about by reactive oxygen intermediates (ROI), as already underlined in experimental animal models.

[Effects of irradiation on fatty acid structure of membrane lipids of the sarcoplasmic reticulum]

It has been shown that single local X-irradiation (0.21 C/kg) of the rabbit hind limb in the early period of acute radiation injury (1 and 14 h) causes a decrease in saturation of sarcoplasmic reticulum membrane lipids. It is mainly connected with a decreased saturation of the total fraction of phosphatidyl serine, phosphatidyl inositol and phosphatidyl choline. The above changes can increase permeability of the sarcoplasmic reticulum membranes for Ca(2+)-ion after X-irradiation.

Light-induced phosphoinositide degradation and light-induced structural alterations in the rat retina are enhanced after chronic lithium treatment

In the rat retina, light-induced break down of polyphosphoinositides and structural alterations of photoreceptor outer segment disk-membranes were enhanced by chronic lithium treatment at doses equivalent to therapeutic levels in humans. Furthermore, the recovery of phosphoinositide baseline levels after light exposure was delayed in lithium treated retinas. Light and lithium may thus affect phospholipid membranes in the retina. Initial processes in the pathogenesis of light damage in the retina are largely unknown to date. The hydrolysis of polyphosphoinositides may possibly constitute such an initial event.

Light- and GTP-activated hydrolysis of phosphatidylinositol bisphosphate in squid photoreceptor membranes

Light stimulates the hydrolysis of exogenous, [3H]inositol-labeled phosphatidylinositol bisphosphate (PtdInsP2) added to squid photoreceptor membranes, releasing inositol trisphosphate (InsP3). At free calcium levels of 0.05 microM or greater, hydrolysis of the labeled lipid is stimulated up to 4-fold by GTP and light together, but not separately. This activity is the biochemical counterpart of observations on intact retina showing that a rhodopsin-activated GTP-binding protein is involved in visual transduction in invertebrates, and that InsP3 release is correlated with visual excitation and adaptation. Using an in vitro assay, we investigated the calcium and GTP dependence of the phospholipase activity. At calcium concentrations between 0.1 and 0.5 microM, some hydrolysis occurs independently of GTP and light, with a light- and GTP-activated component superimposed. At 1 microM calcium there is no background activity, and hydrolysis absolutely requires both GTP and light. Ion exchange chromatography on Dowex 1 (formate form) of the water-soluble products released at 1 microM calcium reveals that the product is almost entirely InsP3. Invertebrate rhodopsin is homologous in sequence and function to vertebrate visual pigment, which modulates the concentration of cyclic GMP through the mediation of the GTP-binding protein transducin. While there is some evidence that light also modulates PtdInsP2 content in vertebrate photoreceptors, the case for its involvement in phototransduction is stronger for the invertebrate systems. The results reported here support the scheme of rhodopsin----GTP-binding protein----phospholipase C activation in invertebrate photoreceptors.

Phosphatidylinositol 4,5-bisphosphate: light-mediated breakdown in the vertebrate retina

Isolated frog (Rana Pipiens) retinas were labeled in the dark with either [32P] PO4-orthophosphate or myo-[2-3H]inositol for 2.5-4 hrs. After washing the retinas with cold buffer, they were exposed to brief flashes of light (5 secs or 15 secs) and their rod outer segments isolated. Upon separation of labeled phospholipids, a specific decrease in label in phosphatidylinositol 4,5-bisphosphate was observed, whereas there was no significant effect on the labeling of phosphatidylinositol 4-phosphate, phosphatidylinositol, or phosphatidic acid. These results are indicative of a light-activated phosphatidylinositol 4,5-bisphosphate-specific phospholipase C in frog rod outer segments.