Lysophosphatidylinositol: A Potential Mediator of 1,25-Dihydroxyvitamin D-Induced Increments in Hepatocyte Cytosolic Calcium*

Abstract

Recent studies in our laboratory demonstrate that 1,25-dihydroxyvitamin D [1,25-(OH)2D] increases hepatocyte cytosolic calcium in the absence of extracellular calcium and activates phospholipase-A-induced deacylation of phosphatidylinositol within 5 min. To determine whether inhibition of phospholipase-A affects D-induced increments in cytosolic calcium and which metabolite of phosphatidylinositol mediates these increments, cytosolic calcium is measured in cultured hepatocytes, loaded with Fura 2AM. Cellular fluorescence is determined at excitation wavelengths of 340 and 380 nm, and cytosolic calcium is calculated. 1,25-(OH)2D treatment for 5 min increases cytosolic calcium levels in the cultured hepatocyte. Inhibition of phospholipase-A with bromophenacylbromide blocks the vitamin D effect on cytosolic calcium, indicating that phospholipase-A activation precedes increments in cytosolic calcium. Neither arachidonic acid nor sodium arachidonate affects cytosolic calcium. In contrast, LPI increases hepatocyte cytosolic calcium in the presence and absence of extracellular calcium. The effect is not blocked by bromophenacylbromide. Neither cell viability nor supernatant fluorescence is altered by LPI, indicating that LPI is increasing fluorescence within the cell. 1,25-(OH)2D treatment for 5 min increases cytosolic pH in cultured hepatocytes. Inhibition of cell alkalinization with amiloride blocks the vitamin D effect on both cytosolic pH and cytosolic calcium, but not the effect of LPI on calcium. NH4Cl increases hepatocyte pH but not cytosolic calcium. The results indicate that phospholipase-A activation is necessary for 1,25-(OH)2D-induced increments in hepatocyte cytosolic calcium and that LPI, a deacylation product of phosphatidylinositol, mediates these increments. Furthermore, inhibition of Na+/H+ exchange, while not blocking the ability of the cell to increase cytosolic calcium in response to LPI, prevents the 1,25-(OH)2D-induced increments in both pH and calcium. The data suggest that the rapid effects of 1,25-(OH)2D on hepatocyte calcium are preceded by cell alkalinization and phospholipase-A activation, and these effects appear to be linked to the Na+/H+ antiport system and not nonspecific cell alkalinization.

GPR55: a new promising target for metabolism?

“…Previous to the description of LPI as the endogenous ligand of GPR55, relevant biological functions had been associated to LPI such as being an inductor of insulin release (Metz 1986), a mediator of hepatic vitamin D3-modulating Ca 2+ metabolism (Baran & Marie Kelly 1988) and a mitogenic modulator in both neuron and endothelial cells (Falasca et al 1995, Corda et al 2002. Nowadays, how GPR55 contributes to these LPI-mediated effects is an object of research.…”

25-Hydroxyvitamin D3-1 alpha-hydroxylase in porcine hepatic tissue: subcellular localization to both mitochondria and microsomes.

“…Recent studies have demonstrated that hepatic tissue has several physiological responses to 1,25(OH)2D3 (16)(17)(18)(19)(20)(21)(22)(23)(24)46) and appears to contain high-affinity receptors for this potent hormone (47). General vitamin D deficiency in the rat has been shown to alter hepatic function as manifested by delayed bromosulfthalein clearance and elevated plasma transaminases (16).…”

“…Because of the important role of 1,25(OH)2D3 in various aspects of hepatic function (16)(17)(18)(19)(20)(21)(22)(23)(24), the possibility that the liver could synthesize in situ this hormonal form of vitamin D3 was investigated. It is reported here that both hepatic mitochondria and microsomes possess a 25(OH)D3 lahydroxylase in significant amounts.…”

Release of the mitogen lysophosphatidylinositol from H-Ras-transformed fibroblasts; a possible mechanism of autocrine control of cell proliferation

“…Other phosphoinositide derivatives of potential biological relevance include those involved in the phosphatidylinositol 3-kinase pathway (Panayotou and Water®eld, 1992), the inositol polyphosphates (Sasakawa et al, 1995), the cyclic inositols (Majerus et al, 1988), the glycerophosphoinositols (Alonso et al, 1988;Valitutti et al, 1991;Iacovelli et al, 1993;Corda and Falasca, 1996) and lysophosphatidylinositol (Falasca and Corda, 1994;Falasca et al, 1995;Metz, 1986;Baran and Kelly, 1987).…”