Inhibition of the action of a stimulatory GDP/GTP exchange protein for smg p21 by acidic membrane phospholipids

Biologically active lipids promote trafficking and membrane association of Rac1 in insulin-secreting INS 832/13 cells

Despite emerging evidence to suggest that glucose-stimulated insulin secretion (GSIS) requires membrane targeting of specific small G proteins (e.g., Rac1), very little is known with regard to the precise mechanisms underlying subcellular trafficking of these proteins in the glucose-stimulated islet beta-cell. We previously reported activation of small G proteins by biologically active lipids via potentiation of relevant GDP/GTP exchange activities within the beta-cell. Herein, we studied putative regulatory roles for these lipids in the trafficking and membrane association of Rac1 in cell-free preparations derived from INS 832/13 beta-cells. Incubation of INS 832/13 cell lysates with polyphosphoinositides (e.g., PIP(2)), phosphatidic acid, phosphatidylcholine, and phosphatidylserine significantly promoted trafficking of cytosolic Rac1 to the membrane fraction. Lysophosphatidic acid, but not lysophosphatidylcholine or lysophosphatidylserine, also promoted translocation and membrane association of Rac1. Arachidonic acid, diacylglycerol, calcium, and cAMP failed to exert any clear effects on Rac1 translocation to the membrane. Together, our findings provide the first direct evidence in support of our recent hypothesis (Kowluru A, Veluthakal R. Diabetes 54: 3523-3529, 2005), which states that generation of biologically active lipids, known to occur in the glucose-stimulated beta-cell, may mediate targeting of Rac1 to the membrane for optimal interaction with its putative effector proteins leading to GSIS.

Purification and characterization of novel plasma membrane phosphatidate phosphohydrolase from rat liver

An N-ethylmaleimide-insensitive phosphatidate phosphohydrolase, which also hydrolyzes lysophosphatidate, was isolated from the plasma membranes of rat liver. The specific activity of an anionic form of the enzyme (53 kDa, pI < 4) was increased 2700-fold. A cationic form of enzyme (51 kDa, pI = 9) was purified to homogeneity, but the -fold purification was low because the activity of the highly purified enzyme was unstable. Immunoprecipitating antibodies raised against the homogeneous protein confirmed the identity of the cationic protein as the phosphohydrolase and were used to identify the anionic enzyme. Both forms are integral membrane glycoproteins that were converted to 28-kDa proteins upon treatment with N-glycanase F. Treatment of the anionic form with neuraminidase allowed it to be purified in the same manner as the cationic enzyme and yielded an immunoreactive protein with a molecular mass identical to the cationic protein. Thus, the two ionic forms most likely represent different sialated states of protein. An immunoreactive 51-53-kDa protein was detected in rat liver, heart, kidney, skeletal muscle, testis, and brain. Little immunoreactive 51-53-kDa protein was detected in rat thymus, spleen, adipose, or lung tissue. This work provides the tools for determining the regulation and function of the phosphatidate phosphohydrolase in signal transduction and cell activation.

Regulation of guanine-nucleotide binding proteins in islet subcellular fractions by phospholipase-derived lipid mediators of insulin secretion

In the accompanying article (Kowluru, A., Rabaglia, M.E., Mose, K.E. and Metz, S.A. (1994) Biochim. Biophys. Acta 1222, 348-359) we identified three specific GTPase activities in islet subcellular fractions; most notably, two of these were enriched in the secretory granules. In the present study, we describe the regulation of GTPase activity in subcellular fractions of normal rat and human islets by insulinotropic lipids with a similar rank order as their insulin-releasing capacity. Arachidonic acid (AA), lysophosphatidylcholine (LPC), or phosphatidic acid (PA) inhibited the GTPase activities significantly (by 60-80%) in islet homogenates; each also selectively inhibited certain GTPases in specific individual fractions. Less insulinotropic fatty acids, such as linoleic acid and oleic acid, inhibited GTPase to a lesser degree, whereas lysophosphatidic acid (LPA), phosphatidylcholine (PC) or palmitic acid, which do not acutely promote secretion, were ineffective. Similar inhibitory effects of these lipids were also demonstrable in fractions of human islets as well as those of transformed beta-cells (HIT cells). The effects of lipids were not attributable to their detergent properties (since several detergents failed to mimic lipid effects) or to inhibition of GTP binding (since they actually increased GTP gamma S binding modestly, and moreover, in reconstituted fractions, they potentiated GDP/GTP exchange activity up to 2-fold). These data indicate that the insulinotropic nature of the lipids might be due, in part, to their ability to maintain G-proteins in their GTP-bound (active) configuration by increasing GTP binding and decreasing its hydrolysis. These studies comprise the first evidence for the regulation by biologically active lipids of endocrine cell G-proteins at a locus distal to plasma membrane events (i.e., on endocrine secretory granules), and provide thereby a possible novel mechanism whereby the activation of islet endogenous phospholipases might culminate in insulin exocytosis.

Biology of the Rap proteins, members of the ras superfamily of GTP-binding proteins

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Microinjection of smg/rap1/Krev-1 p21 into Swiss 3T3 cells induces DNA synthesis and morphological changes

Microinjection of either Ki-rasVal-12 p21 or the GDP-bound form of Ki-ras p21 plus smg GDP dissociation stimulator (GDS), a stimulatory GDP/GTP exchange protein for Ki-ras p21, smg/rap1/Krev-1 p21, and rho p21, into quiescent Swiss 3T3 cells induced DNA synthesis irrespective of the presence or absence of insulin. The guanosine 5'-(3-O-thio)triphosphate (GTP gamma S)-bound form of smg p21B or the GDP-bound form of smg p21B plus smg GDS also induced DNA synthesis but only in the presence of insulin. Either the GDP-bound form of Ki-ras p21 or the same form of smg p21B alone was inactive, but smg GDS alone was slightly active only in the presence of insulin. The morphology of the cells was analyzed by scanning electron, phase-contrast, and confocal laser scanning microscopies. Ki-rasVal-12 p21 induced membrane ruffling irrespective of the presence or absence of insulin. The GTP gamma S-bound form of smg p21B showed the same effect only in the presence of insulin. Either the GDP-bound form of Ki-ras p21, the same form of smg p21B, or smg GDS alone was inactive. Upon microinjection of Ki-rasVal-12 p21, stress fibers markedly decreased and the cells became round and piled up. In contrast, upon microinjection of the GTP gamma S-bound form of smg p21B, stress fibers did not markedly decrease and the cells neither became round nor piled up. These results indicate that both ras p21 and smg p21 are mitogenic in Swiss 3T3 cells but that their actions are slightly different.

Microinjection of smg/rap1/Krev-1 p21 into Swiss 3T3 cells induces DNA synthesis and morphological changes

Microinjection of either Ki-rasVal-12 p21 or the GDP-bound form of Ki-ras p21 plus smg GDP dissociation stimulator (GDS), a stimulatory GDP/GTP exchange protein for Ki-ras p21, smg/rap1/Krev-1 p21, and rho p21, into quiescent Swiss 3T3 cells induced DNA synthesis irrespective of the presence or absence of insulin. The guanosine 5'-(3-O-thio)triphosphate (GTP gamma S)-bound form of smg p21B or the GDP-bound form of smg p21B plus smg GDS also induced DNA synthesis but only in the presence of insulin. Either the GDP-bound form of Ki-ras p21 or the same form of smg p21B alone was inactive, but smg GDS alone was slightly active only in the presence of insulin. The morphology of the cells was analyzed by scanning electron, phase-contrast, and confocal laser scanning microscopies. Ki-rasVal-12 p21 induced membrane ruffling irrespective of the presence or absence of insulin. The GTP gamma S-bound form of smg p21B showed the same effect only in the presence of insulin. Either the GDP-bound form of Ki-ras p21, the same form of smg p21B, or smg GDS alone was inactive. Upon microinjection of Ki-rasVal-12 p21, stress fibers markedly decreased and the cells became round and piled up. In contrast, upon microinjection of the GTP gamma S-bound form of smg p21B, stress fibers did not markedly decrease and the cells neither became round nor piled up. These results indicate that both ras p21 and smg p21 are mitogenic in Swiss 3T3 cells but that their actions are slightly different.

Effect of chlorpromazine on the smg GDS action

A stimulatory GDP/GTP exchange protein for smg p21 (smg GDS) stimulated the binding of guanosine 5'-(3-0-thio) triphosphate (GTP gamma S) to smg p21B. Chlorpromazine (CPZ) inhibited the smg GDS action in a manner competitive with smg GDS and in a manner noncompetitive with smg p21B. In spite of the inhibitory effect of CPZ on the smg GDS action, it counteracted the inhibition of the smg GDS action by acidic phospholipids. These results suggest that CPZ interacts with smg p21B, smg GDS, or both, and thereby inhibits the smg GDS action, and that CPZ also interacts with the acidic phospholipids and thereby counteracts their inhibitory effect on the smg GDS action.