Intracellular Phospholipid Signaling Systems

Cationic amphiphilic drug-induced phospholipidosis

Phospholipidosis, a phospholipid storage disorder, defines an excessive accumulation of intracellular phospholipids. Phospholipids are structural components of mammalian cytoskeleton and cell membranes. The metabolism of this essential cell component is regulated by the individual cell and may be altered by drugs that interact with phospholipids or the enzymes that affect their metabolism. Xenobiotics or their metabolites that induce phospholipidosis include a wide variety of pharmacologic agents, including antibacterials, antipsychotics, antidepressants, antiarrhythmics, antianginals, antimalarials, anorexic agents, cholesterol-lowering agents, and others. Each of these drugs shares several common physiochemical properties: hydrophobic ring structure on the molecule and a hydrophilic side chain with a charged cationic amine group, hence the class term cationic amphiphilic drugs (CADs). This paper reviews the phospholipid metabolism, physiochemical characteristics of CADs, specificity of phospholipidosis in animals and humans, functional effects of phospholipidosis, interaction of CADs with biologic membranes and lysosome metabolism, influence of CADs on phospholipases and phospholipid synthesis, and a proposed mechanism for induction of phospholipidosis in the lung. In human risk assessment, investigators should consider the many factors in evaluating a drug that induces phospholipidosis in animals. These include: the therapeutic class of drug, presence of active metabolites, tissue or organ selectivity in animals and humans, influence of concurrently administered drugs, reversibility of effect, and other factors that increase or decrease the induction of phospholipidosis. Generalities regarding the etiology, incidence, and effect of the drug on a specific host may not be made. Each drug must be evaluated separately to identify the risk when administered for therapeutic effect in humans.

Mechanisms of prostaglandin F2 alpha and histamine-induced contractions in human chorionic vasculature

We investigated the signaling pathways modulating histamine- and prostaglandin F2 alpha (PGF2 alpha)-induced contractions of human chorionic vasculature. Neomycin, a phospholipase C (PLC) inhibitor, attenuated PGF2 alpha and histamine contractile responses 40 and 60%, respectively. AIF4-, a G protein stimulant, induced a strong contraction alone but blocked histamine- and PGF2 alpha-induced contractions. Staurosporine (100 nM), a protein kinase C (PKC) inhibitor, attenuated the PGF2 alpha-dependent contractions by 50% but did not affect the histamine response. However, higher nonspecific inhibitory concentrations of staurosporine (1-2 microM) abolished histamine and PGF2 alpha contractile responses, presumably by inhibiting other protein kinases. Although, the PKC phorbol 12-myristate 13-acetate (PMA) did not affect basal tension or PGF2 alpha-dependent contractions, the histamine response was attenuated by 30%. Sodium nitroprusside (SNP), a guanylyl cyclase stimulant, strongly attenuated histamine- and PGF2 alpha-induced contractions. Tension increases were similarly attenuated by forskolin and isobutylmethylxanthine (IBMX), which increase intracellular cyclic AMP. In vessel rings prelabeled with [3H]myoinositol, PGF2 alpha and histamine increased [3H]inositol phosphate (IP) production 400 and 100%, respectively, indicating that PLC is stimulated by both agonists. Neomycin inhibited histamine- and PGF2 alpha-induced increases in [3H]IP production 60 and 40%, respectively. Staurosporine (0.1-1 microM) and PMA did not affect histamine- or PGF2 alpha-stimulated IP production. AIF4-alone increased IP production but blocked histamine- and PGF(2 alpha)-dependent IP increases. These observations suggest that at least part of the contractile responses due to PGF2 alpha and histamine are associated with stimulation of PLC through an AIF4(-)-sensitive G protein. The role of PKC is variable, because PGF2 alpha but not histamine tension responses were attenuated by PKC inhibition. In addition, therapeutic agents that increase cyclic AMP and cyclic GMP attenuated histamine- and PGF2 alpha-induced contractions in human chorionic vasculature, although histamine responses were relatively more sensitive to these agents.

Cell signaling and estrogens in female rat osteoblasts: a possible involvement of unconventional nonnuclear receptors

Estrogen deficiency is associated with bone loss, and estrogen replacement is an effective treatment of this osteoporotic process. This study examines the early (5-120 s) effects of 17 beta-estradiol on the intracellular calcium and phospholipid metabolism in confluent female rat osteoblasts. The cytosolic free Ca2+ concentration ([Ca2+]i) was determined using fura-2/AM as Ca2+ probe. Cells were labeled with myo-[2-3H]inositol or [14C]arachidonic acid for inositol or lipid determination. Inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) production were determined by either mass measurement or anion-exchange chromatography or by thin-layer chromatography, respectively. 17 beta-Estradiol (1 pM to 1 nM) increased [Ca2+]i in a biphasic manner within 10 s via Ca2+ influx from the extracellular milieu, as shown by the effects of the calcium chelator EGTA and the Ca2+ channel blockers nifedipine and verapamil, and via Ca2+ mobilization from the endoplasmic reticulum (ER), as shown by the effects of thapsigargin. 17 beta-Estradiol (1 pM to 1 nM) induced a biphasic and concomitant increase in IP3 and DAG formation. Estradiol immobilized on bovine serum albumin (BSA) [E-(O-carboxymethyl)oxime BSA] and its derivative (O-carboxymethyl)oxime rapidly increased ([Ca2+]i, IP3, and DAG and were full agonists, although they were less potent than the free estradiol. They had the same action time course and acted via Ca2+ influx and Ca2+ mobilization from ER. Tamoxifen, a potent inhibitor of genomic steroid responses, did not block the rapid increase in Ca2+, IP3, and DAG induced by estradiol. Finally, inhibitor of phospholipase C (neomycin) and pertussis toxin abolished the effects of 17 beta-estradiol on IP3 and DAG formation. These results suggest that female rat osteoblasts bear non-genomic unconventional cell surface receptors for estradiol, belonging to the class of the membrane receptors coupled to a phospholipase C via a pertussis toxin-sensitive G protein.

Oscillations in inositol 1,4,5-trisphosphate and diacyglycerol induced by vitamin D3 metabolites in confluent mouse osteoblasts

For the last 5 years, attention has focused on the nongenomic effects of 1,25-(OH)2D3, but considerably less is known about the mechanisms of the nonnuclear actions of 24,25-(OH)2D3. The present study examines and compares the rapid (5-90 s) effects of 100 pM to 10 nM 24,25-(OH)2D3, 10 pM to 1 nM 1,25-(OH)2D3, and 1-100 nM 25-OHD3 on the formation of inositol phosphates and lipids in confluent mouse osteoblasts. 24,25-(OH)2D3 and 25-OHD3 effects were dose dependent; those of 1,25-(OH)2D3 were dose dependent in a bell-shaped manner. The two dihydroxylated metabolites induced a multiphasic response in inositol 1,4,5-trisphosphate (IP3) formation with three stimulation peaks; the IP3 response to 25-OHD3 was monophasic. The amplitude of the IP3 response to 24,25-(OH)2D3 was greater and its oscillation period was slower than that induced by 1,25-(OH)2D3. The diacylglycerol (DAG) responses to secosteroids showed two stimulation peaks that appeared at different times depending on the secosteroid used. Pretreatment with neomycin totally inhibited the first DAG response; neomycin had no effect on the second peak of DAG induced by 25-OHD3, whereas it partially blocked the second response of DAG to 24,25-(OH)2D3 and 1,25-(OH)2D3. These data show for the first time that 24,25-(OH)2D3 can modulate phospholipid metabolism in confluent mouse osteoblasts as early as 5-10 s. The first pathway used by all three secosteroids is that of the hydrolysis of phosphatidylinositol 4,5-bisphosphate via phospholipase C activation, leading to the formation of the two second messengers, IP3 and DAG, since neomycin totally blocked the response. Thus, the action of these secosteroids on the osteoblast membrane may also implicate several steps of the phosphatidylcholine cycle, according to the metabolite tested. Finally, these data point to a direct interaction of vitamin D metabolites with specific membrane recognition moieties.

Generation of phosphatidic acid during calcium-loading of human erythrocytes. Evidence for a phosphatidylcholine-hydrolyzing phospholipase D

We have studied the mechanism by which calcium-loading of human erythrocytes stimulates phospholipid turnover and generates diacylglycerol and phosphatidic acid. Using quantitative measurement of individual phospholipid classes, we have demonstrated that the amount of phosphatidic acid generated during calcium-loading of intact red cells exceeds the amount of diacylglycerol formed by phospholipase-C-mediated hydrolysis of the polyphosphoinositol lipids and that addition of the diacylglycerol kinase inhibitor, R59022, only partly inhibited this increase. Thus, in contrast to current explanations, the phosphatidic acid generated following calcium-loading of erythrocytes cannot be solely explained by the action of a polyphosphoinositol-lipid-specific phospholipase C with subsequent phosphorylation of diacylglycerol to phosphatidic acid. Our data demonstrate that calcium-loading of intact erythrocytes, but not of red cell ghost membranes, causes a small but significant decrease in the relative amount of phosphatidylcholine (PtdCho). In order to identify the mechanisms responsible for calcium-mediated hydrolysis of PtdCho, we encapsulated Ptd[Me-14C]Cho-containing rat liver microsomes into erythrocytes and studied the generation of [Me-14C]choline and phospho[Me-14C]choline. We found that choline was the only detectable 14C-labeled product. Furthermore, incubation of erythrocytes with calcium under hypotonic conditions and in the presence of [14C]PtdCho vesicles and ethanol resulted in the formation of [14C]phosphatidylethanol. Together, these results suggest that the loss of PtdCho during calcium-loading of human erythrocytes is caused by a previously unrecognized PtdCho-hydrolyzing phospholipase D, resulting in direct generation of phosphatidic acid. Analysis of the molecular species composition of PtdCho, phosphatidic acid, and diradylglycerol, confirm the simultaneous actions of PtdCho-hydrolyzing and polyphosphoinositol-lipid-hydrolyzing phospholipases in calcium-loaded human erythrocytes.

Biology of the protein kinase C family

Protein kinase C (PKC) is composed of a family of isozymes that transduce signals of certain hormones, growth factors, lectins, and neurotransmitters. This review addresses the role of PKC in the regulation of cellular proliferation and its disorders. PKC is directly activated in vivo by the second messenger diacylglycerol, a lipid produced by phospholipase C-catalyzed hydrolysis of phosphatidylinositol and polyphosphoinositides. Diacylglycerol activates PKC by reducing the enzyme's requirement for Ca2+. Phorbol ester tumor promoters and related agents potently activate PKC by a mechanism analogous to that of diacylglycerol, providing evidence that PKC activation is a critical event in tumor promotion. However, the role of PKC activation in tumor promotion is not entirely clear. For example, bryostatin is a potent PKC activator that antagonizes phorbol ester-mediated tumor promotion, and mezerein is a second-stage tumor promoter that potently activates PKC. In addition to studies concerned with tumor promotion, studies of oncogene action also indicate a role for PKC in carcinogenesis. A number of plasma membrane-associated oncogene products and related proteins are PKC substrates, and PKC activation leads to induction of the expression of oncogenes that code for nuclear proteins. PKC is implicated in human breast and colon carcinogenesis. Tumor-promoting bile acids activate PKC, and PKC expression studies in rat colonic epithelial cells and human breast cancer cells indicate a positive role for PKC in the proliferation of the cells. Altered expression of PKC in human colon and breast tumors indicates that PKC isozymes may be useful markers for these diseases.