Regulation of epidermal-growth-factor-receptor signal transduction by cis-unsaturated fatty acids. Evidence for a protein kinase C-independent mechanism

Oleic Acid Could Act as a Channel Blocker in the Inhibition of nAChR: Insights from Molecular Dynamics Simulations

The activation of the muscular nicotinic acetylcholine receptor (nAChR) produces the opening of the channel, with the consequent increase in the permeability of cations, triggering an excitatory signal. Free fatty acids (FFA) are known to modulate the activity of the receptor as noncompetitive antagonists, acting at the membrane-AChR interface. We present molecular dynamics simulations of a model of nAChR in a desensitized closed state embedded in a lipid bilayer in which distinct membrane phospholipids were replaced by two different monounsaturated FFA that differ in the position of a double bond. This allowed us to detect and describe that the cis-18:1ω-9 FFA were located at the interface between the transmembrane segments of α2 and γ subunits diffused into the channel lumen with the consequent potential ability to block the channel to the passage of ions.

Metabolic and Vascular Effect of the Mediterranean Diet

Several studies indicated how dietary patterns that were obtained from nutritional cluster analysis can predict disease risk or mortality. Low-grade chronic inflammation represents a background pathogenetic mechanism linking metabolic risk factors to increased risk of chronic degenerative diseases. A Mediterranean diet (MeDi) style has been reported as associated with a lower degree of inflammation biomarkers and with a protective role on cardiovascular and cerebrovascular events. There is heterogeneity in defining the MedDiet, and it can, owing to its complexity, be considered as an exposome with thousands of nutrients and phytochemicals. Recently, it has been reported a novel positive association between baseline plasma ceramide concentrations and cardiovascular events and how adherence to a Mediterranean Diet-style may influence the potential negative relationship between elevated plasma ceramide concentrations and cardiovascular diseases (CVD). Several randomized controlled trials (RCTs) showed the positive effects of the MeDi diet style on several cardiovascular risk factors, such as body mass index, waist circumference, blood lipids, blood pressure, inflammatory markers and adhesion molecules, and diabetes and how these advantages of the MeDi are maintained in comparison of a low-fat diet. Some studies reported a positive effect of adherence to a Mediterranean Diet and heart failure incidence, whereas some recent studies, such as the PREDIMED study, showed that the incidence of major cardiovascular events was lower among those assigned to MeDi supplemented with extra-virgin olive oil or nuts than among those assigned to a reduced-fat diet. New studies are needed to better understand the molecular mechanisms, whereby the MedDiet may exercise its effects. Here, we present recent advances in understanding the molecular basis of MedDiet effects, mainly focusing on cardiovascular diseases, but also discussing other related diseases. We review MedDiet composition and assessment as well as the latest advances in the genomic, epigenomic (DNA methylation, histone modifications, microRNAs, and other emerging regulators), transcriptomic (selected genes and whole transcriptome), and metabolomic and metagenomic aspects of the MedDiet effects (as a whole and for its most typical food components). We also present a review of the clinical effects of this dietary style underlying the biochemical and molecular effects of the Mediterranean diet. Our purpose is to review the main features of the Mediterranean diet in particular its benefits on human health, underling the anti-inflammatory, anti-oxidant and anti-atherosclerotic effects to which new knowledge about epigenetic and gut-microbiota relationship is recently added.

Fatty Acid Regulation of Voltage- and Ligand-Gated Ion Channel Function

Free fatty acids (FFA) are essential components of the cell, where they play a key role in lipid and carbohydrate metabolism, and most particularly in cell membranes, where they are central actors in shaping the physicochemical properties of the lipid bilayer and the cellular adaptation to the environment. FFA are continuously being produced and degraded, and a feedback regulatory function has been attributed to their turnover. The massive increase observed under some pathological conditions, especially in brain, has been interpreted as a protective mechanism possibly operative on ion channels, which in some cases is of stimulatory nature and in other cases inhibitory. Here we discuss the correlation between the structure of FFA and their ability to modulate protein function, evaluating the influence of saturation/unsaturation, number of double bonds, and cis vs. trans isomerism. We further focus on the mechanisms of FFA modulation operating on voltage-gated and ligand-gated ion channel function, contrasting the still conflicting evidence on direct vs. indirect mechanisms of action.

EGFR Inhibition Blocks Palmitic Acid-induced inflammation in cardiomyocytes and Prevents Hyperlipidemia-induced Cardiac Injury in Mice

Obesity is often associated with increased risk of cardiovascular diseases. Previous studies suggest that epidermal growth factor receptor (EGFR) antagonism may be effective for the treatment of angiotensin II-induced cardiac hypertrophy and diabetic cardiomyopathy. This study was performed to demonstrate if EGFR plays a role in the pathogenesis of hyperlipidemia/obesity-related cardiac injuries. The in vivo studies using both wild type (WT) and apolipoprotein E (ApoE) knockout mice fed with high fat diet (HFD) showed the beneficial effects of small-molecule EGFR inhibitors, AG1478 and 542, against obesity-induced myocardial injury. Administration of AG1478 and 542 significantly reduced myocardial inflammation, fibrosis, apoptosis, and dysfunction in both two obese mouse models. In vitro, EGFR signaling was blocked by either siRNA silencing or small-molecule EGFR inhibitors in palmitic acid (PA)-stimulated cardiomyocytes. EGFR inhibition attenuated PA-induced inflammatory response and apoptosis in H9C2 cells. Furthermore, we found that PA-induced EGFR activation was mediated by the upstream TLR4 and c-Src. This study has confirmed the detrimental effect of EGFR activation in the pathogenesis of obesity-induced cardiac inflammatory injuries in experimental mice, and has demonstrated the TLR4/c-Src-mediated mechanisms for PA-induced EGFR activation. Our data suggest that EGFR may be a therapeutic target for obesity-related cardiovascular diseases.

Oleic acid content is responsible for the reduction in blood pressure induced by olive oil

Numerous studies have shown that high olive oil intake reduces blood pressure (BP). These positive effects of olive oil have frequently been ascribed to its minor components, such as alpha-tocopherol, polyphenols, and other phenolic compounds that are not present in other oils. However, in this study we demonstrate that the hypotensive effect of olive oil is caused by its high oleic acid (OA) content (approximately 70-80%). We propose that olive oil intake increases OA levels in membranes, which regulates membrane lipid structure (H(II) phase propensity) in such a way as to control G protein-mediated signaling, causing a reduction in BP. This effect is in part caused by its regulatory action on G protein-associated cascades that regulate adenylyl cyclase and phospholipase C. In turn, the OA analogues, elaidic and stearic acids, had no hypotensive activity, indicating that the molecular mechanisms that link membrane lipid structure and BP regulation are very specific. Similarly, soybean oil (with low OA content) did not reduce BP. This study demonstrates that olive oil induces its hypotensive effects through the action of OA.

The growth hormone response to repeated bouts of sprint exercise with and without suppression of lipolysis in men

A single 30-s sprint is a potent physiological stimulus for growth hormone (GH) release. However, repeated bouts of sprinting attenuate the GH response, possibly due to negative feedback via elevated systemic free fatty acids (FFA). The aim of the study was to use nicotinic acid (NA) to suppress lipolysis to investigate whether serum FFA can modulate the GH response to exercise. Seven nonobese, healthy men performed two trials, consisting of two maximal 30-s cycle ergometer sprints separated by 4 h of recovery. In one trial (NA), participants ingested NA (1 g 60 min before, and 0.5 g 60 and 180 min after sprint 1); the other was a control (Con) trial. Serum FFA was not significantly different between trials before sprint 1 but was significantly lower in the NA trial immediately before sprint 2 [NA vs. Con: mean (SD); 0.08 (0.05) vs. 0.75 (0.34) mmol/l, P < 0.05]. Peak and integrated GH were significantly greater following sprint 2 compared with sprint 1 in the NA trial [peak GH: 23.3 (7.0) vs. 7.7 (11.9) microg/l, P < 0.05; integrated GH: 1,076 (350) vs. 316 (527) microg.l(-1).60 min(-1), P < 0.05] and compared with sprint 2 in the Con trial [peak GH: 23.3 (7.0) vs. 5.2 (2.3) microg/l, P < 0.05; integrated GH: 1,076 (350) vs. 206 (118) microg.l(-1).60 min(-1), P < 0.05]. In conclusion, suppressing lipolysis resulted in a significantly greater GH response to the second of two sprints, suggesting a potential role for serum FFA in negative feedback control of the GH response to repeated exercise.

Lipid factor (bVLF) from bovine vitreous body evokes in EGFR-T17 cells a Ca2+-dependent K+ current associated with inositol 1,4,5-trisphosphate-independent Ca2+ mobilization

Bovine vitreous lipid factor (bVLF) is a complex phospholipid isolated from bovine vitreous body with strong Ca(2+)-mobilizing activity. In this study, the effects of bVLF on membrane potential were investigated in EGFR-T17 fibroblasts with the whole-cell patch clamp technique on monolayer cells, as well as with the fluorescent dye bis-oxonol as membrane potential-sensitive probe on monolayer and suspension cells. bVLF induced a transient hyperpolarization characterized by an initial peak and subsequent return to resting membrane potential levels within 1-2 min. The increase of [Ca(2+)](i) was concomitant with an outward current responsible for the hyperpolarizing response. Results with: (a) high [K(+)](o) media; (b) the monovalent cation ionophore gramicidin; and (c) substitution of K(+) with Cs(+) in the intracellular solution were consistent with the involvement of K(+) channels. The bVLF-induced hyperpolarization was blocked by the K(+) channel blockers, quinine and tetraethylamonium chloride, and partially affected by 4-aminopyridine. The calcium ionophore ionomycin caused a similar hyperpolarization as bVLF. When intracellular calcium was buffered by adding BAPTA to the pipette solution, bVLF-activated outward current was prevented. Moreover, the hyperpolarization response was strongly reduced at low doses (3 nM) of specific Ca(2+)-activated K(+) channel blockers, charybdotoxin and iberiotoxin. Based on these observations we conclude that bVLF hyperpolarizes the cells via the activation of a Ca(2+)-dependent K(+) current. In addition, it was observed that bVLF did not have a significant effect on intercellular communication measured by a single patch-electrode technique. Thus, membrane potential changes appeared to belong to the earliest cellular responses triggered by bVLF, and are closely associated with phosphatidic acid-dependent [Ca(2+)](i) mobilization.

Membrane permeability changes induce hyperpolarization in transformed lymphoid cells under high-density culture conditions

BACKGROUND

Membrane potential changes in cells from the human lymphoid B cell line, JY, evoked by increasing cell density in culture were investigated, as data published on other cell types are controversial. An attempt was also made to clear the underlying mechanism.

METHODS

Nonadherent JY cells were isolated from high-density plateau-phase cultures (type A cells), medium-density log-phase cultures (type B cells), and low-density lag-phase cultures (type C cells). They were analyzed for transmembrane potential, intracellular free concentration of potassium and sodium, membrane permeability for monovalent cations, cell cycle distribution by measuring DNA content, and glucose uptake.

RESULTS

C type cells proved to be relatively depolarized (-41 +/- 3 mV) and cells obtained from the highest density cultures hyperpolarized (-60 +/- 3 mV). Intracellular concentrations ([K](i) = 92-97 mM and [Na](i) = 34-35 mM) were almost identical for each type of cell. The sodium/potassium permeability constant ratio in the A and C type of cells was 0.047 and 0.094, respectively. High-density culture conditions resulted in a pronounced G(1)-phase arrest.

CONCLUSIONS

Differences in the membrane potential values induced by high-density culture conditions were maintained by changes in the membrane permeability for the monovalent cations.

Selective fatty acid release from intracellular phospholipids caused by PCBs in rat renal tubular cell cultures

The purpose of this study was to explore the influence of different polychlorinated biphenyls (PCBs) upon the release of oleic and palmitic acid from the intracellular lipids, which were previously labeled with [3H]oleic or [3H]palmitic acid, respectively. Studies have been realized with Aroclor 1248 (a commercial PCB mixture with 48% chlorine by weight), and two pure PCB congeners: 3,3',4, 4'-tetrachlorobiphenyl (a non-ortho-substituted planar congener) and 2,2',4,4',5,5'-hexachlorobiphenyl (a di-ortho-substituted nonplanar congener). The treatment of cells with Aroclor 1248 increased [3H]oleic acid release in a concentration-dependent manner. Our results showed that only the di-ortho-substituted congener which prefers a nonplanar configuration stimulated the release of [3H]oleic acid from the intracellular phospholipids to the culture medium, while the exposure of cell cultures to the chosen non-ortho-substituted coplanar congener did not alter the release of [3H]oleic acid to the culture medium. Finally, none of the PCBs studied could increase the release of [3H]palmitic acid from the intracellular stores significantly. The possibility that these differential alterations in the fatty acid release affect cell function during PCB exposure should therefore be postulated.

Activation of epithelial growth factor receptor pathway by unsaturated fatty acids

Nonesterified fatty acids (NEFAs) are acutely liberated during lipolysis and are chronically elevated in pathological conditions, such as insulin resistance, hypertension, and obesity, which are known risk factors for atherosclerosis. The purpose of this study was to investigate the effect and mechanism of action of NEFAs on the epithelial growth factor (EGF) receptor (EGFR). In the ECV-304 endothelial cell line, unsaturated fatty acids triggered a time- and dose-dependent tyrosine phosphorylation of EGFR (polyunsaturated fatty acids [PUFAs] were the most active), whereas saturated FAs were inactive. Although less potent than PUFAs, oleic acid (OA) was used because it is prominent in the South European diet and is only slightly oxidizable (thus excluding oxidation derivatives). EGFR is activated by OA independent of any autocrine secretion of EGF or other related mediators. OA-induced EGFR autophosphorylation triggered EGFR signaling pathway activation (as assessed through coimmunoprecipitation of SH2 proteins such as SHC, GRB2, and SHP-2) and subsequent p42/p44 mitogen-activated protein kinase (as shown by the use of EGFR- deficient B82L and EGFR- transduced B82LK(+) cell lines). OA induced in vitro both autophosphorylation and activation of intrinsic tyrosine kinase of immunopurified EGFR, thus suggesting that EGFR is a primary target of OA. EGFR was also activated by mild surfactants, Tween-20 and Triton X-100, both in vitro (on immunopurified EGFR) and in intact living cells, thus indicating that EGFR is sensitive to amphiphilic molecules. These data suggest that EGFR is activated by OA and PUFAs, acts as a sensor for unsaturated fatty acids (and amphiphilic molecules), and is a potential transducer by which diet composition may influence vascular wall biology.

Inositol 1,4,5-Trisphosphate-independent Ca2+Mobilization Triggered by a Lipid Factor Isolated from Vitreous Body

A complex phospholipid from bovine vitreous body with a strong Ca(2+)-mobilizing activity has been recently isolated to homogeneity by our group. In this work, a sequential analysis of its transmembrane signaling pathway has been undertaken to characterize the intracellular mechanisms responsible for the Ca(2+) rise. The results show that this phospholipid induces, in a dose-dependent manner (ED(50) of around 0.25 microgram/ml), a Ca(2+) mobilization from inositol 1,4,5-trisphosphate-insensitive intracellular stores, with no participation of extracellular Ca(2+). Upon repeated administration, it shows no signs of autologous desensitization, does not induce heterologous desensitization of the L-alpha-lysophosphatidic acid (LPA) receptor but is desensitized by the previous administration of LPA. The Ca(2+)-mobilizing activity requires a membrane protein, is blocked after preincubation of the cells with pertussis toxin and phorbol esters, as well as by U73122 (an inhibitor of phospholipases C/D), R59022 (a diacylglycerol kinase inhibitor), and D609 (which inhibits phosphatidylcholine-specific phospholipase C). Upon administration of this phospholipid, the intracellular levels of phosphatidic acid (PA) rise with a time course that parallels that of the Ca(2+) mobilization, suggesting that PA could be responsible for this Ca(2+) signal. Exposure to AACOCF(3) (a specific inhibitor of phospholipase A(2)) does not modify the Ca(2+) rise, ruling out the possibility that the PA generated could be further converted to LPA by the action of phospholipase A(2). Based on the experimental data obtained, a signaling pathway involving a phosphatidylcholine-specific phospholipase C coupled to diacylglycerol kinase is proposed. This compound may represent a new class of bioactive lipids with a putative role in the physiology of the vitreous body.

Incorporation of exogenous lipids modulates insulin signaling in the hepatoma cell line, HepG2

The lipid content of cultured cells can be experimentally modified by supplementing the culture medium with specific lipids or by the use of phospholipases. In the case of the insulin receptor, these methods have contributed to a better understanding of lipid disorder-related diseases. Previously, our laboratory demonstrated that experimental modification of the cellular lipid composition of an insulin-sensitive rat hepatoma cell line (ZHC) resulted in an alteration in insulin receptor binding and biological action (Bruneau et al., Biochim. Biophys. Acta 928 (1987) 287-296/297-304). In this paper, we have examined the effects of lipid modification in another hepatoma cell line, HepG2. Exogenous linoleic acid (LA, n-6), eicosapentaenoic acid (EPA, n-3) or hemisuccinate of cholesterol (CHS) was added to HepG2 cells, to create a cellular model in which membrane composition was modified. In this model, we have shown that: (1) lipids were incorporated in treated HepG2 cells, but redistributed differently when compared to treated ZHC cells; (2) that insulin signaling events, such as insulin receptor autophosphorylation and the phosphorylation of the major insulin receptor substrate (IRS-1) were altered in response to the addition of membrane lipids or cholesterol derived components; and (3) different lipids affected insulin receptor signaling differently. We have also shown that the loss of insulin receptor autophosphorylation in CHS-treated cells can be correlated with a decreased sensitivity to insulin. Overall, the results suggest that the lipid environment of the insulin receptor may play an important role in insulin signal transduction.

Effects of TGF-beta1 on prolactin synthesis and secretion: an in-vitro study

The hypothalamus exerts a predominantly inhibitory influence on prolactin secretion through dopamine. In addition, the expression of anterior pituitary hormone-gene products are regulated by intrapituitary growth factors. In particular, TGF-beta1 produced in the pituitary regulates lactotroph cell proliferation and prolactin gene-expression. This study characterized the regulation of in-vitro prolactin synthesis and secretion by TGF-beta1 using rat anterior pituitary cells in monolayer culture. Furthermore, we studied the interaction of TGF-beta1 with other signals involved in the neuroregulation of prolactin secretion, such as dopamine and TRH, as well as the importance of different signal transduction pathways in this response. TGF-beta1 inhibited prolactin secretion in a time- and concentration-dependent manner, with half-maximal inhibition occurring at the range of 15-30 pM. The inhibitory effect was observed after 4 h, being maximal after 4 days of exposure of the cells to the peptide. This inhibitory effect was mimicked by TGF-beta2 but not by inhibin, and was not influenced by oestrogens, being similar in male, normal female or oestradiol-treated rats. Prolonged pretreatment of the cells with TGF-beta1(4 days) did not modify GH or TSH secretion nor dopamine-induced inhibition of prolactin secretion, and blunted prolactin responses to TRH, Forskolin, But2-cAMP and to the calcium ionophore A23187. The effect observed after long-term treatment (24 h to 4 days) is essentially caused by a decrease in prolactin synthesis, since TGF-beta1 inhibited prolactin mRNA levels and de novo prolactin protein synthesis. However, in the short term (up to 12 h) TGF-beta1 inhibition of prolactin secretion was associated with an increase in intracellular prolactin content, dissecting a dual mechanism of action of TGF-beta1. The short-term TGF-beta1 effect did not modify Erk-2 phosphorylation, basal or TRH-induced increase in intracellular calcium concentration, but blunted basal and forskolin stimulated cAMP levels. But2-cAMP replacement did not revert the inhibition of prolactin secretion. However, pertussis toxin was able to recover a large percentage of TGF-beta1-induced inhibition of prolactin secretion. This study indicates that TGF-beta1 plays a crucial role as a modulator of lactotroph function, inhibiting prolactin biosynthesis after long-term treatment, as well as, after short-term exposure prolactin secretion at the level of the secretory process, through a mechanism pertussis toxin sensitive but independent of Erk-2 phosphorylation, calcium concentrations or intracellular cAMP.

Cis-unsaturated free fatty acids block VIP-mediated GH and PRL secretion by perturbing the cAMP/protein kinase A pathway

Cis-unsaturated free fatty acids (FFA) like oleic acid are strong blockers of both basal and stimulated GH secretion in vivo by acting directly on the somatotroph cell. Several lines of evidence suggest that this inhibitory action is the result of a perturbation of the function of several plasma membrane integral proteins. It has been reported recently that cis-FFA are able to block several steps in the inositolphosphates/phospholipase C/Ca2+ (InsPs/PLC/Ca2+) signal transduction pathway triggered by the activation of the TRH receptor. In this paper we present evidence showing that the inhibition of growth hormone (GH) and prolactin (PRL) secretion by cis-FFA in vitro is also exerted at several different levels on the cAMP-protein kinase A (cAMP/PKA) pathway triggered by the stimulation of the vasoactive intestinal peptide (VIP) receptor in pituitary clonal cells. By means of a sequential analysis of signal transduction events, we observed that cis-unsaturated FFA; (1) reduce the activity of adenylate cyclase; (2) perturb the activity of protein kinase A; (3) suppress the VIP-triggered Ca2+ influx, and (4) do not perturb VIP binding or the homologous desensitization of the VIP receptor.

Isolation of a bioactive Ca(2+)-mobilizing complex lipid from bovine vitreous body

Vitreous body extracts show a potent Ca(2+)-mobilizing activity on fibroblast cells. This Ca2+ signal is complex, and due to the presence of two different bioactive substances. The first one was identified as acid FGF. The second one was shown to be a low molecular weight substance identified as a complex lipid by a combination of chromatographic and biochemical data. This finding raises the possibility that non-classical substances with growth factor-like activity might play a role in the regulation of proliferative processes in the eye.

cis-FFA do not alter membrane depolarization but block Ca2+ influx and GH secretion in KCl-stimulated somatotroph cells. Suggestion for a direct cis-FFA perturbation of the Ca2+ channel opening

It has been reported that cis-unsaturated free fatty acids (cis-FFA) block intracellular Ca2+ rise in EGFR T17 and GH3 cells by perturbing the generation of Ins(1,4,5)P3. In the present work, it was found that cis-FFA did not alter potassium-induced cell depolarization in GH3 cells, while blocking Ca2+ rise and GH secretion. Interestingly enough, saturated or trans-unsaturated FFA exert the opposite actions, i.e., they block cell depolarization without altering Ca2+ rise and hormone secretion. As depolarization activates GH3 cells via direct opening of Ca2+ channels with no generation of intracellular mediators, these results suggest that cis-FFA act by a direct perturbation of the Ca2+ channel opening.

Differential effect of polyunsaturated fatty acids on cell proliferation during human epithelial in vitro carcinogenesis: involvement of epidermal growth factor receptor tyrosine kinase

Polyunsaturated fatty acids (PUFAs) have been implicated in tumour development and have been shown to influence cell proliferation in vitro. We report here that n-3 and n-6 PUFAs at concentration > 10 microM inhibited the proliferation of a human kidney epithelial cell line (21HKE), which has retained phenotypic characteristics of normal kidney epithelial cells. In contrast, the proliferation was stimulated by n-3 and n-6 PUFAs at concentrations < 10 microM under defined growth conditions. The stimulatory effect of n-3 and n-6 PUFAs was even more profound in the presence of EGF. In human kidney epithelial cell lines reflecting different stages of transformation (11HKE and 1THKEras), the stimulatory effect was abrogated both in the presence and absence of EGF. Saturated fatty acids did not show any stimulatory effect on cell growth. The tyrosine kinase inhibitors genistein and tyrphostin-47 inhibited EGF-induced protein tyrosine phosphorylation dose-dependently in the 21HKE cells, and abolished the growth stimulatory effect of docosahexaenoic acid (DHA). This indicates the involvement of EGF receptor tyrosine kinase activity in the observed increase in cell proliferation.

Regulation of epidermal growth factor receptor activity by crotoxin, a snake venom phospholipase A2 toxin. A novel growth inhibitory mechanism

Crotoxin (CT), a phospholipase A2 (PLA2) derived from the venom of Crotalus durissus terrificus, is a heterodimeric protein composed of subunit B with enzymatic activity and a binding regulatory subunit (A) without enzyme activity. Although the PLA2 activity of CT may be important in its anti-proliferative activity, its cytostatic mechanism is unknown. In this study, we examined the cytostatic effect of PLA2-associated CT activity on squamous carcinoma cells expressing distinct levels of epidermal growth factor receptor (EGFr). CT was most effective in suppressing growth on cells expressing high intrinsic levels of EGFr. Cardiotoxin, another membrane active toxin with no intrinsic PLA2 activity, had no differential anti-proliferative activity on cells expressing high EGFr levels, suggesting a correlation between EGFr expression and CT-directed anti-proliferative activity. Both chemically modified CT (MCT) devoid of PLA2 activity and covalently cross-linked CT (CCT), which is functionally unable to utilize cellular membranes as PLA2 substrate, were also without growth inhibitory activity. No evidence for direct binding of CT to EGFr was found, although pretreatment with EGF was able to partially suppress the anti-proliferative activity of CT. Tyrosine phosphorylation of EGFr, however, was stimulated by CT in intact A431 cells. Tyrosine phosphorylation of EGFr was concentration-dependently stimulated (3- to 8-fold) in cellular membranes of A431 cells treated in vitro with CT but not with anti-proliferatively inactive MCT or CCT. The data provide evidence for transmembrane receptors involved in growth signaling (namely EGFr) as cellular targets and potential effectors of PLA2-mediated anti-proliferative activity of snake venom.

A peptide-based protein-tyrosine phosphatase inhibitor specifically enhances insulin receptor function in intact cells

3S-peptide-I is a synthetic tris-sulfotyrosyl dodecapeptide corresponding to the major site of insulin receptor autophosphorylation that potently inhibits dephosphorylation of the insulin receptor in a cell-free system and in digitonin-permeabilized Chinese hamster ovary (CHO) cells overexpressing the human insulin receptors (CHO/HIRc cells) (Liotta, A. S., Kole, H. K., Fales, H. M., Roth, J., and Bernier, M. (1994) J. Biol. Chem. 269, 22996-23001). In the present study, we found that 3S-peptide-I was not capable of inhibiting dephosphorylation of the epidermal growth factor (EGF) receptors in digitonin-permeabilized CHO cells that overexpress human EGF receptors (CHO/EGF-R cells). Moreover, the addition of a N-stearyl derivative of 3S-peptide-I to intact CHO/HIRc cells caused a concentration-dependent increase in insulin-stimulated phosphorylation of the insulin receptor, with a maximum effect (approximately 2.7-fold) at 50 microM. In contrast, ligand-stimulated EGF receptor phosphorylation in CHO/EGF-R cells was not affected by the presence of stearyl 3S-peptide-I. Furthermore, treatment of CHO/HIRc cells with this N-stearyl peptide led to a significant enhancement of the insulin-induced association of phosphatidylinositol (PI) 3-kinase activity with insulin receptor substrate 1 and the activation of mitogen-activated protein kinase. However, stearyl 3S-peptide-I had no effect on the EGF-stimulated activation of PI-3-kinase and mitogen-activated protein kinase in CHO/EGF-R cells. These data indicate that this tris-sulfotyrosyl dodecapeptide selectively enhances insulin signal transduction by specifically inhibiting dephosphorylation of the insulin receptor in intact cells.

Partial characterization of a putative new growth factor present in pathological human vitreous

BACKGROUND

Several growth factors have been implicated in the development of proliferative eye diseases, and some of those are present in human vitreous (HV). The effects of HV on cellular responses which modulate proliferative cell processes were studied. This study describes the partial characterization of a vitreous factor activity which does not correspond to any of the previously reported growth factors in pathological HV.

METHODS

Vitreous humour was obtained from medical vitrectomies, from patients with PDR and PVR. The biological activity of the vitreous factor was determined by its ability to increase cytosolic calcium concentration ([Ca2+]i), increase production of inositol phosphates, and induce cell proliferation in the cell line EGFR T17. In some experiments other cell lines, such as NIH 3T3, 3T3-L1, FRTL5, A431, PC12, Y79, and GH3, were also employed. Measurement of [Ca2+]i in cell suspensions was performed using the fluorescent Ca2+ indicator fura-2. The activity of the factor present in HV was compared with other growth factors by means of: (a) [Ca2+]i mobilization pattern, (b) sequential homologous and heterologous desensitization of receptors, (c) effects of phorbol esters on their action, and (d) inactivation after treatment with different proteolytic enzymes.

RESULTS

The HV-induced cell proliferation and increases in [Ca2+]i concentration were characterized by a peculiar time pattern. The different approaches used ruled out its identity with PDGF, bFGF, EGF, TGF-beta, IGFs, TNF-alpha, NGF, and other compounds such as ATP, angiotensin I, and bradykinin. Vitreous factor actions are mediated by specific receptors apparently regulated by PKC. This factor is able to induce [Ca2+]i mobilization in most of the cell lines studied, indicating that its effects are not tissue specific.

CONCLUSIONS

These results suggest the presence of a growth factor activity in pathological HV which may be due to the presence of an undescribed growth factor in the eye.

Correlation between the effects of bombesin antagonists on cell proliferation and intracellular calcium concentration in Swiss 3T3 and HT-29 cell lines

Bombesin (BN) acts as an autocrine mitogen in various human cancers. Several pseudononapeptide BN-(6-14) analogs with a reduced peptide bond between positions 13 and 14 have been shown to suppress the mitogenic activity of BN or gastrin-releasing peptide (GRP) when assessed by radioreceptor or proliferation assays and may have significant clinical applications. The search for potent and safe BN antagonists requires the evaluation of a large series of analogs in radioreceptor and proliferation assays. In this paper, we report that the ability of BN analogs to inhibit BN-induced calcium transients in Swiss 3T3 cells shows a high correlation with their inhibitory potency as evaluated by classical proliferation tests. The assay of calcium transients allows a rapid characterization of new BN analogs (in terms of minutes rather than days) and can be adapted as a labor and cost-effective screening step in the selection of potentially relevant BN antagonists for further characterization in cell proliferation systems. We also observed that results from the assay of calcium transients in Swiss 3T3 cells can be correlated with the results of the proliferative response in HT-29 cells, a cell line that does not seem to use the same early transmembrane ionic signal system. This result suggests that the calcium pathway is not mandatory for triggering cell division by the BN receptor.

Heating unsaturated fatty acids in air produces hemagglutinins

When mono-unsaturated fatty acids are heated in air, they form hemagglutinins. When the double bond is delta-6,7 or delta-9,10, the titer is higher than for delta-11,12. Stearic acid does not become a hemagglutinin on heating. Hydroxy-monounsaturated fatty acids, ricinoleic (cis-12-OH-delta-9) and ricinelaidic (trans-12-OH-delta-9) are not hemagglutinins unless they are heated. Oleic acid (delta-9-octadecenoic acid, OA) has a very low agglutination titer but lyses red cells at higher concentrations. Rabbit and rat erythrocytes (RBC) give the highest titers but RBCs of other species are also agglutinated. OA was chosen for further study. Its specific titer against rat RBCs increases with time of heating in air. Thin-layer chromatography (TLC) and mass spectroscopy (MS) show that higher molecular weight compounds are formed and that activity is associated with these materials. Synthetic (oxidation of oleic acid with tert-butyl peroxide) and commercial preparations of oleic acid dimers (Emery and Unichema) and a commercial preparation of oleic trimer mixed with polymer (Emery) have high hemagglutination titers against rat erythrocytes. A cyclic, long-chain dicarboxylic acid, 5(6)-carboxy-4-hexyl-2-cyclohexene-1-octanoic acid (Westvaco) gives a very low titer unless heated and no lysis. Sialidase treatment of the red cells increases the titer. Removal of cations does not alter the titer but addition of Ca2+ or Mg2+ lowers the titer. Light microscopy was used to characterize and visualize the agglutination process with rat RBCs. Agglutination without lysis or fusion is observed for low concentrations of heated oleic acid and C-18 dimers and trimer-polymer preparations, and no large vesicles are seen. We conclude that the oligomeric fatty acids with two or more hydrophobic chains of seven or more carbons are agglutinins at physiological pH. Agglutination by dimer may be the result of the its two hydrophobic side chains inserting into adjacent RBC membranes or the result of dimer inserting completely into RBC membranes and altering their properties. The carboxyl groups may also play a role in the process by interacting with polar headgroups in the RBC membrane.

Diacylglycerol: formation and function in phospholipid-mediated signal transduction

1. Properties, distribution and multiplicity of phosphoinositidases (phospholipase C, PLC) are investigated. 2. Generation of diacylglycerol (DAG) by a variety of enzymes such as phosphoinositide and phosphatidylcholine specific PLC, by a combination of phospholipase D and phosphatidic hydrolase, and by triglyceride lipase is examined. 3. Ca2+ and phospholipid-dependent protein kinase C act as the target of DAG messenger action. 4. There are differences in the formation of DAG in normal and transformal cell.

Role of polyunsaturated fatty acids as signal transducers: amplification of signals from growth factor receptors by fatty acids in mammary epithelial cells

The growth, morphogenesis and differentiation of milk producing epithelial tissues in the developing mammary glands require interaction with extracellular matrices and stimulation by hormones, growth factors and essential fatty acids. In primary culture, the proliferation of mammary epithelial cells (MEC), induced by epidermal growth factor (EGF), is enhanced and sustained by linoleate and its eicosanoid metabolites. Since a combination of linoleic acid (18:2 omega 6) and prostaglandin E2 or cAMP has synergistic effect on EGF-stimulated growth, it is suggested that additional cAMP-dependent protein kinase A (PK-A) independent pathways may also contribute to the linoleate effect on EGF action. Possible involvement of Ca2+/phospholipid-dependent protein kinase C (PK-C) is explored. Both linoleate and arachidonate can activate Type-II and Type-III protein kinase-C in MEC and a PK-C inhibitor can block growth stimulation by EGF and fatty acids. Like 12-O-Tetradecanoly phorbol-13-acetate (TPA), a PK-C activator which also enhances EGF-stimulated growth of MEC, linoleate can phosphorylate a 40-42 KD protein. EGF itself can stimulate transient phosphorylation of the same protein in MEC cultures but when supplemented with linoleate, which does not influence the ligand binding affinity of EGF-receptors, the transient phosphorylation signal in 40-42 KD protein is sustained.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of calcium fluxes in Jurkat T cells by myristic acid. Inhibition is independent of membrane potential and intracellular pH

Treatment of T lymphocytes with mitogenic antibodies against the T-cell receptor/CD3 complex induces within seconds a rise in the concentration of intracellular free Ca2+. We recently reported that free myristic acid, but not its methyl ester, inhibits both the anti-CD3-induced Ca2+ influx across the cell membrane and the Ca2+ release from intracellular stores in Jurkat T cells. Here we show that myristic acid induced a rapid hyperpolarization of the cell membrane potential and a decrease in intracellular pH in Jurkat cells. Lauric acid and palmitic acid caused minor hyperpolarization, whereas other saturated non-esterified fatty acids tested were without effect. Hyperpolarization of the membrane potential in Jurkat cells with valinomycin did not, however, inhibit the anti-CD3-induced Ca2+ signal, and the blocking effect on the Ca2+ signal in myristic acid-treated Jurkat cells was not reversed after normalization of the cell membrane potential by treatment with gramicidin. The inhibitory effect of myristic acid on the Ca2+ fluxes thus cannot be explained by changes in membrane potential. We also present evidence that the blocking effect of myristic acid on the receptor-operated Ca2+ flux is not due to the myristic acid-induced decrease in intracellular pH. Moreover, we demonstrate that myristic acid does not prevent the release of Ca2+ triggered by inositol 1,4,5-trisphosphate from intracellular pools in permeabilized cells. Our findings indicate that myristic acid blocks anti-CD3-induced Ca2+ traffic in Jurkat cells by interfering with the regulation of Ca2+ mobilization, apparently by blocking an early step in signal transduction from the T-cell-antigen receptor/CD3 complex.