Studies of endogenous polyphosphoinositide hydrolysis in human platelet membranes. Evidence that polyphosphoinositides remain inaccessible to phosphodiesterase in the native membrane

Possible mechanisms involved in apoptosis of colon tumor cell lines induced by deoxycholic acid, short-chain fatty acids, and their mixtures

Apoptosis of tumor cells is an important growth-regulating event in tumor masses. In this study we have confirmed that deoxycholic acid (DCA) and the short-chain fatty acids (SCFA) butyrate and propionate induce a time- and concentration-dependent apoptosis in two human colon tumor cell lines: HT-29 and CaCO2. DCA is more potent, inducing effects at low concentration (50 microM) and after 24 hours of incubation, whereas SCFA (4 mM) requires 72-96 hours of treatment. Combining low concentrations of DCA (12.5-25 microM) with butyrate and propionate (4 mM) produces an additive effect on the percentage of apoptotic cells, as demonstrated by flow cytometry and DNA fragmentation. Protein kinase C, protein tyrosine kinase, and gene transcription/translation inhibitors do not significantly modify the rate of apoptosis, whereas the intracellular Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM) completely abolishes the DCA-induced effect without affecting the SCFA-induced apoptosis. Measurement of intracellular Ca2+ by inverted fluorescence microscopy reveals that DCA induces a rapid increase of cytosolic Ca2+ that is abolished when the cells are preincubated with BAPTA-AM, whereas ethyleneglycolbis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid has a minimal effect. In contrast, SCFA does not modify the intracellular Ca2+ concentration. Thus the DCA-induced apoptosis is a Ca(2+)-dependent process, whereas the intracellular signals responsible for the SCFA-induced effect remain unknown. The ionophore activity of DCA could be responsible for the increased intracellular Ca2+, but other mechanisms, such as activation of phospholipase C and phosphoinositide hydrolysis, have to be considered.

In vitro activation and inhibition of rat colonic phospholipase D by fatty acids

The present study was designed to examine the effect of long-chain fatty acids on phospholipase D (PLD) of the proximal colon mucosa of the rat. These fatty acids are present in the colon, in close contact with the membranes which contain the enzyme. The results indicate that unsaturated fatty acids activate the enzyme. At 4 mM, trienes are more potent activators as compared to 18:2 and 18:1. The number of double bonds and the configuration around the double bond were found to be important factors in activation of the enzyme. The metabolism of polyunsaturated fatty acids may be involved in the activation since the presence of antioxidants and cyclooxygenase inhibitors were found to influence the activation of PLD by polyunsaturated fatty acids.

The effects of bile acids on phospholipase C activity in extracts of normal human colon mucosa and primary colon tumors

Phospholipase C (PLC) activity and its response to stimulation by bile acids was assayed in cellular extracts from 16 primary human colon tumors of various Duke's stages and paired adjacent normal mucosal samples. In the absence of bile acid, there was negligible degradation of phosphatidylinositol (PI) 1-stearoyl-2-[14C]-arachiodonoyl by tumor or normal tissue, but the addition of deoxycholic acid (DCA) or taurocholic acid (TCA) resulted in concentration-dependent and time-dependent stimulation of diacylglycerol (DAG) formation at optimal concentrations of 2 mM DCA and 4 mM TCA. Triton X-100 (0.125-1.0%) inhibited rather than enhanced the PI-degrading activity of these extracts, indicating that the stimulatory effects of DCA and TCA were not simply due to a detergent effect. Under the same assay conditions there was only a small amount of labeled monoacylglycerol or free arachidonic acid produced by extracts incubated in the absence or presence of DCA or TCA. No major differences in DAG production from PI were seen between paired samples of normal colon mucosa and primary colon tumors, in assays done in the presence of 2 mM TCA. Extracts from tumors in the distal part of the colon had higher activity than those from the proximal colon. This was also true for the extent of release of free arachidonic acid from labeled PI. Under the same conditions, labeled phosphatidylcholine or phosphatidylethanolamine did not serve as substrates for the colon mucosa or tumor extracts. Nor was there significant hydrolysis of the labeled DAG (1-stearoyl-2-14C-arachidonoylglycerol) by normal colon mucosa or tumor extracts, in the absence or presence of DCA or TCA. On the other hand, a low level of DAG lipase activity was detected in the presence of Triton X-100. These findings provide the first evidence that normal human colon mucosa and primary colon tumors contain a PI-specific PLC activity that is markedly stimulated by bile acids. Our results also suggest that bile acids may enhance colon carcinogenesis by acting on this enzyme system, thereby influencing signal transduction pathways in the target cells.

TRH: mediator of prolactin in the prostate?

Thanks to our preoccupation with androgen ablative therapy, no significant progress has been made in combating prostate cancer (PCa) in 50 years. Also, there have been only limited advances in medical treatment of benign prostatic hyperplasia (BPH). The intent of this essay is to explore the mode of participation of prolactin (Prl) in prostatic physiology in the hope that such knowledge will reveal new avenues through which both BPH and PCa can be opposed-even prevented. An especially novel aspect of this study is the recognition of the presence and androgen- and prolactin-dependent concentration of the tripeptide, thyrotropin-releasing hormone (TRH) in prostatic tissue. It is hypothesized that, whereas TRH is the hypothalamic stimulus of hypophyseal Prl secretion, it may, in the prostate, serve as the mediator of Prl's independent and androgen-dependent controls of the gland's growth and function. Through an overview of these relationships, methods are suggested both for their study and for their adaptation to early detection and prevention of imminent pathogenesis.

Taurodeoxycholate activates potassium and chloride conductances via an IP3-mediated release of calcium from intracellular stores in a colonic cell line (T84)

Whole-cell patch-clamp techniques and fluorescence measurements of intracellular Ca2+ concentration, (Ca2+)i, were used to investigate the mechanism of taurodeoxycholate (TDC) stimulation of Cl- secretion in the T84 colonic cell line. During perforated whole-cell recordings, the cell membrane voltage was alternately clamped to EK and ECl. Initially, TDC (0.75 mM) stimulated inward nonselective cation currents that were composed of discrete large conductance single-channel events. This initial response was followed by activation of K+ and Cl- currents with peak values of 385 +/- 41 pA and 98 +/- 28 pA, respectively (n = 12). The K+ and Cl- currents oscillated while TDC was present and returned to baseline levels upon its removal. The threshold for activation of the oscillatory currents was 0.1 mM TDC. Taurocholate, a bile acid that does not stimulate colonic Cl- secretion, induced no current response. The TDC-induced currents could be activated in Ca(2+)-free bathing solutions. Preincubation of cells with the Ca2+ chelator, bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethy)-ester (20 microM), (BAPTA-AM), eliminated the K+ and Cl- current responses, although the nonselective cation channel events were still present. Replacement of bath Na+ with NMDG+ inhibited the TDC-induced nonselective cation current but did not affect the K+ or Cl- currents. TDC induced a transient (Ca2+)i rise of 575 +/- 70 nM from a baseline of 71 +/- 5 nM (n = 15); thereafter, (Ca2+)i either plateaued or oscillated. TDC-induced (Ca2+)i oscillations were observed in the absence of bath Ca2+; however, removal of bath Ca2+ during the TDC response caused (Ca2+)i to return to near baseline values. Simultaneous K+ current and (Ca2+)i measurements confirmed that the initial nonselective cation current was independent of (Ca2+)i, while K+ current oscillations were in phase with the (Ca2+)i oscillations. TDC induced inositol monophosphate (IP) accumulation, reflecting production of inositol 1,4,5-trisphosphate (IP3) during TDC stimulation. The response to TDC during standard whole-cell patch-clamp was similar to that observed with perforated whole-cell recordings, except the nonselective cation current was prolonged. When heparin (1 mg/ml) was added to the pipette under these conditions, the Ca(2+)-activated currents were inhibited, but the nonselective cation currents were unaffected. These data suggest that TDC induces a Ca(2+)-independent nonselective cation conductance, perhaps by directly permeabilizing the plasma membrane. TDC stimulates Cl- secretion by activating K+ and Cl- conductances via an IP3-mediated release of Ca2+ from intracellular stores.

Association of alpha-phosphatidylinositol-specific phospholipase C with phospholipid vesicles

The alpha isoform of phosphatidylinositol-specific phospholipase C (alpha-PI-PLC, Mr 62,000) was purified from bovine brain. Enzyme activity was dependent on calcium, sodium cholate and showed the anticipated specificity for the phosphatidylinositols. Calcium interaction with this protein, investigated by gel filtration chromatography, showed no detectable binding at calcium concentrations adequate to activate the enzyme. Association of alpha-PI-PLC with phospholipid vesicles was studied by light scattering, fluorescence energy transfer and gel-filtration chromatography. The enzyme readily associated with vesicles of high charge density, with vesicles of crude acidic phospholipids and with PIP2. Interaction was characterized by a rapid association followed by slower addition of more protein to the phospholipid. Complexes containing 20-30 percent protein (by weight) were readily obtained. Calcium had only a small effect on this interaction. The protein-phospholipid complexes appeared to bind less calcium than a similar amount of phospholipid alone. Thus, alpha-PI-PLC did not appear to be a calcium-binding protein in either its free or membrane-associated states. Although alpha-PI-PLC showed the highest propensity to bind to phospholipids, a number of other proteins also associated with phospholipids under the conditions used. Thus, whether or not the observed interaction of alpha-PI-PLC with membranes was specific and biologically important or whether it was a process common to many proteins, was not known. Knowledge of this interaction may enhance our understanding of possible mechanisms for protein-membrane interactions in general.

Characterization of partially purified phospholipase C from human platelet membranes

A phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]-hydrolytic activity was found to be present in the human platelet membrane fraction, with 20% of the total activity of the homogenate. The membrane-associated phospholipase C activity was extracted with 1% deoxycholate (DOC). The DOC-extractable phospholipase C was partially purified approx. 126-fold to a specific activity of 0.58 mumol of PtdIns-(4,5)P2 cleaved/min per mg of protein, by Q-Sepharose, heparin-Sepharose and Ultrogel AcA-44 column chromatographies. This purified DOC-extractable phospholipase C had an Mr of approx. 110,000, as determined by Ultrogel AcA-44 gel filtration. The enzyme exhibits a maximal hydrolysis for PtdIns-(4,5)P2 at pH 6.5 in the presence of 0.1% DOC. The addition of 0.1% DOC caused a marked activation of both PtdIns(4,5)P2 and phosphatidylinositol (PtdIns) hydrolyses by the enzyme. The enzyme hydrolysed PtdIns(4,5)P2 and PtdIns in a different Ca2+-dependent manner; the maximal hydrolyses for PtdIns(4,5)P2 and PtdIns were obtained at 4 microM- and 0.5 mM-Ca2+ respectively. In the presence of 1 mM-Mg2+, PtdIns(4,5)P2-hydrolytic activity was decreased at all Ca2+ concentrations examined, but PtdIns-hydrolytic activity was not affected.

Phospholipase C from human sperm specific for phosphoinositides

Human sperm lysates were incubated in the presence of 1-[14C]stearoyl-2-acyl-sn-glycero-3-phosphocholine, 1-[14C]stearoyl-2-acyl-sn-glycero-3-phosphoethanolamine or 1-[14C]stearoyl-2-acyl-sn-glycero-3-phosphoinositol. Only the latter substrate was hydrolyzed to a significant extent, with a concomitant formation of 1-[14C]stearoyl-2-acyl-sn-glycerol. Furthermore, incubation of phosphatidyl[3H]inositol under the same conditions was accompanied by the formation, in roughly equal amounts, of [3H]inositol 1-phosphate and [3H]inositol 1:2-cyclic monophosphate. Finally [32P]phosphatidylinositol 4-phosphate and [32P]phosphatidylinositol 4,5-bisphosphate were degraded into [32P]inositol 1,4-bisphosphate and [32P]inositol 1,4,5-trisphosphate, respectively. The phosphoinositide-specific phospholipase C was activated by calcium (optimal concentration 5-10 mM) and inhibited by EGTA, although endogenous calcium supported a half-maximal activity. The enzyme displayed an optimal pH of 6.0 and an apparent Km of 0.08 mM. Its specific activity was around 10 nmol/min per mg protein, which is approximately the same as that found in human blood platelets. Subcellular fractionation revealed that 55% of the enzyme was solubilized under conditions where 80% of acrosin appeared in the supernatants. The majority of the particulate phospholipase C activity (37% of total) was found in the 1000 X g pellet, which contained only 8% of total acrosin activity. Further fractionation of spermatozoa into heads and tails indicated no specific enrichment of phospholipase C activity in any of these two fractions. However, owing to a 4-fold higher protein content in the head compared to the tail fraction, it is concluded that about 80% of particulate phospholipase C activity is located in sperm head. The physiological significance of this enzyme is discussed in relation to a possible role in acrosome reaction and (or) in egg fertilization.

How far does phospholipase C activity depend on the cell calcium concentration? A study in intact cells

The dependence of phospholipase C activity on the cytosolic Ca2+ concentration ([Ca2+]i) was studied in intact liver cells treated with the Ca2+-mobilizing hormone vasopressin, or not so treated. Phospholipase C (PLC) activity was estimated from the formation of [3H]inositol trisphosphate (InsP3) and the degradation of [3H]phosphatidylinositol 4,5-bisphosphate (PtdInsP2). The [Ca2+]i of the cells was clamped from 29 to 1130 nM by quin2 loading. This wide concentration range was obtained by loading the hepatocytes with a high concentration of the Ca2+ indicator in low-Ca2+ medium or by using the Ca2+ ionophore ionomycin in medium containing Ca2+. In resting cells, in which [Ca2+]i was 193 nM, treatment with 0.1 microM-vasopressin which stimulates liver PLC maximally, tripled InsP3 content and raised [Ca2+]i to 2 microM within 15 s. Lowering [Ca2+]i partially decreased cell InsP3 content as well as the ability of vasopressin to stimulate InsP3 formation maximally. At 29 nM, the lowest Ca2+ concentration obtained in isolated liver cells, basal InsP3 content was 64% of that measured in control cells. Addition of vasopressin no longer affected [Ca2+]i, but significantly increased InsP3 by 200%, although less than in the controls (300%). The maintenance of the greater part of the PLC response at constant [Ca2+]i indicated that, in the liver, InsP3 formation does not result from an increase in [Ca2+]i. The effects of lowering [Ca2+]i were reversible. When low cell [Ca2+]i was restored to a normal value, resting InsP3 content and the ability of vasopressin to stimulate InsP3 formation maximally by 300% were also restored. Raising [Ca2+]i from 193 to 1130 nM had little effect on the InsP3 content or the vasopressin-mediated increase in InsP3. In agreement with the stimulation of PLC activity by vasopressin, cell [3H]PtdInsP2 and total PtdInsP2 were degraded by application of this hormone for 15 s. In contrast, when [Ca2+]i was lowered to 29 nM, basal [3H]PtdInsP2 and total PtdInsP2 were increased by about 30%, [3H]PtdInsP2 was further increased by vasopressin, but total PtdInsP2 was not changed. These results show that, in intact hepatocytes, PLC is little affected by [Ca2+]i concentrations above 193 nM, but is partially dependent on Ca2+ below that value. They suggest that, in addition to activating PLC activity, vasopressin might stimulate PtdInsP2 synthesis, presumably via phosphatidylinositol-phosphate kinase, and that this pathway might predominate in cells with low [Ca2+]i.

Role of activation of protein kinase C in the stimulation of colonic epithelial proliferation and reactive oxygen formation by bile acids

Deoxycholate (DOC), chenodeoxycholate, 12-O-tetradecanoyl phorbol-13-acetate (TPA), or 1-oleoyl-2-acetyl-glycerol (OAG) activated colonic epithelial protein kinase C as reflected by translocation from the soluble to the particulate cell fraction. Activation of protein kinase C was correlated with stimulation of enhanced proliferative activity of colonic mucosa and reactive oxygen production. TPA and OAG, but not DOC, directly activated soluble protein kinase C in vitro. However, DOC rapidly increased labeled inositol phosphate and diacylglycerol accumulation in colonic epithelial cells. Retinoic acid inhibited protein kinase C activity and suppressed DOC-, TPA-, and OAG-induced increases in reactive oxygen production. The results support a role for protein kinase C in the stimulation of colonic epithelial proliferative activity and reactive oxygen production induced by bile acids, TPA and OAG. In contrast to TPA and OAG, which activate protein kinase C directly, bile acids appear to activate protein kinase C indirectly by increasing the diacylglycerol content of colonic epithelium.

Biochemical characterization of plasma membranes and intracellular membranes isolated from human platelets using Percoll gradients

Two kinds of membranes (plasma membranes and intracellular membranes) have been separated from human platelets by fractionation on Percoll gradients (successively at pH 7.4 and pH 9.6). On alkaline Percoll gradient, plasma membranes floated at low density, as shown with specific markers such as [3H]concanavalin A and monoacylglycerol lipase, whereas intracellular membranes sedimented in the higher densities and displayed a 5.6-12.4-fold enrichment in NADH diaphorase, antimycin insensitive NADH-cytochrome-c oxidoreductase and Ca2+-ATPase. Another criterion allowing differentiation of two membrane populations of human platelets was their lipid composition, which showed a cholesterol/phospholipid molar ratio of 0.5 in plasma membranes against 0.2 in intracellular membranes. Phospholipid analysis of the two kinds of membranes displayed also quite different profiles, since phosphatidylcholine increased from 30-32% in the plasma membrane to 52-66% in the intracellular membranes. This was at the expense of sphingomyelin (20-23% in plasma membrane, against 6.8-7.7% in intracellular membranes) and of phosphatidylserine (12-13% in plasma membrane, against 2-6% in intracellular membranes). Other striking differences between plasma membranes and intracellular membranes were obtained by SDS-polyacrylamide gel electrophoresis, which revealed the absence of actin and myosin in the intracellular membrane, whereas both proteins were present in significant amounts in plasma membranes. Finally, intracellular membranes but not plasma membranes were able to incorporate calcium. These results suggest that intracellular membrane fractions are derived from the dense tubular system and plasma membranes should correspond to the whole surface membrane of human platelets.