Signaling role of CD36 in platelet activation and thrombus formation on immobilized thrombospondin or oxidized low-density lipoprotein

BACKGROUND AND OBJECTIVE

Platelets abundantly express glycoprotein CD36 with thrombospondin-1 (TSP1) and oxidized low-density lipoprotein (oxLDL) as proposed ligands. How these agents promote platelet activation is still poorly understood.

METHODS AND RESULTS

Both TSP1 and oxLDL caused limited activation of platelets in suspension. However, immobilized TSP1 and oxLDL, but not LDL, strongly supported platelet adhesion and spreading with a major role of CD36. Platelet spreading was accompanied by potent Ca(2+) rises, and resulted in exposure of P-selectin and integrin activation, all in a CD36-dependent manner with additional contributions of α(IIb) β(3) and ADP receptor stimulation. Signaling responses via CD36 involved activation of the protein tyrosine kinase Syk. In whole blood perfusion, co-coating of TSP1 or oxLDL with collagen enhanced thrombus formation at high-shear flow conditions, with increased expression on platelets of activated α(IIb) β(3), P-selectin and phosphatidylserine, again in a CD36-dependent way.

CONCLUSIONS

Immobilized TSP1 and oxLDL activate platelets partly via CD36 through a Syk kinase-dependent Ca(2+) signaling mechanism, which enhances collagen-dependent thrombus formation under flow. These findings provide novel insight into the role of CD36 in hemostasis.

Skeletal muscle fatty acid transport and transporters

While it has long been assumed that long chain fatty acids (LCFA) can freely diffuse across the plasma membrane, recent work has shown that LCFA uptake also involves a protein-mediated mechanism. Three putative LCFA transporters have been identified (FABPpm, FATP, and FAT/CD36), and all are expressed in rodent and human muscles. In a new model system (giant vesicles), we have demonstrated that (a) LCFA transport rates are scaled with the oxidative capacity of heart and muscle, (b) only FABPpm and FAT/CD36, but not FATP1, correlate with vesicular LCFA transport, and (c) LCFA transport can be increased by increasing (1) the FAT/CD36 protein of muscle (chronic adaptation) or (2) via the translocation of FAT/CD36 from an intracellular pool to the plasma membrane during muscle contraction (acute adaptation).

Increased rates of fatty acid uptake and plasmalemmal fatty acid transporters in obese Zucker rats

Giant vesicles were used to study the rates of uptake of long-chain fatty acids by heart, skeletal muscle, and adipose tissue of obese and lean Zucker rats. With obesity there was an increase in vesicular fatty acid uptake of 1.8-fold in heart, muscle and adipose tissue. In some tissues only fatty acid translocase (FAT) mRNA (heart, +37%; adipose, +80%) and fatty acid-binding protein (FABPpm) mRNA (heart, +148%; adipose, +196%) were increased. At the protein level FABPpm expression was not changed in any tissues except muscle (+14%), and FAT/CD36 protein content was altered slightly in adipose tissue (+26%). In marked contrast, the plasma membrane FAT/CD36 protein was increased in heart (+60%), muscle (+80%), and adipose tissue (+50%). The plasma membrane FABPpm was altered only in heart (+50%) and adipose tissues (+70%). Thus, in obesity, alterations in fatty acid transport in metabolically important tissues are not associated with changes in fatty acid transporter mRNAs or altered fatty acid transport protein expression but with their increased abundance at the plasma membrane. We speculate that in obesity fatty acid transporters are relocated from an intracellular pool to the plasma membrane in heart, muscle, and adipose tissues.

Electrostimulation enhances FAT/CD36-mediated long-chain fatty acid uptake by isolated rat cardiac myocytes

We investigated palmitate uptake and utilization by contracting cardiac myocytes in suspension to explore the link between long-chain fatty acid (FA) uptake and cellular metabolism, in particular the role of fatty acid translocase (FAT)/CD36-mediated transsarcolemmal FA transport. For this, an experimental setup was developed to electrically stimulate cardiomyocytes in multiple parallel incubations. Electrostimulation at voltages > or =170 V resulted in cellular contraction with no detrimental effect on cellular integrity. At 200 V and 4 Hz, palmitate uptake (measured after 3-min incubation) was enhanced 1.5-fold. In both quiescent and contracting myocytes, after their uptake, palmitate was largely and rapidly esterified, mainly into triacylglycerols. Palmitate oxidation (measured after 30 min) contributed to 22% of palmitate taken up by quiescent cardiomyocytes and, after stimulation at 4 Hz, was increased 2.8-fold to contribute to 39% of palmitate utilization. The electrostimulation-mediated increase in palmitate uptake was blocked in the presence of either verapamil, a contraction inhibitor, or sulfo-N-succinimidyl-FA esters, specific inhibitors of FAT/CD36. These data indicate that, in contracting cardiac myocytes, palmitate uptake is increased due to increased flux through FAT/CD36.

Coordinately regulated expression of FAT/CD36 and FACS1 in rat skeletal muscle

Protein-mediated fatty acid uptake and intracellular fatty acid activation are key steps in fatty acid metabolism in muscle. We have examined (a) the abundance of fatty acid translocase (FAT/CD36) mRNA (a fatty acid transporter) and long-chain acyl CoA synthetase (FACS1) mRNA in metabolically heterogeneous muscles (soleus (SOL), red (RG) and white gastrocnemius (WG)), and (b) whether FAT/CD36 and FACS1 mRNAs were coordinately up-regulated in red (RTA) and white tibialis muscles (WTA) that had been chronically stimulated for varying periods of time (0.25, 1, 6 and 24 h/day) for 7 days. FAT/CD36 mRNA and FACS1 mRNA abundance were scaled with (a) the oxidative capacity of muscle (SOL > RG > WG) (p < 0.05), (b) the rates of fatty acid oxidation in red and white muscles, and (c) fatty acid uptake by sarcolemmal vesicles, derived from red and white muscles. In chronically stimulated muscles (RTA and WTA), FAT/CD36 mRNA and FACS1 mRNA were up-regulated in relation to the quantity of muscle contractile activity (p < 0.05). FAT/CD36 mRNA and FACS1 mRNA up-regulation was highly correlated (r = 0.98). The coordinated expression of FAT/CD36 and FACS is likely a functional adaptive response to facilitate a greater rate of fatty acid activation in response to a greater rate of fatty acid transport, either among different types of muscles or in muscles in which capacity for fatty acid metabolism has been enhanced.

Involvement of membrane-associated proteins in the acute regulation of cellular fatty acid uptake

The transport of long-chain fatty acids across cellular membranes most likely occurs to some extent by passive diffusion and additionally is facilitated by a number of membrane-associated and cytoplasmic proteins. In this overview we focus on the involvement of the membrane proteins fatty acid translocase (FAT/CD36), plasma membrane fatty acid-binding protein (FABPpm) and fatty acid-transport protein (FATP). Newly obtained evidence is presented that in skeletal muscle, fatty acid uptake is subject to short-term regulation by translocation of FAT/CD36 from intracellular stores to the plasma membrane, analogous to the regulation of muscular glucose uptake by GLUT-4 translocation. These new findings establish a significant role of membrane-associated proteins in the cellular fatty acid-uptake process. Possible implications for the uptake and transport of long-chain fatty acids by the brain are discussed.

Fatty acid transport proteins facilitate fatty acid uptake in skeletal muscle

In view of the importance of long chain fatty acids (LCFAs) to many cellular processes, it may be desirable to regulate the LCFA disposition in the cell. Such regulation may be present at the level of the plasma membrane, since a number of putative LCFA transport proteins have been cloned. The development of a model system of giant vesicles has proven to be important in identifying the metabolic role of the LCFA transport system. LCFA transport rates and transporters (FABPpm and FAT/CD36) are scaled with oxidative capacity of heart and muscle. FAT/CD36 is a critical LCFA transport protein in muscle. With chronic contraction the increase in this protein also results in an increase in LCFA transport. Most importantly, LCFA transport is also increased acutely by muscle contraction, involving the translocation of FAT/CD36 from intracellular depots to the surface of the muscle cell. The acute (minutes) and chronic (days) regulation of LCFA transporters and transport by muscle may be an important mechanism for LCFA utilization during exercise and adaptable with training and with a metabolic disease such as type 2 diabetes.

Muscle palmitate uptake and binding are saturable and inhibited by antibodies to FABP(PM)

Studies show that uptake of long-chain fatty acids (LCFA) across the plasma membranes (PM) may occur partly via a carrier-mediated process and that the plasma membrane fatty acid-binding protein (FABP(PM)) may be a component of this system. To test the hypothesis that FABP(PM) is involved in transsarcolemmal transport of LCFA in muscle, we measured palmitate uptake in giant sarcolemmal vesicles and palmitate binding to PM proteins in rat muscles, (1) in the presence of increasing amounts of unbound palmitate and (2) in the absence or presence of antibody to FABP(PM). Both palmitate uptake and binding were found to be saturable functions of the unbound palmitate concentration with calculated Vmax values of 10.5 +/- 1.2 pmol/mg protein/15 sec and 45.6 +/- 2.9 nmol/mg protein/15 min and Km values of 12.8 +/- 3.8 and 18.4 +/- 1.8 nmol/L, respectively. The Vmax values for both palmitate uptake and binding were significantly decreased by 75-79% in the presence of a polyclonal antibody to the rat hepatic FABP(PM). Antibody inhibition was found to be dose-dependent and specific to LCFA. Glucose uptake was not affected by the presence of the antibody to FABP(PM). Palmitate uptake and binding were also inhibited in the presence of trypsin and phloretin. These results support the hypothesis that transsarcolemmal LCFA transport occurs in part by a carrier-mediated process and that FABP(PM) is a component of this process in muscle.

Acute regulation of fatty acid uptake involves the cellular redistribution of fatty acid translocase

We used muscle contraction, which increases fatty acid oxidation, as a model to determine whether fatty acid transport is acutely regulated by fatty acid translocase (FAT/CD36). Palmitate uptake by giant vesicles, obtained from skeletal muscle, was increased by muscle contraction. Kinetic studies indicated that muscle contraction increased V(max), but K(m) remained unaltered. Sulfo-N-succinimidyl oleate, a specific inhibitor of FAT/CD36, fully blocked the contraction-induced increase in palmitate uptake. In giant vesicles from contracting muscles, plasma membrane FAT/CD36 was also increased in parallel with the increase in long chain fatty acid uptake. Further studies showed that like GLUT-4, FAT/CD36 is located in both the plasma membrane and intracellularly (endosomally). With muscle contraction, FAT/CD36 at the surface of the muscle was increased, while concomitantly, FAT/CD36 in the intracellular pool was reduced. Similar responses were observed for GLUT-4. We conclude that fatty acid uptake is subject to short term regulation by muscle contraction and involves the translocation of FAT/CD36 from intracellular stores to the sarcolemma, analogous to the regulation of glucose uptake by GLUT-4.

Endurance training increases FFA oxidation and reduces triacylglycerol utilization in contracting rat soleus

We examined the effects of 8 wk of intense endurance training on free fatty acid (FFA) transporters and metabolism in resting and contracting soleus muscle using pulse-chase procedures. Endurance training increased maximal citrate synthase activity in red muscles (+54 to +91%; P </= 0.05) but failed to increase cytosolic fatty acid binding protein content, mRNA for fatty acyl-CoA synthase, and the putative FFA transporters or transport of palmitic acid into giant sarcolemmal vesicles. At rest, only triacylglycerol (TG) synthesis was significantly increased by training (+100.9 +/- 8.7 vs. +66.6 +/- 6.7 nmol/g wet wt; P </= 0.05). Muscle contraction increased TG synthesis (+46%; P </= 0.05) and palmitate oxidation (+115%; P </= 0.05) in untrained rats. Endurance training further enhanced synthesis of monoacylglycerol (MG), diacylglycerol (DG) and TG during contraction (+36, +69 and +71%, respectively; P </= 0.05), as well as exogenous palmitate oxidation (+41%; P </= 0.05) relative to untrained rats. Compared with those in untrained rats, TG breakdown and oxidation during contraction were reduced after training by 49 and 30%, respectively (P </= 0.05). In conclusion, endurance training 1) increases FFA oxidation and incorporation into endogenous lipid pools during contraction and 2) reduces the rate of intramuscular TG utilization during contraction when exogenous FFA availability is adequate. The enhanced FFA uptake subsequent to training appears to be independent of altered maximal transport rates of FFA into the muscle cell.

Cellular fatty acid transport in heart and skeletal muscle as facilitated by proteins

Despite the importance of long-chain fatty acids (FA) as fuels for heart and skeletal muscles, the mechanism of their cellular uptake has not yet been clarified. There is dispute as to whether FA are taken up by the muscle cells via passive diffusion and/or carrier-mediated transport. Kinetic studies of FA uptake by cardiac myocytes and the use of membrane protein-modifying agents have suggested the bulk of FA uptake is due to a protein component. Three membrane-associated FA-binding proteins were proposed to play a role in FA uptake, a 40-kDa plasma membrane FA-binding protein (FABPpm), an 88-kDa FA translocase (FAT/CD36), and a 60-kDa FA transport protein (FATP). In cardiac and skeletal myocytes the intracellular carrier for FA is cytoplasmic heart-type FA-binding protein (H-FABP), which likely transports FA from the sarcolemma to their intracellular sites of metabolism. A scenario is discussed in which FABPpm, FAT/CD36, and H-FABP, probably assisted by an albumin-binding protein, cooperate in the translocation of FA across the sarcolemma.

Protein-mediated palmitate uptake and expression of fatty acid transport proteins in heart giant vesicles

Giant sarcolemmal vesicles were isolated from rat heart and hindlimb muscles for a) characterization of long-chain fatty acid transport in the absence of metabolism and b) comparison of fatty acid transport protein expression with fatty acid transport. Giant vesicles contained cytosolic fatty acid binding protein. Palmitate uptake was completely divorced from its metabolism. All palmitate taken up was recovered in the intravesicular cytosol as unesterified FA. Palmitate uptake by heart vesicles exhibited a K m of 9.7 nm, similar to that of muscle (K m = 9.7 nm). Vmax (2.7 pmol/mg protein/s) in heart was 8-fold higher than in muscle (0.34 pmol/mg protein/s). Palmitate uptake was inhibited in heart (55-80%) and muscle (31-50%) by trypsin, phloretin, sulfo-N-succinimidyloleate (SSO), or a polyclonal antiserum against the 40 kDa plasma membrane fatty acid binding protein (FABPpm). Palmitate uptake by heart and by red and white muscle vesicles correlated well with the expression of fatty acid translocase (FAT/CD36) and fatty acid binding protein FABPpm, which may act in concert. The expression of fatty acid transport protein (FATP), was 10-fold lower in heart vesicles than in white muscle vesicles. It is concluded that long-chain fatty acid uptake by heart and muscle vesicles is largely protein-mediated, involving FAT/CD36 and FABPpm. The role of FATP in muscle and heart remains uncertain.

Skeletal muscle fatty acid transport and transporters

Long-chain fatty acids (LCFAs) are an important energy source for many tissues. The dogma that LCFAs are freely diffusible has been challenged. It is now known that LCFAs are transported into many tissues. Our studies have shown that LCFAs are also transported into skeletal muscle and into the heart. In recent years a number of putative fatty acid transport proteins have been identified. These are known as plasma membrane fatty acid binding protein (FABPpm, 43 kDa), fatty acid translocase (FAT, 88 kDa) and fatty acid transporter protein (FATP, 63 kDa). All three proteins are present in skeletal muscle and in the heart. The existence of an LCFA transport system in muscle may be essential 1) to facilitate the rapid and regulatable transport of LCFA to meet the metabolic requirements of working muscles and 2) to cope with an increase in circulating LCFAs in some pathological conditions (e.g. diabetes). There is now some evidence that metabolic changes and chronically increased muscle activity can increase the transport of LCFAs and increase the expression of putative LCFA transporters.

Stable transfection of fatty acid translocase (CD36) in a rat heart muscle cell line (H9c2)

Fatty acid translocase (FAT/CD36) is a membrane protein putatively involved in the transmembrane transport of long-chain fatty acids. We tested the hypothesis that expression of this protein in H9c2, a rat heart cell line normally not expressing FAT, would increase cellular palmitate uptake. We were able to stably transfect H9c2 cells with FAT, yielding 15 cell lines showing varying levels of FAT expression. The uptake and metabolism of palmitate was first studied in the non-transfected H9c2 cells and in two FAT-transfected cell lines. In each case, uptake of palmitate was found to be linear in time for at least 30 min and the uptake rate was saturable with increasing palmitate concentrations. Using conditions under which the maximal capacity of intracellular palmitate handling was not fully utilized, we tested 7 out of 15 FAT-transfected cell lines with varying FAT expression levels. No significant correlation was found between the level of FAT expression and the rate of palmitate uptake. In conclusion, we found that palmitate uptake by H9c2 cells occurs mainly by passive diffusion. Fatty acid translocase (FAT) transfection did not significantly increase the palmitate uptake rate, raising the possibility that H9c2 cells lack a protein (or set of proteins) that acts as an obligatory partner of FAT in long-chain fatty acid transport from the extracellular compartment to the cytoplasm.

Palmitate transport and fatty acid transporters in red and white muscles

We performed studies 1) to investigate the kinetics of palmitate transport into giant sarcolemmal vesicles, 2) to determine whether the transport capacity is greater in red muscles than in white muscles, and 3) to determine whether putative long-chain fatty acid (LCFA) transporters are more abundant in red than in white muscles. For these studies we used giant sarcolemmal vesicles, which contained cytoplasmic fatty acid binding protein (FABPc), an intravesicular fatty acid sink. Intravesicular FABPc concentrations were sufficiently high so as not to limit the uptake of palmitate under conditions of maximal palmitate uptake (i.e., 4.5-fold excess in white and 31.3-fold excess in red muscle vesicles). All of the palmitate taken up was recovered as unesterified palmitate. Palmitate uptake was reduced by phloretin (-50%), sulfo-N-succinimidyl oleate (-43%), anti-plasma membrane-bound FABP (FABPpm, -30%), trypsin (-45%), and when incubation temperature was lowered to 0 degrees C (-70%). Palmitate uptake was also reduced by excess oleate (-65%), but not by excess octanoate or by glucose. Kinetic studies showed that maximal transport was 1.8-fold greater in red vesicles than in white vesicles. The Michaelis-Menten constant in both types of vesicles was approximately 6 nM. Fatty acid transport protein mRNA and fatty acid translocase (FAT) mRNA were about fivefold greater in red muscles than in white muscles. FAT/CD36 and FABPpm proteins in red vesicles or in homogenates were greater than in white vesicles or homogenates (P < 0.05). These studies provide the first evidence of a protein-mediated LCFA transport system in skeletal muscle. In this tissue, palmitate transport rates are greater in red than in white muscles because more LCFA transporters are available.

Role of membrane-associated and cytoplasmic fatty acid-binding proteins in cellular fatty acid metabolism

A number of membrane-associated and cytoplasmic fatty acid-binding proteins (FABPs) are now being implicated in the cellular uptake and intracellular transport of long-chain fatty acids (FA). These proteins each have the capacity of non-covalent binding of FA, are present in tissues actively involved in FA metabolism, and are upregulated in conditions of increased cellular FA metabolism. To date, five distinct membrane FABPs have been described, ranging in mass from 22 to 88 kDa and each showing a characteristic tissue distribution. Evidence for involvement in cellular fatty acid uptake has been provided for several of them, because it was recently found that isolated cell lines transfected with 88-kDa putative fatty acid translocase (FAT; homologous to CD36) or with 63-kDa fatty acid-transport protein show an increased rate of FA uptake. The (at least nine) FABPs of cytoplasmic origin belong to a family of small (14-15 kDa) lipid binding proteins, all having a similar tertiairy structure but differing in binding properties and in tissue occurrence. The biological functions of the various FABPs, possibly exerted in a concerted action among them, comprise solubilization and compartmentalization of FA, facilitation of the cellular uptake and intracellular trafficking of FA, and modulation of mitosis, cell growth, and cell differentiation. In addition, the FABPs have been suggested to participate in and/or modulate FA-mediated signal transduction pathways and FA regulation of gene expression, and to prevent local high FA concentrations thereby contributing to the protection of cells against the toxic effects of FA. In conclusion, long-chain fatty acids are subject to continuous interaction with multiple proteins, which interplay influences their cellular metabolism.

Molecular mechanism of cellular uptake and intracellular translocation of fatty acids

The molecular mechanism of the transport of long-chain fatty acids across cellular membranes and the necessity and precise functioning of specific proteins in this process are still unclear. Various alternative mechanisms have been proposed. Studies with artificial phospholipid bilayers support the concept that fatty acids may enter and traverse the plasma membrane without the involvement of proteins. On the other hand, a number of membrane-associated fatty acid-binding proteins (FABPs) have been described which putatively function as acceptors for fatty acids released from albumin or from lipoproteins. Albumin binding proteins located at the outer cell surface could play an additional role in the delivery of fatty acids. The subsequent transmembrane translocation of fatty acids could take place by a membrane protein acting as a translocase, or by simple diffusion of fatty acids through either the phospholipid bilayer or a pore or channel formed by one or more membrane fatty acid transporters. At the inner side of the plasma membrane, the fatty acid is bound to a cytoplasmic FABP, which serves to buffer the intracellular aqueous fatty acid concentration. The direction of fatty acid migration through the plasma membrane most likely is governed by the transmembrane gradient of fatty acid concentration, assisted to some extent and in selected tissues by co-transport of sodium ions. The intracellular transport of fatty acids from the plasma membrane to the sites of metabolic conversion (oxidation, esterification) or subcellular target (signal transduction) is greatly facilitated by cytoplasmic FABPs. In conclusion, cellular uptake and intracellular translocation of long-chain fatty acids is a multi-step process that is facilitated by various membrane-associated and soluble proteins. The mechanism of cellular uptake of fatty acids probably involves both a passive and carrier-mediated transmembrane translocation.

Autophagic proteolysis: control and specificity

The rate of proteolysis is an important determinant of the intracellular protein content. Part of the degradation of intracellular proteins occurs in the lysosomes and is mediated by macroautophagy. In liver, macroautophagy is very active and almost completely accounts for starvation-induced proteolysis. Factors inhibiting this process include amino acids, cell swelling and insulin. In the mechanisms controlling macroautophagy, protein phosphorylation plays an important role. Activation of a signal transduction pathway, ultimately leading to phosphorylation of ribosomal protein S6, accompanies inhibition of macroautophagy. Components of this pathway may include a heterotrimeric Gi3-protein, phosphatidylinositol 3-kinase and p70S6 kinase. Recent evidence indicates that lysosomal protein degradation can be selective and occurs via ubiquitin-dependent and -independent pathways.

Uptake and metabolism of palmitate by isolated cardiac myocytes from adult rats: involvement of sarcolemmal proteins

The precise mechanism of uptake of long-chain fatty acids (FA) by cardiac myocytes is incompletely understood. We examined the involvement of sarcolemmal proteins in the initial uptake of FA by isolated rat cardiac myocytes, and the relation between initial uptake and metabolism. Cardiac myocytes were incubated in the presence of 90 microns [1-14C]palmitate complexed to 300 microns bovine serum albumin (BSA), presenting a physiologically relevant condition. During initial palmitate uptake (3 min), 56% of the intracellularly sequestered palmitate was esterified, and an additional 21% converted into oxidation intermediates. Varying the palmitate/BSA molar ratio revealed saturation kinetics with the apparent Km for cellular palmitate uptake (435 micro M) to be comparable to those for esterification (465 micro M) and oxidation (222 micro M). Varying the BSA concentration at a fixed palmitate/BSA molar ratio also showed saturation of uptake at increasing concentrations, with an apparent Km for BSA of 23 micro M. Changes in palmitate metabolism induced by changes in glucose utilization were accompanied by identical effects on palmitate uptake. Addition of lactate also inhibited both oxidation and uptake of palmitate, but had no effect on esterification. Virtually complete inhibition of palmitate oxidation by etomoxir inhibited palmitate uptake for 50%, while decreasing esterification by 33%. In the presence of phloretin and trypsin, palmitate uptake and metabolism were inhibited 76-88%, and in the presence of sulfo-N-succinimidyloleate by 53%. It is concluded that a) the bulk of sarcolemmal palmitate translocation occurs by membrane-associated FA-binding proteins, most likely assisted by albumin binding proteins without regulatory function, and b) palmitate uptake is most likely driven by its rapid intracellular metabolic conversion.

The role of the intralysosomal pH in the control of autophagic proteolytic flux in rat hepatocytes

Current methods to estimate changes in intralysosomal pH in hepatocytes do not discriminate between lysosomes and other intracellular acidic compartments. To obtain selective information on the change in lysosomal function in response to a change in lysosomal pH we have used the liberation of fluorescent 4-methoxy-2-naphthylamide from low concentrations of lysyl-alanyl-4-methoxy-2-naphthylamide, a substrate of lysosomal dipeptidylpeptidase II. Using permeabilized and intact hepatocytes, the activity of this enzyme in response to manipulations meant to increase the intralysosomal pH was compared with intralysosomal protein degradation and with the accumulation of [14C]chloroquine as a pH indicator of intracellular acidic compartments. The data show that changes in intralysosomal pH are indicated by changes in dipeptidylpeptidase II activity and that these are mainly due to a pH-dependent change in substrate accumulation in the lysosomes. Subsequently, the method was applied to establishing the extent to which an increase in intralysosomal pH can contribute to the inhibition of autophagic proteolysis in intact hepatocytes caused by a decrease in intracellular ATP, by an increase in amino acid concentration and by hypo-osmotic cell swelling. The following observations were made. (a) Moderate changes in intracellular ATP do not affect the lysosomal pH. (b) Hypo-osmotic cell swelling, which promotes inhibition of proteolysis by amino acids in freshly isolated hepatocytes, does not affect the lysosomal pH. (c) In addition to their known inhibitory effect on autophagic sequestration, amino acids (leucine in particular) can increase the lysosomal pH and thus inhibit intralysosomal protein degradation directly. (d) Low concentrations of the acidotropic agent methylamine increase the lysosomal pH without having an effect on autophagic proteolytic flux. It is concluded that autophagic proteolysis is not controlled by changes in the lysosomal pH.

Phosphorylation of ribosomal protein S6 is inhibitory for autophagy in isolated rat hepatocytes

In rat hepatocytes, autophagy is known to be inhibited by amino acids. Insulin and cell swelling promote inhibition by amino acids. Each of the conditions leading to inhibition of autophagic proteolysis was found to be associated with phosphorylation of a 31-kDa protein that we identified as ribosomal protein S6. A combination of leucine, tyrosine, and phenylalanine, which efficiently inhibits autophagic proteolysis, was particularly effective in stimulating S6 phosphorylation. The relationship between the percentage inhibition of proteolysis and the degree of S6 phosphorylation was linear. Thus, inhibition of autophagy and phosphorylation of S6 are under the control of the same signal transduction pathway. Stimulation of S6 phosphorylation by the presence of amino acids was due to activation of S6 kinase and not to inhibition of S6 phosphatase. The inhibition by amino acids of both autophagic proteolysis and autophagic sequestration of electro-injected cytosolic [14C]sucrose was partially prevented by rapamycin, a compound known to inhibit activation of p70 S6 kinase. In addition, rapamycin partially inhibited the rate of protein synthesis. We conclude that the fluxes through the autophagic and protein synthetic pathways are regulated in an opposite manner by the degree to which S6 is phosphorylated. Possible mechanisms by which S6 phosphorylation can cause inhibition of autophagy are discussed.

Cell swelling and the sensitivity of autophagic proteolysis to inhibition by amino acids in isolated rat hepatocytes

In the isolated perfused rat liver, autophagic proteolysis is inhibited by hypo-osmotic perfusion media [Häussinger, D., Hallbrucker, C., vom Dahl, S., Lang, F. & Gerok, W. (1990) Biochem. J. 272, 239-242]. Here we report that in isolated hepatocytes, incubated in the absence of amino acids to ensure maximal proteolytic flux, proteolysis was not inhibited by hypo-osmolarity while the synthesis of glycogen from glucose, a process known to be very sensitive to changes in cell volume [Baquet, A., Hue, L., Meijer, A. J., van Woerkom, G. M. & Plomp, P. J. A. M. (1990) J. Biol. Chem. 265, 955-959], was stimulated under identical conditions. However, in isolated hepatocytes, hypo-osmolarity increased the sensitivity of autophagic proteolysis to inhibition by low concentrations of amino acids. The anti-proteolytic effect of hypo-osmolarity in our experiments was not due to stimulation of amino-acid transport into the hepatocytes: neither the consumption of most amino acids, nor the rate of urea synthesis was appreciably affected by hypo-osmotic incubation conditions. In the course of these studies we also found that hypo-osmolarity increased the affinity of protein synthesis for amino acids. In the presence of amino acids the intracellular level of ATP was not much affected. However, because of cell swelling under these conditions the intracellular concentration of ATP decreased. It is proposed that a small part of the inhibition of proteolysis by amino acids may be due to this fall in ATP concentration.

Autophagic degradation of peroxisomes in isolated rat hepatocytes

Degradation of the peroxisomal enzymes fatty acyl-CoA oxidase and catalase was studied in hepatocytes isolated from rats treated with clofibrate and from control rats. Hepatocytes were incubated in the absence of amino acids in order to ensure maximal flux through the autophagic pathway and in the presence of cycloheximide to inhibit protein synthesis. (1) Degradation of the two peroxisomal enzymes in hepatocytes from clofibrate-fed rats, but not in hepatocytes from control rats, was much faster than that of other intracellular enzymes. This increased degradation of the peroxisomal enzymes was almost completely prevented by 3-methyladenine, an inhibitor of macroautophagic sequestration. (2) The increased degradation of the peroxisomal enzymes was also inhibited by a long-chain (C16:0) and a very-long-chain (C26:0) fatty acid, but not by C12:0, a medium-chain fatty acid, or by C8:0, a short-chain fatty acid. These results provide direct evidence for the proposal that autophagic sequestration can be highly selective [(1987) Exp. Mol. Pathol. 46, 114-122]. It is concluded that preferential autophagy of peroxisomes is prevented when these organelles are supplied with their fatty acid substrates.