Glucose transport and sensing in the maintenance of glucose homeostasis and metabolic harmony (original) (raw)
Glucose-stimulated insulin secretion (GSIS) in the pancreatic β cell is the most extensively studied model of cell-autonomous glucose sensing in mammalian systems. Glucose enters the islet β cell via facilitated diffusion through GLUT2, the predominant glucose transporter in β cells. In pancreatic β cells, glucokinase, not glucose transport, is rate limiting for glucose metabolism at physiological glucose concentrations (24). In addition, glucokinase, unlike other hexokinases, is not allosterically inhibited by glucose-6-phosphate. Thus, glycolytic flux is proportional to the extracellular glucose concentration. Increases in the ATP/ADP ratio generated as a result of this glucose flux are thought to lead to β cell depolarization via closure of ATP-sensitive potassium channels (K ATP channels) and subsequent insulin secretion (for review, see ref. 25).
However, ATP generation via glycolysis is not the only factor regulating GSIS in the β cell. The pancreatic β cell is relatively unique in that it expresses low levels of lactate dehydrogenase and high levels of pyruvate carboxylase, an anaplerotic enzyme (26). Anaplerosis refers to biochemical reactions that increase the net carbon content of the TCA cycle (27). Flux through pyruvate carboxylase leads to an accumulation of carbon within the TCA cycle, whereas flux through pyruvate dehydrogenase does not (Figure 1). With the latter enzyme, the 2 carbons added to the TCA cycle as acetyl-CoA are fully oxidized to CO2 and H2O with one turn of the cycle. As the mitochondrial matrix cannot act as a sink for TCA cycle intermediates, carbon entering the cycle through pyruvate carboxylase must leave the mitochondria as TCA intermediates to maintain a stable, functional level of TCA intermediates. Evidence is accumulating that one or more of these TCA cycle intermediates exported from the mitochondria to maintain balance between anaplerosis and cataplerosis (involving net loss of carbon from the TCA cycle) is likely to serve as a metabolic coupling factor for GSIS (28). Therefore, while glucose plays a role in providing a substrate for energy generation, specific intracellular glucose-derived metabolites appear to have independent roles as signaling molecules (Figure 1).
Malonyl-CoA. Malonyl-CoA derived from citrate exported from the mitochondria is one such potential cataplerotic metabolite implicated in glucose sensing in numerous tissues, including the β cell (reviewed in ref. 29). Malonyl-CoA, an intermediate in de novo fatty acid synthesis, is an allosteric inhibitor of carnitine palmitoyltransferase 1 (CPT1), which regulates the transport of long-chain fatty acyl-CoAs into the mitochondria (Figure 1) (30). Thus, in the presence of abundant glucose, malonyl-CoA accumulates and fatty acid oxidation is inhibited. Malonyl-CoA generation has been implicated in glucose sensing in the liver to regulate ketogenesis (31) and the hypothalamus to regulate nutrient intake (32, 33).
K ATP channels. K ATP channel closure–induced membrane depolarization as a mechanism for glucose sensing is not reserved only for the pancreatic β cell. Glucose-sensing hypothalamic neurons were initially identified in the 1960s based upon discrete sets of neurons that could be either electrically inhibited or excited by elevations in extracellular glucose (34). Neuronal glucose sensing has long been implicated in the regulation of autonomic nervous system–mediated changes in glucose homeostasis, particularly the counterregulatory response to hypoglycemia (35, 36). Seino and colleagues demonstrated that absence of functional K ATP channels as a result of knockout of the Kir6.2 subunit of the channel complex abolishes the glucose responsiveness of ventromedial hypothalamic neurons and is associated with loss of glucagon secretion in response to neuroglycopenia (37). While K ATP-mediated glucose sensing appears operative in some glucose-responsive neurons, not all glucose-responsive neurons require K ATP channels for glucose sensing. For instance, Kir6.2 deficiency does not abolish glucose-induced excitation of glucose-sensing neurons in the arcuate nucleus (38). Recently, Rossetti and colleagues showed that in the presence of basal (low physiologic) insulin levels, icv infusion of glucose in rats directly suppresses endogenous glucose production without any detectable changes in glucoregulatory hormones (39). The suppressive effect of intracerebral glucose on endogenous glucose production could be inhibited with icv administration of the K ATP channel blocker glibenclamide, suggesting the same glucose sensor that plays a pivotal role in glucose homeostasis in the β cell may play a similar role in the hypothalamus (39).
KATP channels sense glucose via changes in cellular energy status, but they are not the exclusive cellular mechanism for sensing energy status. AMP-activated protein kinase (AMPK) is a ubiquitous, evolutionarily conserved sensor of cellular energy status that is activated by increases in the cellular AMP/ATP ratio (40) (Figure 1). AMPK, when activated, phosphorylates a diverse set of enzymes and alters expression of numerous genes which result in increased energy availability and inhibition of energy-consuming processes within the cell. In certain tissues and situations, AMPK may also act as a glucose sensor. The energetic stress (e.g., ATP depletion) imposed by exercise in skeletal muscle leads to AMPK activation, which promotes fatty acid oxidation (41) and may contribute to increased insulin-independent glucose uptake (Figures 1 and 2), though this remains controversial (42). Recently, AMPK activity has been implicated in glucose sensing in the hypothalamus and in mediating the counterregulatory response to insulin-induced hypoglycemia (Figure 2) (43, 44).
Intertissue glucose sensing and communication. Glucose is sensed in numerous tissues and cell types, including the hypothalamus, hepatocytes, the hepatoportal sensor, pancreatic islets, and possibly muscle and adipose tissue, each of which communicates with other tissues via hormones, neural pathways, or changes in the utilization of substrate. Pink lines represent neural mediated communication. Black lines represent glucose- or hormone-mediated communication.
Sirtuin 1. The redox status of a cell, reflected by the lactate/pyruvate ratio, may be regulated independently of the energy status (45). Sirtuin 1 (SIRT1), the mammalian homolog of the NAD +-dependent histone deacetylase Sir2, which regulates lifespan in Saccharomyces cerevisiae and Caenorhabditis elegans in response to caloric intake, has recently been shown to play a key role in regulating gluconeogenic and glycolytic pathways in mouse liver. Treatment of primary hepatocytes with pyruvate increased, and glucose decreased, protein levels of SIRT1 (46) (Figure 1). The mechanism mediating glucose/pyruvate sensing is uncertain, though the resulting change in SIRT1 protein levels is posttranscriptional. The combination of increased SIRT1 protein levels along with increased NAD + accumulation in fasted livers stimulates the SIRT1-mediated deacetylation of PPARγ, coactivator 1-α (PGC-1α), which increases expression of gluconeogenic enzymes specifically, without affecting PGC-1α–regulated mitochondrial genes (46).
The transcriptional changes mediated by the SIRT1/PGC-1α pathway may impact hepatic glucose production over the medium to long term, but the liver can also autoregulate its glucose production in response to a glucose load in the short term (reviewed in ref. 47). For instance, hyperglycemia causes a 60% reduction in net hepatic glucose output within 30 minutes in the conscious 36-hour-fasted dog in the setting of basal levels of insulin and glucagon (48, 49). Hepatic glucose output is determined by the rates of net glycogenolysis, gluconeogenesis, and glucose cycling (the futile cycling between glucose and glucose-6-phosphate). It appears that all 3 of these parameters may change to restore euglycemia as a direct result of changes in ambient glucose levels (50). After short-term fasting, when glycogen stores are sufficient, hyperglycemia appears to reduce glucose production predominantly by decreasing the glycogenolytic rate and increasing glucose cycling. Glucose-6-phosphate, the first glucose metabolite in glycolysis, may account for some of these effects, since it is a potent allosteric activator of glycogen synthase that catalyzes glycogen synthesis (Figure 1) (51). In this case, glucose-6-phosphate resulting from glucose metabolism is the signal reporting glucose flux, while glycogen synthase is the sensor.
Carbohydrate response element–binding protein. Other glucose metabolites have also been suggested to have signaling roles. Carbohydrate response element–binding protein (ChREBP) is a transcription factor that mediates glucose-responsive changes in gene expression in liver and possibly other tissues (52). In the presence of glucose, ChREBP translocates from the cytosol to the nucleus, and its DNA binding/transcription activity is additionally stimulated, resulting in transcription of glycolytic and lipogenic enzymes. It has been proposed that xylulose-5-phosphate, a metabolite of glucose generated via the pentose phosphate pathway, activates a specific isoform of protein phosphatase 2A, which dephosphorylates ChREBP, contributing to its translocation and activation (53) (Figure 1).
Hexosamines. In addition, glucose metabolites in the hexosamine biosynthetic pathway have also been implicated as cellular signaling molecules. This pathway metabolizes glucose to uridine diphospho-_N_-acetyl glucosamine (UDP-GlcNAc), which is used in the synthesis of glycosaminoglycans, proteoglycans, and glycolipids. The amidation of fructose-6-phosphate to glucosamine-6-phosphate is the rate-limiting step in the pathway and is catalyzed by glutamine:fructose-6-phosphate amidotransferase (GFAT) (54). GFAT activity correlates with insulin sensitivity and adiposity (55). Transgenic overexpression of GFAT in mouse liver results in hyperlipidemia, obesity, and impaired glucose tolerance (56). Combined overexpression of GFAT in muscle and adipose tissue results in hyperleptinemia and insulin resistance, though explanted muscle from these mice did not demonstrate insulin resistance ex vivo (57). Adipose-specific overexpression of GFAT causes impaired glucose tolerance and skeletal muscle insulin resistance, again demonstrating that genetic modifications in adipose tissue may impact whole-body glucose homeostasis (58). Interestingly, in muscle-specific GLUT1 overexpressors, where increased glucose flux into muscle is constitutive, GFAT activity and the concentration of UDP-hexoses are increased. In contrast, neither GFAT activity nor UDP-hexose concentrations are increased in adipose/muscle-specific GLUT4 overexpressors (59). The differences in hexosamine production may account for the fact that insulin resistance develops in the GLUT1 overexpressors but not in the GLUT4 overexpressors.
Hepatoportal sensor. Hepatoportal glucose sensing (Figure 2) describes a well-characterized phenomenon by which a glucose gradient established between the portal vein and the hepatic artery is sensed, resulting in increased hepatic glucose uptake, increased peripheral glucose disposal, inhibition of counterregulatory hormone secretion, and inhibition of food intake leading to hypoglycemia (60, 61). The actions of this sensor appear to depend on autonomic afferents to the CNS (62, 63). However, the precise cellular composition of this sensor and the mechanism by which it senses and communicates remain uncertain.