The significant contribution of adipose-derived circulating factors to physiological regulation has long been recognized. Factors such as leptin, adiponectin, and tumor necrosis factor alpha (TNF-α) feature frequently as characters in exercise physiology research. The purpose of this brief review is to introduce a new character to the cast: pigment epithelium-derived factor (PEDF).
PEDF is a 50 kd multifunctional glycoprotein, first identified in human retinal epithelial cells (28) and curiously described as a serine protease inhibitor; curious because one of its defining features is an inactive c-reactive loop that renders it unable to directly inhibit serine proteases (18). Since this initial description, it has been confirmed that PEDF is secreted directly from adipocytes (4,6) and is thought to be an important determinant of oxidative stress (1,35), inflammation (30), and angiogenesis (35). Further, it has been linked with metabolic syndrome (3,27,32), insulin resistance (6,16,23), and visceral adiposity (29,31), is predictive of future clinical events in patients with heart failure (22), and has been proposed as a potential therapeutic target for cancer treatment (14,15). The foci of this brief review are PEDF‘s role as a regulator of insulin sensitivity, current evidence for its regulation by the sympathetic nervous system, and consideration of its candidacy in the cross talk between adipose tissue and skeletal muscle.
PEDF AND INSULIN RESISTANCE
Serum PEDF concentration is high in adult humans with type 2 diabetes (9,19). Further, it is an independent predictor of metabolic syndrome (3) and is associated statistically with indirect measures of insulin resistance, including the homeostasis model assessment (16,29). However, until recently, there was little evidence in humans for a link between robust measures of insulin sensitivity and circulating PEDF. Using the hyperinsulinemic-euglycemic clamp technique, considered by many to be the gold-standard measure of insulin sensitivity, we (21) reported on the inverse relation between the circulating PEDF and insulin sensitivity (Fig. 1A). In addition, we also reported on the inverse association between PEDF and metabolic flexibility, the ability to adjust the relative reliance on a specific substrate in response to changing nutrient supply (Fig. 1B). Our observations were corroborated a short while later by reports of similar correlations between PEDF and insulin sensitivity, as determined using the intravenous glucose tolerance test (23). Noteworthy, in this study (23) and one other (29), following diet and/or surgery-induced weight loss, the subsequent improvement in insulin sensitivity was associated with the magnitude of decrease in PEDF.
The most compelling mechanistic evidence for a link between PEDF and insulin resistance comes from a comprehensive series of animal experiments (4). When recombinant PEDF was administered acutely to lean mice, insulin sensitivity was decreased appreciably. This decrease was attributed to activation of proinflammatory signaling, that in turn inhibited insulin signal transduction. In a separate experiment involving lean mice, 5 d of PEDF administration led to insulin resistance, increased plasma free fatty acid concentration, increased skeletal muscle and liver triacyglycerol content, and the accumulation of the fatty acid metabolites ceramide and diacyglycerol in skeletal muscle. Finally, when a PEDF-neutralizing antibody was administered to obese mice over 5 days, insulin sensitivity was augmented, whereas skeletal muscle and liver triacyglycerol content and skeletal muscle ceramide and diacyglycerol were decreased.
Conversely, PEDF has been shown to protect against insulin resistance. In the human hepatoma cell line, Hep3B, insulin sensitivity was decreased dramatically after exposure to advanced glycation end-products (AGE) (33). Exposure to AGE-activated Ras-related C3 botulinum toxin substrate 1 (Rac-1) and impaired insulin signaling; PEDF inhibited this AGE-elicited action (34). Based on these data, it has been speculated that PEDF does not contribute to the development of insulin resistance; rather, it may be released to protect tissues from insulin resistance (33). Although this idea could explain the human correlation studies, it is clearly opposed by the observations of increased insulin sensitivity in obese mice when PEDF was neutralized and increased insulin resistance in lean mice after PEDF administration (4).
These apparent contradictory observations may be explained by recent studies using adipose triglyceride lipase (ATGL) knockout mice (ATGL−/−) (2). When PEDF was administered to wild-type mice, adipose tissue lipolysis was increased, whereas the ability of skeletal muscle to oxidize fat was attenuated, as was insulin sensitivity. In contrast, when PEDF was administered to ATGL−/− mice, no change in lipid metabolism (lipolysis or oxidation) or insulin sensitivity was detected, leading the authors to conclude that ATGL is necessary for the unfavorable effects of PEDF on insulin action. Adipose triglyceride lipase is present only in low abundance, if at all, in Hep3B cells; thus, this may explain why PEDF did not induce insulin resistance when administered to this cell line.
REGULATION OF PEDF BY THE SYMPATHETIC NERVOUS SYSTEM
Given the proposed link between PEDF and various chronic diseases, identification of the biological processes responsible for its regulation may be important. In this regard, there is some evidence to suggest PEDF is under partial control by the sympathetic nervous system. In the sympathectomized retina of rats, PEDF gene expression and protein content were decreased compared with the intact contralateral eye (12). In cultured retinal pigment epithelial cells treated with norepinephrine (12) and in cultured human choroidal endothelial cells treated with the nonselective beta-adrenergic receptor agonist, isoproterenol (26), PEDF protein expression was increased. Thus, based on these animal and cell culture data, it seems that the sympathetic nervous system may regulate PEDF and that the association is positive. That is, increased sympathetic activity leads to greater PEDF.
Contrary to these data (12,26), we have demonstrated in healthy adult humans that short-term inhibition of the sympathetic nervous system does not affect circulating PEDF (17). Six days of transdermal administration of the centrally acting alpha-2-adrenergic receptor agonist, clonidine, decreased skeletal muscle sympathetic nerve activity, plasma catecholamine concentration, and resting heart rate without changing serum PEDF concentration. In another human study, we performed graded, intravenous administration of isoproterenol to stimulate beta-adrenergic receptors and found that circulating PEDF concentration was decreased (21). Recently, we repeated this observation as part of a different study, in a separate group of adults (Bell C., unpublished, 2010; Fig. 2). Possible explanations to account for the differences between our data and those of previous studies (12,26) may relate to the interaction of PEDF with ATGL and its prolipolytic properties. During short-term (16 h) fasting, PEDF protein is upregulated; after refeeding, PEDF protein is decreased (2). During systemic beta-adrenergic receptor stimulation, rates of lipolysis are increased (24), and substrate (i.e., fatty acid) is made available. Our observation of decreased PEDF during beta-adrenergic receptor stimulation in humans may be due to increased lipolysis. This could be confirmed with quantification of PEDF before and during beta-adrenergic receptor stimulation on two separate occasions: once during normal conditions and again after administration of a pharmacological inhibitor of lipolysis (such as acipimox). One might speculate that during inhibition of lipolysis, PEDF would not change in response to beta-adrenergic receptor stimulation.
Aside from the sympathetic nervous system, several other regulators of PEDF have been investigated. In human primary adipocytes, secretion of PEDF was increased in response to insulin, decreased with hypoxia and troglitazone, and unaffected by TNF-α (6). The insulin-mediated secretion of PEDF fits well with the observation of greater serum PEDF concentration reported in adults with type 2 diabetes (9,19). Further, PEDF gene expression, measured in human adipose tissue biopsies, was increased with overfeeding and decreased with caloric restriction; plasma PEDF concentration also was increased or decreased with overfeeding or caloric-restriction, respectively (7).
PEDF IN ADIPOSE/MUSCLE CROSS TALK
Several studies have suggested that excessive adiposity may attenuate the physiological adaptations to exercise training. For example, compared with mice fed a low-fat diet, mice chronically fed a high-fat diet demonstrated impaired load-induced skeletal muscle hypertrophy (25). It has been proposed that these attenuated responses in part may be attributed to cross talk between adipose tissue and skeletal muscle; that is, excessive adipose tissue leads to higher concentrations of circulating proinflammatory factors that may inhibit protein synthesis in skeletal muscle (10,20). PEDF may be another candidate for adipose/muscle cross talk. Peripheral vasculature adaptations to exercise training include angiogenesis; vascular endothelial growth factor (VEGF) is thought to be critical to these adaptations (13). PEDF has been shown to have antiangiogenic properties, facilitated primarily by decreasing VEGF expression (8) and also by inducing apoptosis in endothelial cells (5). The angiogenic response to exercise of diabetic mice is less than that of healthy mice (11). Given the high serum concentrations of PEDF associated with type 2 diabetes (9,19), it seems plausible that the absence of a postexercise VEGF response in diabetic mice may be at least partially mediated by the inhibitory effects of high PEDF.
SUMMARY AND UNIFYING WORKING HYPOTHESIS
In the spirit of the journal, the following unifying hypothesis is presented with the goal of advancing a new and testable integrated model (Fig. 3). Prolonged positive energy balance is associated with increased PEDF gene expression in adipose tissue and increased serum PEDF concentration (7). Based on animal data (2), increased PEDF, via ATGL, stimulates lipolysis while decreasing the capacity of skeletal muscle to oxidize fat. This leads to increased ectopic lipid accumulation (e.g., increased skeletal muscle and liver triacyglycerol content and the accumulation of the fatty acid metabolites such as ceramide and diacyglycerol in skeletal muscle) and eventually insulin resistance (4). Resistance to insulin usually results in higher circulating plasma insulin concentrations. Administration of insulin to adipocytes is known to promote secretion of PEDF (6), and increased PEDF stimulates lipolysis (2). Thus, an adipocyte gene, in response to chronic excessive caloric intake, has the potential to initiate a cascade of events that may contribute to the development of type 2 diabetes in weight-gaining adults. Exercise seems like an obvious intervention; however, high circulating PEDF may attenuate some of the favorable physiological adaptations to exercise, possibly by its inhibitory effect on VEGF.
In summary, PEDF is a multifunctional, adipose-derived glycoprotein, thought to be capable of regulating oxidative stress, inflammation, and angiogenesis. It has been linked with an assortment of chronic diseases, including metabolic syndrome, diabetes, cancer, and vascular disease, and it seems to have been overlooked by exercise physiology; at the time of submission, a PubMed search for studies of PEDF and exercise returned only one study (21). Maybe it is time to consider adding a new adipokine to your current list of favorites.
The author receives funding from the American Diabetes Association's Amaranth Diabetes Fund, Defense Advanced Research Projects Agency, and Office of Naval Research. The work of other researchers is recognized; regretfully, the reference limitations imposed by Exercise and Sport Sciences Reviews prevent the inclusion of these citations.
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