The role of protease inhibitors in the pathogenesis of HIV-associated lipodystrophy: cellular mechanisms and clinical implications

Abstract

Metabolic complications associated with HIV infection and treatment frequently present as a relative lack of peripheral adipose tissue associated with dyslipidemia and insulin resistance. In this review we explain the connection between abnormalities of intermediary metabolism, observed either in vitro or in vivo, and this group of metabolic effects. We review molecular mechanisms by which the HIV protease inhibitor (PI) class of drugs may affect the normal stimulatory effect of insulin on glucose and fat storage. We then propose that both chronic inflammation from HIV infection and treatment with some drugs in this class trigger cellular homeostatic stress responses with adverse effects on intermediary metabolism. The physiologic outcome is such that total adipocyte storage capacity is decreased, and the remaining adipocytes resist further fat storage. The excess circulating and dietary lipid metabolites, normally "absorbed" by adipose tissue, are deposited ectopically in lean (muscle and liver) tissue, where they impair insulin action. This process leads to a pathologic cycle of lipotoxicity and lipoatrophy and a clinical phenotype of body fat distribution with elevated waist-to-hip ratio similar to the metabolic syndrome.

Figure 1

Protease inhibitors inhibit adipocyte glucose uptake. Insulin binds to the adipocyte insulin receptor and induces the mobilization of transporters for glucose (GLUT4) and fatty acids* (FATP1) from the cytoplasm to the cell membrane. The major effect of PIs on adipocytes is direct inhibition of adipocyte glucose uptake, resulting in a significant reduction in triglyceride (TG) synthesis (Parker et al. 2005). The graph shows the inhibition by increasing concentrations of ATV and LPV of insulin-stimulated glucose uptake (ten minutes of exposure to drug) and triglyceride synthesis (sixteen hours of exposure to drug) in cultured, fully differentiated human subcutaneous adipocytes, using previously described methods (Noor et al. 2006; Parker et al. 2005). Reduced triglyceride synthesis contributes to adipocyte shrinkage and lipoatrophy. Proteasome inhibition in adipocytes (Parker et al. 2005) will additionally cause the accumulation of unfolded proteins and induce the ER stress response (Ron and Walter 2007), which, together with glucose deprivation, may be the dominating mechanism for adipocyte degeneration and lipoatrophy. (*Note: fatty acids metabolized by adipocytes are mainly derived from free fatty acids bound to albumin or lipoprotein lipase [LPL]-mediated hydrolysis of triacylglycerol-rich lipoprotein particles such as chylomicrons or very-low-density lipoproteins [VLDL] [Large et al. 2004]).

Figure 2

Shared structural features of the HIV protease inhibitors. Squares represent the core peptidomimetic structure found within all HIV PIs.

Figure 3

Protease inhibitors inhibit pancreatic β-cell glucose-stimulated insulin release. Glucose uptake via the GLUT2 transporter is the key driver of insulin release by the pancreatic β cell. Unlike the GLUT4 glucose transporter in adipocytes and muscle, the GLUT2 transporter is not inhibited by PIs in our studies with the MIN6 insulinoma cell line, but insulin release is nevertheless inhibited (Flint et al. 2005). In the β cell, elevated glucose increases the ATP:ADP ratio, closing the ATP-dependent potassium channel, depolarizing the cell membrane, and leading to the opening of voltage-dependent calcium channels (VDCC). Increasing intracellular concentrations of calcium trigger the release of insulin. Protease inhibitors, like ritonavir, may block the calcium channel (Flint et al. unpublished data), inhibiting the signaling cascade that leads to the release of insulin.

Figure 4

Protease inhibitors stimulate hepatic lipid synthesis by maintaining the nuclear activity of the SREBP regulatory transcription factors. GLUT2 is the major hepatocyte glucose transporter, but unlike GLUT4 in the adipocyte, GLUT2 appears to be unaffected by the PIs (Flint et al. 2005). There is also no evidence that PIs affect the transport of fatty acids by FATP2 or FATP5. Instead, PIs affect hepatocyte metabolism by extending the activity of transcription factors involved in regulating lipid synthesis (Parker et al. 2005). The most important of these factors are the sterol response element binding proteins (SREBPs) that, in the resting state, remain in the endoplasmic reticulum membrane in close association with SCAP, the sterol-sensing escort protein (Horton et al. 2002). When sterols are depleted, however, the SREBP-SCAP complex moves from the endoplasmic reticulum into the Golgi, where Site 1 and 2 proteases (S1P, S2P) clip the active basic helix-loop-helix (bHLP) domain of SREBP out of the membrane. The SREBP transcription factor then enters the nucleus and binds to response elements of lipogenic genes, inducing their transcription and ultimately increasing the synthesis of cholesterol and triglycerides. SREBP half-life in the nucleus is determined by its rate of degradation by nuclear proteasomes (Hirano et al. 2001). PIs inhibit proteasome proteolysis, extending the half-life of SREBP, thus further increasing the synthesis of hepatic lipids (Parker et al. 2005; Riddle et al. 2001).

Figure 5

Early direct effects of HIV protease inhibitors on intermediary metabolism. Glucose uptake inhibition leading to reduced metabolism dominates the effect in myocytes and adipocytes, but stimulation of lipogenesis is the direct and dominant effect in hepatocytes. The acquired insulin insensitivity in peripheral tissues is exacerbated by reduced pancreatic β-cell function. Once triggered, the cascade is also likely to be further aggravated by inflammatory mediators produced in the presence of chronic HIV infection. The result is hyperlipidemia, coupled with lipo-toxicity in hepatocytes and lipoatrophy/insulin resistance in fat tissues. These processes together can completely explain the lipodystrophy and “metabolic syndrome” observed in HIV patients treated with PIs. However, treatment normally combines drugs from several antiviral classes, including the NRTIs, NNRTIs, and more recently, drugs targeting novel mechanisms such as HIV attachment. It is unfortunate that several of the NRTIs, especially the thymidine analogs, can also inhibit the differentiation of new adipocytes from preadipocytes (Caron et al. 2004; Roche et al. 2002), though there is no evidence that they affect metabolism in the mature adipocyte. It is possible that this mechanism is responsible for reports of lipoatrophy in patients who have not been treated with PIs (Mallal et al. 2000). The combined effect of NRTIs and PIs may be responsible for the reported high prevalence of lipodystrophy, and especially lipoatrophy, in HIV patients (Estrada et al. 2006; Miller et al. 2003; Samaras et al. 2007).