Systemic loss of caveolin-1 leads to a complex metabolic phenotype including a substantial decrease in metabolic inflexibility. This is also tightly associated with lower adiponectin levels and, in particular, lower levels of the HMW form of adiponectin. In fact, mice overexpressing adiponectin and mice lacking caveolin-1 display metabolic phenotypes at two opposite extremes of the spectrum of metabolic responses. At least part of this complex metabolic phenotype of the caveolin-1 null mouse is a compensatory response to severe metabolic dysregulation at the level of adipocytes. Importantly, reconstituting caveolin-1 expression in the endothelium in the context of a full body caveolin-1 null mouse did not restore the metabolic phenotype, but did improve cardiovascular and pulmonary hypertension phenomena, hence eliminating another likely cell type that could have contributed to a metabolic phenotype (Murata et al., 2007
). Caveolin-1 null adipose tissue is insensitive to both insulin and adrenergic agonists likely due to the lack of caveolin-1, the key structural protein of caveolae in adipocytes that stabilizes the receptors and facilitates signaling (Liu et al., 2002
). As judged by the enhanced response to the phosphodiesterase inhibitor enoximone, downstream signaling at the level of adrenergic agonists and glucagon appears to function properly, even in the absence of caveolin-1. Nevertheless, a high degree of metabolic inflexibility persists in the caveolin-1 null mouse. Our data suggest that this is not only due to a very complex set of changes that lead to the inability to accurately gauge the metabolic needs of the system, but is also caused by altered mitochondrial function.
Lack of caveolin-1 is associated with altered mitochondrial function at multiple levels. Caveolin-1 null MEFs display an increased dependence on glucose and a higher mitochondrial membrane potential. In vivo
, we observe elevated circulating levels of H2
and an increased build-up of lactate upon PEPCK-inhibition in caveolin-1 null mice. In caveolin-1 null adipose tissue, we observe increased oxidative damage and an increased susceptibility to HFD-induced adipocyte death. Gene expression analysis also shows altered expression of many mitochondrial and redox-sensitive genes in adipose tissue, but not in the liver. At this point, we do not know whether the altered mitochondrial function is a consequence of the metabolic changes in the caveolin-1 null mice or if caveolin-1 deficiency directly has an impact on mitochondrial function. There are several potential mechanisms that can lead to the altered mitochondrial function observed in caveolin-1 null cells. One possibility is that the increased local levels of adipose tissue FFAs, driven by widespread elevated lipolysis in caveolin-1 null adipocytes, directly induce mitochondrial dysregulation and ROS production (a mechanism recently reviewed in (Vigouroux et al., 2011
)). Several additional studies have highlighted that the rate of mitochondrial peroxide generation is significantly higher when basal respiration is driven by fatty acid oxidation compared to carbohydrate-based substrates (Anderson et al., 2007
; St-Pierre et al., 2002
). Previous studies from our laboratory show that FFAs can lead to an increase in mDIC, higher levels of which in turn elevate the mitochondrial potential (Das et al., 1999
). In line with a cause / effect relationship, caveolin-1 null MEFs and adipose tissue display elevated mDIC levels. Increased membrane potential per se
can be the cause for increased ROS production, leading to mitochondrial and metabolic dysfunction. Thus, the phenomenon of increased mitochondrial membrane potential may not necessarily directly depend on excess FFA levels in these cells. A second possibility is the mechanism suggested by Bosch and colleagues. They report an increase in mitochondrial cholesterol accumulation and a reduction in mitochondrial GSH in caveolin-1 deficient cells and suggest that this leads the mitochondrial impairments in these cells (Bosch et al., 2011
). It is also possible that intracellular dysregulation of lipid species other than cholesterol plays a role for the mitochondrial phenotype. Sphingolipids, such as ceramides, may show an altered subcellular distribution and play a role in this context. However, we mainly detect signs of mitochondrial alteration in adipose tissue, but not in other tissues where caveolin-1 is also expressed at high levels, such as the lung. This suggests that altered intracellular lipid partitioning alone cannot fully explain the mitochondrial alterations in caveolin-1 null mice. Hence, our present hypothesis that best explains our findings is that adipose tissue-resident cells are more vulnerable to mitochondrial changes induced by caveolin-1 deficiency through the elevated exposure of FFAs.
The elevated RER in caveolin-1 null mice suggests that the preference for glucose is not only an in vitro
phenomenon, but also evident at the whole system level. Despite the increased use of glucose, the caveolin-1 null mice are not only able to maintain, but have, in fact, higher fasting glucose levels than the wildtype controls. This is due to a vastly increased capacity for hepatic glucose production. In particular, we observe an increased capacity to produce glucose from glycerol in the caveolin-1 null mice. However, the elevated levels of circulating urea also suggest that the glucose overproduced in the caveolin-1 null mouse is in part generated from an enhanced amino acid catabolism. In general, enhanced amino acid catabolism leads to muscle wasting. However, caveolin-1 null mice do not have reduced muscle mass. There may well be several mechanisms underlying this complex phenotype, but we chose to focus on secondary adaptations due to the severe defects observed in the adipose tissue (this study and (Cohen et al., 2004
; Cohen et al., 2003
; Razani et al., 2002
). Our rationale to this approach was mainly that the liver-specific caveolin-1 null mice lack a substantial liver phenotype, and that the reported muscle phenotype of the caveolin-1 null mouse appears to be more pronounced as the mice age (Schubert et al., 2007
From gene expression profile analysis of adipose tissue, we found that the pathway for BCAA catabolism is down-regulated. On the contrary, our leucine tracer studies showed that leucine is more effectively taken up and used in the lipogenic pathway in caveolin-1 null adipose tissue compared to wildtypes. Thus, the transcriptional reduction of BCAA metabolizing enzymes result from a negative feed-back mechanism and is not the cause of the higher circulating BCAA levels in the caveolin-1 null mice. Moreover, we confirm that excess dietary BCAAs feed into the lipogenic pathway in mature adipocytes (Frerman et al., 1983
). BCAAs are not only an important metabolic building block and precursor, but have been shown to stimulate anabolic pathways through the mTOR-pathway. Furthermore, isoleucine prevents accumulation of triglycerides in liver and muscle through a mechanism that involves up-regulation of PPARα and UCPs, resulting in increased fatty acid oxidation (Nishimura et al., 2010
). On the other hand, elevated BCAAs in diet-induced obesity have recently been shown to induce insulin resistance as a consequence of chronic activation of the mTOR pathway (Newgard et al., 2009
). All these effects assigned to BCAAs fit very well with the phenotype of the caveolin-1 null mouse. The muscles of the caveolin-1 null mouse display enhanced mitochondrial proliferation (Schubert et al., 2007
) and elevated levels of phospo-mTOR, and the brown adipose depot has elevated levels of UCP1 (Mattsson et al., 2010
). However, we do not exclude that caveolin-1 may also play a direct role for insulin resistance and mitochondrial proliferation in the skeletal muscle in caveolin-1 null mice as also suggested by Oh, Schubert and colleagues (Oh et al., 2008
; Schubert et al., 2007
). The main findings of our study are summarized in .
What is the relevance of the caveolin-1 null mouse for human biology and disease? There are a few reports on caveolin-1 deficiency, leading to lipodystrophy, but this condition is very rare (Garg and Agarwal, 2008
). However, a relative decrease in caveolin-1 and “caveolar dysfunction” has been suggested to play a role in the metabolic syndrome (Fernandez-Real et al., 2010
; Venugopal et al., 2004
). Obese subjects have also been shown to display elevated levels of BCAAs (Newgard et al., 2009
). Moreover, several studies suggest that mitochondrial dysfunction in adipose tissue contributes to metabolic disturbances associated with obesity (De Pauw et al., 2009
; Maassen, 2006
). It may be that enhanced glucose production in insulin resistant states is further aggravated by factors and metabolic alterations originating from suboptimal mitochondrial function in adipose tissue. We do not yet fully understand the underlying mechanism for the lower adiponectin levels in the caveolin-1 null mice, but the altered mitochondrial function (leading to an altered energetic state of the adipocyte) and the increased levels of reactive oxygen species are likely contributing factors (Koh et al., 2007
; Sun et al., 2009
). In this study, we discovered a striking positive correlation in between pyruvate and adiponectin, which may be related to the antioxidant capacity of pyruvate. Further detailed studies are required to fully understand the regulation of adiponectin levels as well as the specific role of mitochondrial alterations in adipose tissue for systemic metabolic disease.
Many tumor cells lose caveolin-1 in the early phase of transformation, and caveolin-1 is well-known for its tumor suppressor activity through inhibition of several anabolic pathways (Williams and Lisanti, 2005
). However, a high rate of proliferation requires enhanced aerobic glycolysis as opposed to oxidative phosporylation. Thus, altered mitochondrial function associated with loss of caveolin-1 may contribute to an increase in glycolysis and thereby enable proliferation. The overall metabolic environment of the full body caveolin-1 null knockout may therefore predispose cells to transformation. In fact, Lisanti and colleagues have proposed a model where loss of caveolin-1 in tumor-associated fibroblasts increases oxidative stress and causes increased tumor growth (Trimmer et al., 2011
). We support that hypothesis and the full caveolin-1 null mouse indeed displays elevated circulating H2
levels. Moreover, tumor growth depends on high influx of nutrients such as glucose and BCAAs and is inhibited by pyruvate. This demand is met by an upregulation of various nutrient transporters, and a selective decrease in the Sodium-Coupled Monocarboxylate Transporter 1 (SMCT1) that primarily transports butyrate and pyruvate, which are inhibitors of histone deacetylases and thereby can induce tumor cell apoptosis (Ganapathy et al., 2009
). Thus, the increased levels of BCAAs and glucose, and the reduced levels of pyruvate may also promote tumor progression in the caveolin-1 null mouse.
In summary, caveolin-1 null mice are lean and muscular, but show a decreased metabolic flexibility and an increased predisposition for malignant transformation, ultimately leading to a shorter life span (Park et al., 2003
). Our data suggest that the complex phenotype of the caveolin-1 null mouse to a large extent depends on metabolic and mitochondrial alterations at the level of the adipocyte. The metabolic adaptations that occur in the caveolin-1 null mice may serve as a model for the effects of changes in mitochondrial function observed in obesity as well as in cancer metabolism.