Adipose tissue harbors a major pool of free cholesterol18
but its role in regulating circulating HDL-C is poorly understood. In this work, we present the first evidence that adipocytes transfer cholesterol to HDL in vivo
as well as in vitro
. We identified a differentiation-dependent role for ABCA1 and SR-BI, but not ABCG1, in adipocyte cholesterol efflux to apoA-I and mature HDL respectively and provide experimental evidence that both ABCA1 and SR-BI can regulate adipocyte cholesterol transfer to HDL in vivo
. Finally, we show that adipocyte inflammation down-regulates transporters and impairs adipocyte cholesterol efflux to HDL. As adipose inflammation is a hallmark of central obesity and type-2 diabetes, loss of adipocyte-lipidation of HDL may directly contribute to lower HDL-C in these adipose inflammatory states.
Lipidation of HDL particles in vivo
involves the coordinated effect of several tissues6, 7
likely involving cell-specific transporter functions. ABCA1 plays a major role in generation of nascent HDL particles5, 36
and maintenance of plasma HDL-C through integrated hepatic and peripheral tissue actions. Dramatic reductions in HDL-C levels are observed in the absence of ABCA16, 7
primarily because of loss of hepatic lipidation of liver-secreted apoA-I. However, peripheral ABCA1 also contributes to HDL-C6
ABCA1. Although macrophage cholesterol efflux to HDL plays a major role in attenuating atherosclerosis, macrophages contain a very small pool of cholesterol and do not regulate circulating HDL-C in vivo4
. Because adipose tissue contains a large pool of free cholesterol15, 16
, we hypothesized that adipocytes may play a unique role in cholesterol transfer to HDL both in vitro
and in vivo
. In fact, a role for adipose or involvement of non-ABCA1 transporters in peripheral lipidation of HDL has not been demonstrated.
Our findings support a model of adipocyte-specific regulation of cholesterol efflux to HDL acceptors. We identified a role for ABCA1 and SR-BI transporters in efflux to apoA-I and HDL respectively and demonstrated marked up-regulation of these proteins during adipocyte differentiation. Although ABCG1 promotes macrophage cholesterol efflux to mature HDL8, 9, 38
we found no evidence that ABCG1 protein is expressed in mature adipocytes or plays a role in adipocyte cholesterol efflux to HDL.
Using a modified version of our published macrophage-to-HDL reverse cholesterol transport model12, 26
, we demonstrate that adipocytes are capable of transferring cholesterol to circulating HDL. Indeed, the timecourse and extent of cholesterol label movement onto HDL was similar to macrophages26, 39, 40
. Further, cholesterol movement from IP-injected adipocytes to HDL was increased in apoA-I transgenic and reduced in apoA-I null mice. Adipocyte deficiency of ABCA1 or SR-BI reduced tracer movement onto HDL in vivo
. Overall, our data provide indirect evidence for adipocyte regulation of HDL-C in vivo
and suggest a role for SR-BI and ABCA1, but not ABCG1, in this process.
Recent studies suggest an underappreciated role for adipocyte cholesterol in adipose function and pathophysiologies19, 41, 42,43, 44
. Zhao et al21
showed that primary adipocytes, isolated from rabbits fed a high-cholesterol diet or treated with statins, had altered cholesterol efflux to HDL that correlated with changes in SR-BI expression. They did not prove, however, that SR-BI was causal. Verghese and colleagues42
demonstrated that enhanced adipocyte cholesterol efflux to HDL occurs during lipolysis without change in SR-BI and ABCA1 expression. It is possible, however, that modulation of transporter function45–47
or membrane localization40
rather than change in protein level could mediate this efflux. Our studies provide novel data that go beyond prior correlative studies. We addressed directly the role of specific transporters and performed in vivo
studies examining the potential for adipocytes and specific transporters to transfer cholesterol to HDL in vivo
. Future work with adipose-specific, conditional modulation of ABCA1 and SR-BI in rodent models is required to confirm the importance of these transporters and adipose regulation of HDL cholesterol mass in vivo
Our in vivo
experimental model does have limitations including its non physiological nature, use of exogenous cells and reliance on cholesterol tracer rather than mass. The peritoneal space is a convenient experimental location in which cells are exposed to extracellular fluid that has many of the characteristics of extracellular fluid in other tissues. Importantly, this model has provided fundamental insights into the macrophage reverse cholesterol transport process12, 26
. Work by Sehayek and colleagues48
demonstrates that the subcutaneous administration of macrophages provides a similar pattern of reverse cholesterol transport as the peritoneal cavity arguing against any unique properties for the peritoneum. Although adipocytes do not occur as single cells in the peritoneum, IP-injection of labeled adipocytes resulted in cholesterol-label movement to plasma HDL that was remarkably similar to that published for macrophages. Therefore, we doubt a systematic difference between adipocyes and macrophages in the intraperitoneal model.
We examined the impact of an inflammatory adipocytokine on adipocyte cholesterol efflux in order to explore if loss of adipocyte HDL-lipidation is one possible mechanism for reduced HDL-C in adipose-inflammatory settings33, 49
. Because adipocyte susceptibility to inflammation34, 35
depends on adipocyte maturity, we examined TNFα effects during differentiation. TNFα impaired cholesterol efflux most in partially-differentiated adipocytes coincident with greatest suppression of ABCA1 and SR-BI. This is consistent with work by Chung et al.
who reported that endotoxin impaired glucose transport maximally in partially-differentiated adipocytes34
. We also found that endotoxemia down-regulated adipose SR-BI and ABCA1 in vivo
. Thus, despite increased adipose mass and adipose cholesterol in obesity, attenuation of adipocyte-mediated HDL-lipidation may directly contribute to lower HDL-C in metabolic syndrome and type-2 diabetes ().
Figure 6 As adipose inflammation is a hallmark of central obesity and type-2 diabetes, loss of adipocyte lipidation of HDL may directly contribute to lower HDL-C levels in these inflammatory, insulin resistant states. Despite greater adipose mass and cholesterol (more ...)
In conclusion, adipocytes support transfer of cholesterol to HDL in vivo. This process is mediated by ABCA1 and SR-BI, but not ABCG1, and is attenuated in inflamed adipocytes. Our findings suggest adipocyte-specific cholesterol transporter functions and a role for mature adipose in maintenance of HDL-C levels. Conversely, adipose inflammation may attenuate adipocyte lipidation of HDL leading to lower HDL-C in metabolic syndrome and type-2 diabetes.
Adipose tissue harbors a major pool of free cholesterol but its role in regulating circulating HDL-C is poorly understood. In this work, we present the first evidence that adipocytes transfer cholesterol to HDL in vivo as well as in vitro. We identified a differentiation-dependent role for the lipid-transporters ABCA1 and SR-BI, but not ABCG1, in adipocyte cholesterol efflux to apoA-I and mature HDL respectively. We also provide experimental evidence that both ABCA1 and SR-BI can regulate adipocyte cholesterol transfer to HDL in vivo. Finally, we show that adipocyte inflammation down-regulates transporters and impairs adipocyte cholesterol efflux to HDL. Our findings suggest a role for mature adipose in directly maintaining HDL-C levels. Conversely, adipose inflammation may attenuate adipocyte lipidation of HDL and may directly contribute to lower HDL-C in adipose inflammatory states such as central obesity and type-2 diabetes. Thus adipose tissue cholesterol homeostasis may be a direct therapeutic target for modulation of HDL levels in vivo.