Excessive lipid accumulation in the liver (hepatic steatosis) is a hallmark of nonalcoholic fatty liver disease (NAFLD), which contributes to insulin resistance, type 2 diabetes, and cardiovascular disease. Obesity and aging, demographic characteristics of widespread national prevalence especially in the VA patient population, are two independent drivers of NAFLD. However, the pathogenetic mechanisms involved in the development of NAFLD are not well understood and as such there is an absence of proven therapies. Catecholamines acting via beta-adrenergic receptors ( B-ARs) regulate lipid and glucose homeostasis. In studies from our laboratory and others, obesity and aging have been demonstrated to increase B-AR membrane content and associated adenylyl cyclase responsiveness in liver. In preliminary studies we demonstrated that activation of B-AR signaling, mainly via B2-AR subtype, increases hepatic lipid accumulation in livers of young rodents, whereas inhibition of B-AR signaling in hepatocytes from old rodents reduces cellular lipid accumulation. In comparative studies of wild type (wt) and whole-body B2-AR knockout (ko) mice, we observed that old ko mice exhibited lower levels of liver steatosis and hepatic lipin-1 mRNA levels than the wt animals. Lipin-1 is a key regulator of hepatic lipid metabolism with distinct functions depending on its subcellular localization;in the endoplasmic reticulum (ER) it functions as a triglyceride (TG) synthetic enzyme, whereas in the nucleus it acts as a transcriptional coactivator that enhances fatty acid oxidation. We also observed that in vivo pharmacological activation of B-ARs increases hepatic TG and lipin-1 protein levels while reducing expression of factors (apoB, microsomal transfer protein) involved in secretion of very low density lipoprotein (VLDL)-TGs from the liver. Increased B-AR signaling was also found to reduce expression of PPAR?, a transcription factor that promotes fatty acid oxidation by mechanisms which may involve interactions with lipin-1 in the nucleus. Based on these findings, we hypothesize that enhanced hepatic B2-AR activation is a major contributor to increased fat accumulation in liver. The overall goal of this study is to elucidate the mechanisms by which hepatic B2-AR signaling promotes lipid accumulation in liver. To test this hypothesis, we propose the following Specific Aims, which will be pursued using the B2-AR ko mouse model and in vivo and in vitro techniques:
Aim 1) : Determine whether increased hepatic B2-AR signaling augments TG synthesis by inducing lipin-1 transcription and translocation to the endoplasmic reticulum (ER). We hypothesize that in vivo and in vitro overexpression of B2-ARs will increase lipin-1 expression and translocation to the ER, leading to enhanced TG synthesis. We also hypothesize that the increase in lipin-1 transcription and its translocation to the ER will be prevented by inhibiting hepatic B2-AR signaling.
Aim 2) Investigate whether increased hepatic B2-AR signaling decreases B-oxidation of fatty acids by inhibiting interaction of lipin-1 with PPAR? in the nucleus. We hypothesize that in vivo and in vitro overexpression of B2-ARs will reduce fatty acid B-oxidation by suppressing the interaction of lipin-1 with nuclear PPARa We also hypothesize that the translocation of lipin-1 out of the nucleus and suppression of fatty acid B-oxidation will be prevented by B2-AR ablation.
Aim 3) Determine whether B2-AR signaling reduces VLDL assembly and secretion in liver. We hypothesize that in vivo and in vitro overexpression of B2-ARs will reduce assembly and secretion of VLDL-TGs in liver by decreasing factors involved in VLDL secretion such as apoB and microsomal triglycleride transfer protein. We also hypothesize that inhibition of hepatic VLDL secretion will be prevented by B2-AR ablation. With obesity and aging on the rise, especially in the VA patient population, it is vitally important to study the factors responsible fr the development of NAFLD. This study is innovative as the role of B2-AR signaling in development of hepatic steatosis has not yet been investigated. The study is expected to have an important impact by defining new molecular targets for the prevention and/or treatment of NAFLD and its complications.
Obesity and aging, demographic characteristics of widespread national prevalence especially in the VA patient population, are two independent drivers of non alcoholic fatty liver disease (NAFLD). It is estimated that one-third of adults are diagnosed with NAFLD which is strongly associated with insulin resistance and cardiovascular disease. A hallmark of NAFLD is excessive lipid accumulation in the liver (hepatic steatosis). With an increasing population of combat and elderly veterans at risk for developing NAFLD, it is vitally important to understand the factors responsible for increased hepatic fat accumulation and the development of NAFLD. The overall goal of this study is to elucidate novel mechanisms by which hepatic beta 2-adrenergic receptor signaling promotes excessive lipid accumulation in liver. The results of this study are expected to have an important impact by defining new molecular targets for the prevention and/or treatment of NAFLD and its complications.