The mechanisms by which nutrient and hormonal signals regulate the storage and breakdown triglycerides and glycogen are still not completely understood. We are interested in the molecular mechanisms which control lipid and glycogen levels in muscle, liver and adipose tissue. We are particularly interested in the mechanisms by which insulin and amino acids modulate glycogen and lipid synthesis.
One of the pathways that lie at the intersection between nutrient levels and metabolic responses is the mTORC1 pathway. This protein kinase complex is regulated by a variety of anabolic signals including energy status (via AMPK), amino acids and growth factors such as insulin. mTORC1 can integrate these signals and alters metabolism to either store or use those nutrients. Some examples of mTORC1-dependent changes include:
- Promoting triglyceride and glycogen synthesis.
- mTORC1 can promote lipid synthesis by several mechanisms. These include regulating the transcription factors Lipin and SREBP1c. We have shown that SREBP is important for mTORC1 regulation of glycogen in the liver (see Lu et al.) and are investigating the role of the mTORC1/SREBP1c axis in adipose and muscle tissues as well)
- Altering energy utilization
- Another way by which mTORC1 can alter nutrient homeostasis is to promote nutrient utilization in energy consuming tissues. The up-regulation of metabolism is a normal response to energy excess, a process known as diet-induced thermogenesis. Several recent papers have shown that mTORC1 in muscle can increase energy expenditure, and we are investigating the mechanisms by which this happens as a way to promote negative energy balance.
- Generate new muscle and adipose tissue
- mTORC1 also plays an essential role in the formation of new muscle tissue (myogenesis) and adipocytes (adipogenesis). A recent paper from our group showed that this mechanism is conserved even in fruit flies (see Hatfield et al.)
To study this we are using a variety of mouse and cell culture models where we can test the effects of manipulating nutrient sensing pathways to determine the effects of these on glycogen and triglyceride storage.
Who is Working on This?
What sources of funding support this project?
Obesity is a public health epidemic wordwide and affects nearly a third of adult Americans. Several devastating co-morbidities are associated with obesity, including insulin resistance/type II diabetes and non-alcoholic steatohepatitis/non-alcoholic fatty liver disease. Paradoxically, in obese states, lipid storage is not suppressed, in spite of resistance to insulin action. This finding that has important consequences for the management of obesity and its complications. An emerging molecular mechanism linking obesity to excessive lipid storage is the mTORC1/SREBP1c pathway, which our group and others have identified as both activated with obesity, and as a key regulator of lipogenesis and new adipocyte formation. This study will test the hypothesis that mTORC1 activation in the obese state elevates lipid levels, due to activation of both adipogenesis and lipogenesis. To accomplish this, we have developed several new innovative models to test specific aspects of this hypothesis. First, we have generated adipose-specificTsc1 knockout mice as a model of chronic adipose mTORC1 activation. The chronic elevations in mTORC1 signaling in these mice are associated with elevated fat mass and increased hepatic steatosis, likely due to enhanced de novo lipogenesis in adipose tissue. We will determine the molecular changes resulting from chronic mTORC1 elevation, and identify the molecular mechanisms underlying these mTORC1-dependent increases in lipid storage. Elevated adiposity may is caused by increased adipogenesis, so we will determine the molecular mechanisms by which mTORC1 positively regulates adipogenesis. We will specifically evaluate the hypothesis that mTORC1 regulates PPARγ mRNA stability via a miRNA-dependent mechanism. To test the role of mTORC1 in the liver, we will study both activation and inhibition this kinase via ablation of the essential mTORC1 component Rptor, and Tsc1 respectively in adult mouse livers. This approach will allow us to evaluate whether mTORC1 is necessary and sufficient for the development and maintenance of hepatic steatosis in adult liver tissues for the first time. This is an important gap in our knowledge, since in obesity-associated liver disease, mTORC1 is not activated during development, but co-incident with elevations in adiposity. We will explore the physiological significance of a positive feedback loop in SREBP1c using genome-edited rats. This key mTORC1 target plays an important role in de novo lipogenesis and the amplification of SREBP1c action by a transcriptional feed-forward circuit has been proposed to be an important component of both diet-induced hepatic steatosis and obesity. By deleting only the relevant SRE at the endogenous Srebf1 locus, we can test the importance of this circuit in a controlled and direct manner. Importantly, these rats will also allow us to separate the direct activation of SREBP1c by mTORC1 and other signals, from the confounding effects of positive feedback. Together these studies will answer fundamental mechanistic questions regarding how mTORC1 and SREBP1c regulate adipogenesis and lipogenesis, providing insights into potential routes of therapeutic intervention for obesity and liver disease.