Towards a better understanding of the molecular mechanisms regulating liver metabolism in response to fasting

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The liver is a central organ that regulates systemic metabolism in animals. In response to fasting, hepatocytes turn on several catabolic processes to support the function of the brain and peripheral tissues. Although the comprehension of the molecular mechanisms regulating the metabolic adaptation of the liver to fasting has greatly improved, how exactly hepatocytes trigger these catabolic processes to match the nutritional state is still incompletely characterized. We hypothesize that key players regulating the response to fasting still remain to be identified. In order to identify these new regulators, we took advantage of a simple approach that we successfully used in the past to identify proteins controlling fat cell development. Briefly, we used a hepatocyte cell line (FAO) and prepared plates in which a single cell per well was isolated by serial dilutions. From these plates, we amplified new clonal cultures of FAO cells and measured glucose production of each new line as an indicator of their natural potential to trigger catabolism upon fasting. Interestingly, we observed that some clonal lines produced low amounts of glucose (Low) while others produced a lot (High). In order to identify the genes responsible for this differential response, we performed microarray analyzes to compared Low and High lines. Hydroxysteroid dehydrogenase type-like 2 (Hsdl2), was identified as a gene highly expressed in in catabolic hepatocytes. HSDL2 is a poorly characterized protein. Our preliminary experiments revealed that hepatic Hsdl2 expression is highly induced by fasting in mice. In cultured hepatocytes, withdrawal of the serum, which mimics a fasting state, also increased the expression of Hsdl2. Confirming these findings, we observed that insulin, which is a major player in regulating the fasting response, strongly repressed the expression of Hsdl2 in FAO cells. Together, these results indicate that HSDL2 is activated by fasting and may thus play a role in this process. Building upon my previous work and expertise on the liver, metabolism, cell signaling and cell biology, this research program aims to improve the understanding of the molecular mechanisms controlling the hepatic response to fasting. The objectives of this proposal for the next 5 years are to: 1) Define the localization and the metabolic functions of HSDL2. 2) Define the post-translational modifications controlling the activity of HSDL2. 3) Define the metabolic impacts of hepatic HSDL2 loss/overexpression in vivo. 4) Identify additional proteins regulating liver metabolism in response to fasting. Studying the molecular mechanisms controlling liver metabolism in response to fasting will improve our understanding of the biological processes triggered to ensure survival when nutrients are scarce. In addition to improving the basic knowledge of liver physiology, this project could potentially reveal general pathways controlling metabolism in mammalian cells.