Mechanisms and Regulations of Neutral Lipid Metabolism - PROJECT SUMMARY/ABSTRACT The biosynthesis of neutral lipids including triacylglycerols (TG) and sterol esters (SE) is a ubiquitous mechanism for storing metabolic energy and lipid substrates (e.g., fatty acids) in eukaryotes. However, excessive neutral lipid production leads to obesity in humans, which in turn contributes to the pathogenesis of metabolic syndrome and various types of cancer. Given the physiological role of this lipid anabolic pathway in directing metabolic energy and lipid metabolites from utilization (e.g., membrane synthesis) into storage, it must be precisely regulated. Our research program will discover and elucidate the underlying regulation mechanisms at the mechanistic level. To generate neutral lipids, two homologous integral membrane acyltransferases, DGAT1 and ACAT1, catalyze the terminal and rate-limiting steps in TG and SE production, respectively. Interestingly, due to the insoluble nature of TG/SE, both enzymes adopt an unusual multi-pass transmembrane topology to mediate TG/SE biosynthesis in the plane of the membrane at the endoplasmic reticulum (ER). This unique mode of membrane-embedded catalysis is crucial for spatially confining TG/SE formation within a stringent hydrophobic environment, thereby preventing their exposure to the cytoplasmic aqueous phase. To ensure overall cellular lipid homeostasis, our central hypothesis is that the enzymatic activities of DGAT1 and ACAT1 undergo rigorous regulations. Our preliminary data indicate that the intrinsically disordered segment at the cytoplasmic N-terminal region of DGAT1 effectively suppresses its activity in vivo. While this observation suggests an auto-inhibitory regulation, our studies also reveal that DGAT1 undergoes substantial activation through interactions with specific lipid ligands. Based on the available evidence on ACAT1 biochemical features and its high degree of structural homology with DGAT1, we hypothesize that both enzymes undergo significant activation/inhibition regulations through similar mechanisms. However, the molecular basis underpinning these regulations uniquely occurring in the lipid bilayer remain largely unknown. Using a combination of structural biology, lipid biochemistry, and cell biology approaches, we will elucidate how these regulations are implemented at the molecular and cellular level. In addition, DGAT1 and ACAT1 are members of a large enzyme family known as membrane-bound acyltransferase (MBOATs). This class of enzymes have emerged as critical therapeutic targets demonstrated by their promise in treating lipid associated disorders and certain types of cancer using pharmacological inhibitors. However, the mechanisms of MBOAT inhibitors remain poorly defined. Building on our recent elucidation of the DGAT1 inhibition mechanism, our research program will also unravel the molecular principles governing the actions of MBOAT inhibitors. We anticipate that the insights gained from our program will establish a new conceptual framework to greatly advance our understanding of lipid metabolism and potentially inspire novel therapeutic strategies for treating lipid-associated disorders to improve human health.
