Stress-responsive HILPDA is necessary for thermoregulation during fasting

Abstract

Hypoxia-Inducible Lipid-Droplet Associated protein (HILPDA) has been shown to localize to lipid droplets in nutrient responsive cell types such as hepatocytes and adipocytes. However, its role in the control of whole-body homeostasis is not known. We sought to measure cell-intrinsic and systemic stress responses in a mouse strain harboring whole body Hilpda deficiency. We generated a genetically engineered mouse model of whole body HILPDA deficiency by replacing the coding HILPDA exon with Luciferase. We subjected the knockout animals to environmental stresses and measured whole-animal metabolic and behavioral parameters. Brown adipocyte precursors were isolated and differentiated in vitro to quantify the impact of HILPDA ablation in lipid storage and mobilization in these cells. HILPDA knockout animals are viable and fertile, but show reduced ambulatory activity and oxygen consumption at regular housing conditions. Acclimatization at thermoneutral conditions abolished the phenotypic differences observed at 22°C. When fasted, HILPDA KO mice are unable to maintain body temperature and become hypothermic at 22°C, without apparent abnormalities in blood chemistry parameters or tissue triglyceride content. HILPDA expression was upregulated during adipocyte differentiation and activation in vitro, however, it was not required for lipid droplet formation in brown adipocytes. We conclude that HILPDA is necessary for efficient fuel utilization suggesting a homeostatic role for Hilpda in sub-optimal environments.

HILPDA Regulates Lipid Metabolism, Lipid Droplet Abundance, and Response to Microenvironmental Stress in Solid Tumors

“…HILPDA is a much weaker ATGL inhibitor than the structurally related G0S2 protein in enzymatic assays with purified proteins (32); and we and others have shown that it does not directly regulate lipolysis in adipocytes (24,26,29). It is thus probable that specific upstream stimuli are required for HILPDA activation and/or its weak ATGL inhibitory activity becomes necessary with respect to other lipases and their inhibitors.…”

“…These adaptive functions of HILPDA support the growth of model tumors. HILPDA has been implicated in lipid turnover in mice (24)(25)(26)(27)(28)(29), and recently two independent reports provided evidence of physical interaction between HILPDA and the ratelimiting triglyceride hydrolase ATGL (31,32), which appears to be responsible for its protumorigenic activity (31).…”

“…Transformed MEFs from HILPDA WT and KO embryos were established by our group (29). HIF1a WT and KO MEFs have been described previously (33).…”

“…By using whole body genetic ablation of HILPDA in mice, we recently reported that HILPDA participates in thermoregulation under stress in vivo (29). In order to delineate the function of HILPDA at the cellular level, we isolated MEFs from HILPDA WT and KO embryos (hereafter referred to as WTs and KOs), immortalized them with SV40 T-antigen and transformed them with H-ras (29). Based on the reported lipid droplet localization of HILPDA (23), we hypothesized that its loss may affect lipid droplet number and/or size.…”

“…HILPDA (formerly known as HIG2) expression is induced under hypoxic conditions and the protein localizes to lipid droplets (22,23); however, its mechanistic function remains poorly defined. At the organismal level, we and others have shown that mouse HILPDA plays physiologic roles in lipid storage, systemic glucose metabolism, and body temperature defense, and that its protein expression responds to both hypoxia and hypoxiaindependent stimuli, such as PPAR and cAMP signaling (24)(25)(26)(27)(28)(29). HILPDA mRNA is highly overexpressed in lipid-rich tumor types, including colorectal and renal clear cell carcinomas, and positively regulates model tumor growth (23,30,31).…”