Hypoxia-Inducible Lipid Droplet–Associated Is Not a Direct Physiological Regulator of Lipolysis in Adipose Tissue

Abstract

Triglycerides are stored in specialized organelles called lipid droplets. Numerous proteins have been shown to be physically associated with lipid droplets and govern their function. Previously, the protein hypoxia-inducible lipid droplet-associated (HILPDA) was localized to lipid droplets and was suggested to inhibit triglyceride lipolysis in hepatocytes. We confirm the partial localization of HILPDA to lipid droplets and show that HILPDA is highly abundant in adipose tissue, where its expression is controlled by the peroxisome proliferator-activated receptor γ and by β-adrenergic stimulation. Levels of HILPDA markedly increased during 3T3-L1 adipocyte differentiation. Nevertheless, silencing of Hilpda using small interfering RNA or overexpression of Hilpda using adenovirus did not show a clear impact on 3T3-L1 adipogenesis. Following β-adrenergic stimulation, the silencing of Hilpda in adipocytes did not significantly alter the release of nonesterified fatty acids (NEFA) and glycerol. By contrast, adenoviral-mediated overexpression of Hilpda modestly attenuated the release of NEFA from adipocytes following β-adrenergic stimulation. In mice, adipocyte-specific inactivation of Hilpda had no effect on plasma levels of NEFA and glycerol after fasting, cold exposure, or pharmacological β-adrenergic stimulation. In addition, other relevant metabolic parameters were unchanged by adipocyte-specific inactivation of Hilpda. Taken together, we find that HILPDA is highly abundant in adipose tissue, where its levels are induced by peroxisome proliferator-activated receptor γ and β-adrenergic stimulation. In contrast to the reported inhibition of lipolysis by HILPDA in hepatocytes, our data do not support an important direct role of HILPDA in the regulation of lipolysis in adipocytes in vivo and in vitro.

Stress-responsive HILPDA is necessary for thermoregulation during fasting

“…These may, for example, include compensations such as change in body posture and blood flow or changes in skeletal muscle thermogenesis through uncoupling of the SERCA pump by Sarcolipin (Rowland, et al 2015). It is also possible that, in our genetic background, BAT function below thermoneutrality is altered when Hilpda is absent, similar to what has been seen in the adipose tissue-specific KO model (Dijk et al 2017; DiStefano et al 2016) In agreement with recently published reports, we did not find a cell-autonomous defect of HILPDA KO adipocytes in their differentiation capacity or lipolysis (Dijk et al 2017; DiStefano et al 2016). Hilpda was, however, required for body temperature defense after fasting, and the hypothermia of the KO mice was associated with rebound hyperphagia.…”

“…It is now evident that additional stresses and transcription factors, such as PPARs, cAMP and nutrient imbalance induce Hilpda expression (this work) and (Dijk et al 2017; DiStefano et al 2015; DiStefano et al 2016; Gimm et al 2010; Mattijssen et al 2014; Wang, et al 2010). The mechanistic details of Hilpda-dependent lipid accumulation are not known yet and require additional biochemical investigation.…”

“…In support of this model, Hilpda has been reported to negatively regulate lipolysis and triglyceride secretion in the liver, but not in adipose tissue (Dijk et al 2017; DiStefano et al 2015; DiStefano et al 2016; Mattijssen et al 2014). Cell-context and the particular lipid droplet proteome may also determine distinct ways of lipid ester storage dynamics (Beller, et al 2008; Guo, et al 2008; Khor, et al 2014) and therefore the significance of Hilpda in these processes.…”

“…Previous reports have shown that Hilpda is expressed in human and murine adipocytes and responds to PPAR- and b-adrenergic signaling (Dijk et al 2017; DiStefano et al 2016). To determine the regulation of Hilpda expression and its impact on adipocyte differentiation in our genetically engineered model we isolated the stromal vascular fraction (SVF) from interscapular BAT and differentiated it in vitro (Fig.…”

“…Genetic manipulation in murine liver has implicated HILPDA in triglyceride accumulation by mechanisms involving PPARα-dependent Hilpda induction, triglyceride secretion and regulation of lipolysis, although direct inhibition of lipase activity was not found (DiStefano, et al 2015; Mattijssen, et al 2014). Additionally, deletion of Hilpda in WAT and/or BAT did not impact lipolysis, despite its robust induction during adipocyte differentiation in vitro and nutritional manipulations in vivo (Dijk, et al 2017; DiStefano, et al 2016). …”