Abstract
The Liver X receptor (LXR) is an important regulator of carbohydrate and lipid metabolism in humans and mice. We have recently shown that activation of LXR regulates cellular fuel utilization in adipocytes. In contrast, the role of LXR in human adipocyte lipolysis, the major function of human white fat cells, is not clear. In the present study, we stimulated in vitro differentiated human and murine adipocytes with the LXR agonist GW3965 and observed an increase in basal lipolysis. Microarray analysis of human adipocyte mRNA following LXR activation revealed an altered gene expression of several lipolysis-regulating proteins, which was also confirmed by quantitative real-time PCR. We show that expression and intracellular localization of perilipin1 (PLIN1) and hormone-sensitive lipase (HSL) are affected by GW3965. Although LXR activation does not influence phosphorylation status of HSL, HSL activity is required for the lipolytic effect of GW3965. This effect is abolished by PLIN1 knockdown. In addition, we demonstrate that upon activation, LXR binds to the proximal regions of the PLIN1 and HSL promoters. By selective knock-down of either LXR isoform, we show that LXRα is the major isoform mediating the lipolysis-related effects of LXR. In conclusion, the present study demonstrates that activation of LXRα up-regulates basal human adipocyte lipolysis. This is at least partially mediated through LXR binding to the PLIN1 promoter and down-regulation of PLIN1 expression.
Highlights
Besides lipases, the lipid droplet-coating proteins are known important regulators of lipolysis [3]
Our present study demonstrates for the first time that agonist activation of liver X receptor (LXR) increases basal lipolysis and affects the expression and function of several lipolysis-regulating proteins in human adipocytes
We show that PLIN1 is an LXR target gene, and we conclude that LXR is a regulator of human adipocyte lipolysis
Summary
Subjects and Cell Culture—Human preadipocytes were differentiated as previously described [23, 24]. 3T3-L1 cells were cultured and differentiated to adipocytes as previously described [26]. Two different total HSL antibodies were used with similar results: 4107 from Cell Signaling and a kind gift from Prof. Immunostaining—To examine the localization of PLIN1 and HSL in the same cell we performed a sequential double immunocytochemical staining, as previously described [6]. RNAi—In vitro-differentiated preadipocytes were treated with targeting siRNA or non-targeting control siRNA as previously described [20], medium was collected, and cells were lysed for RNA or protein. Chromatin Immunoprecipitation (ChIP)—In vitro differentiated adipocytes were treated with GW3965 for 6 h, and ChIP assay was performed as previously described [33].
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