Hormone-sensitive lipase, the rate-limiting enzyme of intracellular TG hydrolysis, is a major determinant of fatty acid mobilization in adipose tissue as well as other tissues. It plays a pivotal role in lipid metabolism, overall energy homeostasis, and, presumably, cellular events involving fatty acid signaling. Detailed knowledge about its structure and regulation may provide information regarding the pathogenesis of such human diseases as obesity and diabetes and may generate concepts for new treatments of these diseases. The current review summarizes the recent advances with regard to hormone-sensitive lipase structure and molecular mechanisms involved in regulating its activity and lipolysis in general. A summary of the current knowledge regarding regulation of expression, potential involvement in lipid disorders, and role in tissues other than adipose tissue is also provided.

Estrogens, and in particular 17β-estradiol (E2), play a pivotal role in sexual development and reproduction and are also implicated in a large number of physiological processes, including the cardiovascular system. Both acetylcholine-induced and flow-dependent vasodilation are preserved or potentiated by estrogen treatment in both animal models and humans. Indeed, E2 increases the endothelial production of nitric oxide and prostacyclin and prevents early atheroma through endothelial-mediated mechanisms. Furthermore, whereas it prevents endothelial activation, E2 potentiates the ability of several subpopulations of the circulating or resident immune cells to produce proinflammatory cytokines. The balance between these 2 actions could determine the final effect in a given pathophysiological process. E2 also promotes endothelial healing, as well as angiogenesis. Estrogen actions are essentially mediated by 2 molecular targets: estrogen receptor-α (ERα) and ERβ. The analysis of mouse models targeted for ERα or ERβ demonstrated a prominent role of ERα in vascular biology. ERα directly modulates transcription of target genes through 2 activation functions (AFs), AF-1 and AF-2. Interestingly, an AF-1-deficient ERα isoform can be physiologically expressed in the endothelium and appears sufficient to mediate most of the vasculoprotective actions of E2. In contrast, AF-1 is necessary for the E2 actions in reproductive targets. Thus, it appears conceivable to uncouple the vasculoprotective and sexual actions with appropriate selective ER modulators.

Short-term exposure to 17β-estradiol (E2) in vitro has been reported to decrease the production of proinflammatory cytokines by LPS-activated macrophages through estrogen receptor α (ERα)-dependent activation of the PI3K pathway. In the present study, we confirm that in vitro exposure of mouse peritoneal macrophages to E2 enhanced Akt phosphorylation and slightly decreased LPS-induced cytokine production. In striking contrast, we show that chronic administration of E2 to ovariectomized mice markedly increases the expression of IL-1β, IL-6, IL-12p40, and inducible NO synthase by resident peritoneal macrophages in response to LPS ex vivo. These results clearly indicate that short-term E2 treatment in vitro does not predict the long-term effect of estrogens in vivo on peritoneal macrophage functions. We show that this in vivo proinflammatory effect of E2 was mediated through ERα. Although the expression of components of the LPS-recognition complex remained unchanged, we provided evidences for alterations of the TLR4 signaling pathway in macrophages from E2-treated mice. Indeed, E2 treatment resulted in the inhibition of PI3K activity and Akt phosphorylation in LPS-activated macrophages, whereas NF-κB p65 transcriptional activity was concomitantly increased. Incubation of macrophages with the PI3K inhibitor wortmanin enhanced proinflammatory cytokine gene expression in response to TLR4 activation, and abolishes the difference between cells from placebo- or E2-treated mice, demonstrating the pivotal role of the PI3K/Akt pathway. We conclude that the macrophage activation status is enhanced in vivo by E2 through ERα and, at least in part, by the down-modulation of the PI3K/Akt pathway, thereby alleviating this negative regulator of TLR4-signaling.

Mechanisms accounting for gender dimorphism during immune responses are still poorly understood. Since invariant natural killer T (iNKT) cells exert important regulatory functions through their capacity to produce both T helper 1 (Th1) and Th2 cytokines, we addressed the question of whether these activities could be modulated by sexual hormones. We found that in vivo challenge with the specific ligand of iNKT cells, α-galactosylceramide (α-GalCer), induced significantly higher concentrations of interferon γ (IFN-γ) in the serum of female than in that of male mice, while interleukin 4 (IL-4) production was not modified. In support of a crucial role of ovarian hormones in this phenomenon, a significant decrease of serum IFN-γ concentrations occurred in ovariectomized females, in response to treatment with α-GalCer, while orchidectomy affected neither IFN-γ nor IL-4 serum concentrations in males. The implication of estrogens in this selective enhancement of IFN-γ production by iNKT cells was demonstrated by (1) the increased α-GalCer–induced IFN-γ synthesis by iNKT cells upon both in vitro and in vivo exposure to estradiol and (2) the abolition of the sex-linked difference in α-GalCer–induced IFN-γ release in estrogen receptor α-deficient mice. These results provide the first evidence that estrogens influence iNKT cells leading to this gender dimorphism in their cytokine production profile.

The human hormone-sensitive lipase (HSL) gene encodes a 786-aa polypeptide (85.5 kDa). It is composed of nine exons spanning approximately 11 kb, with exons 2-5 clustered in a 1.1-kb region. The putative catalytic site (Ser423) and a possible lipid-binding region in the C-terminal part are encoded by exons 6 and 9, respectively. Exon 8 encodes the phosphorylation site (Ser551) that controls cAMP-mediated activity and a second site (Ser553) that is phosphorylated by 5'-AMP-activated protein kinase. Human HSL showed 83% identity with the rat enzyme and contained a 12-aa deletion immediately upstream of the phosphorylation sites with an unknown effect on the activity control. Besides the catalytic site motif (Gly-Xaa-Ser-Xaa-Gly) found in most lipases, HSL shows no homology with other known lipases or proteins, except for a recently reported unexpected homology between the region surrounding its catalytic site and that of the lipase 2 of Moraxella TA144, an antarctic psychrotrophic bacterium. The gene of lipase 2, which catalyses lipolysis below 4 degrees C, was absent in the genomic DNA of five other Moraxella strains living at 37 degrees C. The lipase 2-like sequence in HSL may reflect an evolutionarily conserved cold adaptability that might be of critical survival value when low-temperature-mobilized endogenous lipids are the primary energy source (e.g., in poikilotherms or hibernators). The finding that HSL at 10 degrees C retained 3- to 5-fold more of its 37 degrees C catalytic activity than lipoprotein lipase or carboxyl ester lipase is consistent with this hypothesis.

Full-length 66-kDa estrogen receptor α (ERα) stimulates target gene transcription through two activation functions (AFs), AF-1 in the N-terminal domain and AF-2 in the ligand binding domain. Another physiologically expressed 46-kDa ERα isoform lacks the N-terminal A/B domains and is consequently devoid of AF-1. Previous studies in cultured endothelial cells showed that the N-terminal A/B domain might not be required for estradiol (E2)-elicited NO production. To evaluate the involvement of ERα AF-1 in the vasculoprotective actions of E2, we generated a targeted deletion of the ERα A/B domain in the mouse. In these ER α AF-1 ⁰ mice, both basal endothelial NO production and reendothelialization process were increased by E2 administration to a similar extent than in control mice. Furthermore, exogenous E2 similarly decreased fatty streak deposits at the aortic root from both ovariectomized 18-week-old ER α AF-1 ^+/+ LDLr ^−/− (low-density lipoprotein receptor) and ER α AF-1 ⁰ LDLr ^−/− mice fed with a hypercholesterolemic diet. In addition, quantification of lesion size on en face preparations of the aortic tree of 8-month-old ovariectomized or intact female mice revealed that ERα AF-1 is dispensable for the atheroprotective action of endogenous estrogens. We conclude that ERα AF-1 is not required for three major vasculoprotective actions of E2, whereas it is necessary for the effects of E2 on its reproductive targets. Thus, selective ER modulators stimulating ERα with minimal activation of ERα AF-1 could retain beneficial vascular actions, while minimizing the sexual effects.