Characterization of the Functional Interaction of Adipocyte Lipid-binding Protein with Hormone-sensitive Lipase

Abstract

Hormone-sensitive lipase (HSL)1 is an intracellular neutral lipase that is highly expressed in adipose and steroidogenic tissues (1). The enzyme has broad substrate specificity, displaying hydrolytic activity against triacylglycerol, diacylglycerol, and cholesteryl ester (2). Observations from HSL-null mice have shown that HSL is responsible for ϳ50% of the neutral triglyceride lipase activity and all of the neutral cholesteryl ester hydrolase activity in white adipose tissue (3). Thus, HSL plays an important role in regulating lipolysis and the release of fatty acids from adipose tissue. The sequence of HSL is unrelated to other mammalian lipases, but it shares sequence and structural similarity with several bacterial and fungal lipases (4 -11). This structural similarity is based on the ability to model a large portion of the C-terminal ϳ450 amino acids of HSL as an ␣/␤ hydrolase (7); however, the initial ϳ320 amino acids of the protein share no sequence or structural homology with any known proteins. Within the C-terminal region of the protein lies a 150-amino acid sequence that contains a number of sites phosphorylated in response to lipolytic stimulation (7,12,13). In this regard HSL is unique among lipases for the ability of its activity to be up-regulated by phosphorylation. In addition to phosphorylation, HSL activity appears to be regulated by oligomerization, with the dimeric enzyme exhibiting markedly increased activity (14).Utilizing a yeast two-hybrid screen of a rat adipose tissue library, we previously demonstrated that HSL specifically interacts with adipocyte lipid-binding protein (ALBP or aP2) and identified the N-terminal 300 amino acids of HSL as the region responsible for this interaction (15). ALBP is highly expressed in adipose tissue and is a member of the family of intracellular fatty acid-binding proteins that bind fatty acids, retinoids and other hydrophobic ligands (16). It has been proposed that intracellular fatty acid-binding proteins function to sequester fatty acids, thus serving as an intracellular buffer or participating in facilitating the movement of fatty acids within the cell. In view of our observation that HSL and ALBP interact, we proposed that ALBP might prevent feedback inhibition of HSL by high local concentrations of free fatty acids released at the site of hydrolysis. Consistent with this view, adipocytes from ALBP-null mice exhibit markedly reduced basal and stimulated lipolysis both in situ and in vivo (17,18). In the present studies we have addressed the functional significance of the interaction of HSL with ALBP and provide evidence that the interaction of ALBP with HSL constitutes an additional mechanism whereby the hydrolytic activity of HSL is regulated. Furthermore, we have explored the identification of the sequences in HSL that mediate its interaction with ALBP.

Effects of obesity and weight loss on the expression of proteins involved in fatty acid metabolism in human adipose tissue

“…22 Whether KLBP has a similar role remains to be established. However, due to differing fatty acid binding affinities and surface charge patterns of ALBP and KLBP it is likely that these two FABPs will interact differently with intracellular targets such as HSL.…”

Interaction of Hormone-sensitive Lipase with Steroidogeneic Acute Regulatory Protein

“…Thus, it appears that sites within the first 62-amino acid N terminus and within the region between amino acids 182 and 205/221 are able to mediate the interaction of StAR with HSL. StAR Increases HSL Hydrolytic Activity-The interaction of ALBP with HSL is reported to increase the hydrolytic activity of the enzyme (13). The ability of ALBP to increase substrate hydrolysis by HSL is not due entirely to the binding and sequestration of fatty acids by ALBP but is dependent on the physical interaction of ALBP with HSL, suggesting that the protein-protein interaction causes a conformational change or steric effect on the enzyme.…”

“…The interaction of HSL with adipocyte lipid-binding protein (ALBP) occurs through amino acid residues within the N-terminal domain; the physical interaction of ALBP with HSL increases the hydrolytic activity of HSL and protects HSL from product inhibition by fatty acids (13). In the same way that the interaction of HSL with ALBP might help to facilitate the trafficking of fatty acids in adipose cells, HSL might interact with specific cholesterol carrier proteins in the adrenal and, thus, facilitate intracellular cholesterol trafficking to mitochondria for steroidogenesis.…”

Adipocyte fatty acid binding protein: a novel adipokine involved in the pathogenesis of metabolic and vascular disease?

“…AFABP transfers NEFA to membranes via collisional interaction, and lysine residues in the helical cap domain are vital for this collisional transfer process [18]. Furthermore, AFABP increases the hydrolytic activity of hormone-sensitive lipase (HSL) via a specific protein-protein interaction [19]. Work using mutational analysis coupled with fluorescence resonance energy transfer suggests that interaction between AFABP and HSL depends on AFABP-bound NEFA and HSL phosphorylation [20].…”