Interaction of the Adipocyte Fatty Acid-binding Protein with the Hormone-sensitive Lipase

Abstract

. interacts with the activated, phosphorylated HSL and that the association is likely to be regulatory; either delivering FA to inhibit HSL (facilitating feedback inhibition) or affecting multicomponent complex formation on the droplet surface. Adipocyte fatty acid-binding protein (AFABP)2 , also known as aP2, is the predominant member of the multigene family of intracellular lipid-binding proteins found in adipose tissue (1). The fatty acid-binding proteins (FABPs) are low molecular weight (15 kDa) abundant, ubiquitously expressed proteins that bind and sequester hydrophobic ligands such as free fatty acids and/or closely related compounds facilitating their solubilization and trafficking between various aqueous compartments of the cell. Whereas the members of the multigene family share only 20 -70% sequence identity, they fold into a structurally conserved ␤-barrel that produces an internal water-filled cavity that functions as the fatty acid binding site (1).Lipolysis in adipocytes (defined as the regulated release of fatty acids from the cell) is facilitated via a series of regulatory phosphorylation events linking receptor-mediated increases in cAMP to the phosphorylation of several key proteins including perilipin A and the hormone-sensitive lipase (HSL) resulting in an increase in triacylglycerol, diacylglycerol, and monoacylglycerol hydrolysis and efflux of fatty acid from the adipocyte (2). Perilipin A phosphorylation results in a dynamic restructuring of the lipid droplet surface, potentially providing access to the lipid droplets for lipases (3,4). Recent structure-function studies have indicated that diacylglycerol is the predominant glyceride substrate for HSL in vivo and is provided by the action of the upstream adipose triglyceride lipase (5). HSL function is under multidimensional regulation by hormones and catecholamines, and several investigators have shown that protein kinase A-dependent phosphorylation of Ser 659 and Ser 660 is required for both the translocation of HSL to the lipid droplet and increased hydrolytic activity (6,7,8). In addition, AMPKdependent phosphorylation of HSL (Ser 565 ) also results in activation of the lipase and is necessary for translocation (7,9). Concomitant with HSL phosphorylation, the phosphorylation of perilipin facilitates the translocation of HSL from the cytosol to the lipid droplet and access to the substrate (10). As such, lipolysis is a multicomponent highly regulated process linking a number of hydrolytic events leading toward fatty acid release from the adipocyte.AFABP/aP2-null mice exhibit decreased basal and hormone-stimulated lipolysis both in situ and in vivo, but demonstrate rates of fatty acid influx identical to those of wildtype mice, suggesting that the protein facilitates diffusion of fatty acids from the site of lipid hydrolysis (droplet surface) to the plasma membrane as part of lipid efflux from the cell (11). Importantly, AFABP/aP2-null mice accumulate intracellular FFA, suggesting that lipid release (lipolysis) from adipocytes is attenuat...

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

“…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]. In accordance with these findings, a small molecular inhibitor blocking NEFA binding to AFABP also antagonises physical interaction with HSL [21].…”

Functional expression of glucose-dependent insulinotropic polypeptide receptors is coupled to differentiation in a human adipocyte model

“…FABP4 is believed to have two functions in lipolysis. 26 The first is to facilitate free fatty acid diffusion from the site of hydrolysis (lipid droplet) to the plasma membrane, as part of lipid efflux from the cell. The second is to attenuate hormone sensitive lipase activity with which it interacts on the surface of the lipid droplet.…”

X‐ray crystallographic analysis of adipocyte fatty acid binding protein (aP2) modified with 4‐hydroxy‐2‐nonenal

“…As stated previously, the affinity of AFABP for fatty acids is decreased upon 4-HNE-modification and it is therefore unlikely that modified AFABP bind additional fatty acids. 9 It remains undetermined if 4-HNE-modified AFABP facilitates heterotypic protein-protein association as does AFABP bound with fatty acid, which interacts with the hormone sensitive lipase (HSL) 25 and Janus kinase 2 (JAK2) 36 and regulates cellular metabolism and signalling.…”

“…The helixturn-helix motif of AFABP and other FABP isoforms are important for fatty acid transfer from FABPs to membranes 24 and four surface-exposed charged residues in this motif represent a protein-protein interaction domain. 25,26 Physiological ligands for AFABP are long-chain fatty acids and carboxylate derivatives that bind to the protein noncovalently through acyl chain hydrophobic and entropic interactions and salt bridges between the carboxylate and Arg126 and Tyr128. AFABP binds avidly to a variety of long-chain fatty acids (K d % 5-50 nM) with the affinity declining substantially when the fatty acid is less than 12 carbons.…”