Phosphorylation of HSL

Apart from fhree PKA phosphorylation sites described above, HSL is phosphorylated in vivo in hormonally quiescent cells at a site named the basal site [235], corresponding to Ser-565 in rat HSL [232, 234], i.e. two residues carboxy-terminal to Ser-563. Glycogen synfhase kinase-4 [237], Ga /calrrioduliri-dcpendent kinase II, and AMP-activated protein kinase (AMPK) [238] phosphorylate Ser-565 in vitro without any direct effect on enzyme activity (Fig. 11.5). AMPK has been proposed to be the physiologically relevant kinase [238], based on its involvement in ofher aspects of lipid metabolism and on its proposed role as fuel gauge [239]. Furthermore, because phosphorylation by AMPK prevented subsequent phosphorylation of Ser-563, and vice versa, it was proposed fhat phosphorylation of Ser-565 exerts an antilipolytic role [238]. This proposal is supported by experiments showing that preincu- 

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In agreement with the above findings, perilipin A acts cooperatively with HSL in hpolysis when both proteins are introduced into CHO cells. Thus, the abihty of perilipin to regulate hpolysis of TAG in the core of LD appears to be manifested with any hpase that has access to the LD surface. The enhancement of hpolytic activity by perihpin A and the endogenous hpase of CHO cells is clearly time-depen-dent, requiring at least 30 min after stimulation to become manifest In contrast when both HSL and perihpin are introduced into CHO cells, the hpolytic activation occurs with no detectable lag. Phosphorylation of HSL is assumed to foster this immediate reaction, in contrast to the endogenous hpase, which is apparently not a PKA substrate. 

The binding of lipotransin to HSL appears to be controlled by several factors. The interaction was significantly increased after phosphorylation of HSL by PKA [266]. Perhaps Ser-659 and Ser-660 control the binding of HSL to hpotransin, providing... 

Inhibition of the translocation of HSL from the cytosol to LD, for instance, by upregulation of perilipin expression or blockade of PKA-dependent phosphorylation of HSL and perilipin, or enhancement of the HSL-lipotransin interaction during inhibition of the lipotransin ATPase. 

An additional mechanism controlling lipolysis specifically in humans has recently come to light (J. Galitsky, 2000). Atrial natriuretic peptide has a potent stimulatory effect on lipolysis in human adipocytes, where administration of the peptide was associated with increased phosphorylation of HSL and perilipin. This effect was blocked by inhibition of cGMP-dependent protein kinase I. At this point it is unclear whether this kinase phospho-rylates HSL and perilipin directly or if it has an indirect effect mediated through PKA. 

L.Y. Xue, J. He, N.L. Oleinick (1997). Rapid tyrosine phosphorylation of HSl in the response of mouse lymphoma L5178Y-R cells to photodynamic treatment sensitized by the phthalocyanine Pc 4. Photochem. Photobiol., 66, 105-113. 

Fig. 11.6 (Hormonal) regulation of TAG mobilization in adipocytes by translocation of HSL and perilipin as well as the molecular mechanisms involved (phosphorylation by PKA, dephosphorylation by protein phospha-...

Cortisol promotes the liberation of fatty acids from adipose tissue by inducing and maintaining the synthesis of hormone-sensitive lipase (HSL), an effect supported by GH. The actual activity of HSL is controlled by those hormones that trigger its phosphorylation (glucagon, catecholamines) or dephosphorylation (insulin, PGE). 

Neural NE and plasma E are both stimulators of adipocyte lipolysis, an effect that is mediated by )8,- and 63-adrenergic receptors and involves cAMP-mediated phosphorylation of hormone-sensitive lipase (HSL). There are regional differences in 3-adrenergic receptor density (visceral fat has more than subcutaneous fat), which determines that catecholamine is physiologically important. Note that receptors are low-affinity receptors that require high concentrations of catecholamines. They respond better to neural stimulation because of the higher local concentration of NE at the fat tissue site. 

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