Nutritional and hormonal control of lipolysis in isolated gilthead seabream (Sparus aurata) adipocytes

Abstract

-We examined the effects of diet composition and fasting on lipolysis of freshly isolated adipocytes from gilthead seabream (Sparus aurata). We also analyzed the effects of insulin, glucagon, and growth hormone (GH) in adipocytes isolated from fish fed with different diets. Basal lipolysis, measured as glycerol release, increased proportionally with cell concentration and time of incubation, which validates the suitability of these cell preparations for the study of hormonal regulation of this metabolic process. Gilthead seabream were fed two different diets, FM (100% of fish meal) and PP (100% of plant protein supplied by plant sources) for 6 wk. After this period, each diet group was divided into two groups: fed and fasted (for 11 days). Lipolysis was significantly higher in adipocytes from PP-fed fish than in adipocytes from FM-fed fish. Fasting provoked a significant increase in the lipolytic rate, about threefold in isolated adipocytes regardless of nutritional history. Hormone effects were similar in the different groups: glucagon increased the lipolytic rate, whereas insulin had almost no effect. GH was clearly lipolytic, although the relative increase in glycerol over control was lower in isolated adipocytes from fasted fish compared with fed fish. Together, we demonstrate for the first time that lipolysis, measured in isolated seabream adipocytes, is affected by the nutritional state of the fish. Furthermore, our data suggest that glucagon and especially GH play a major role in the control of adipocyte lipolysis.fish; nutritional and hormonal regulation; insulin; glucagon; growth hormone; fasting ADIPOSE TISSUE PLAYS A CENTRAL role in energy homeostasis in storing lipids in the form of triacylglycerols and in mobilizing them via breakdown into free fatty acids (FFAs) and glycerol (34). Adipose tissue is one of the most important lipid stores in several teleosts, although in some species liver or muscle also constitutes lipid storage organs (33). In salmonids, adipose tissue is distributed primarily in the abdominal cavity, associated with the mesenteric and pyloric ceca (31). In gilthead seabream, adipose tissue is also located periviscerally. It is known that fish adiposity changes seasonally and is affected by trophic status. High-fat feeds can lead to increases in visceral fat (4), resulting in reduced product yield and quality of cultured fish (7). In gilthead seabream, replacing fish meal with plant protein seems to alter lipid metabolism and results in smaller fat depots (10).Endocrine control of adipose tissue mobilization and storage remains almost unexplored in fish, although insulin and glucagon are clearly involved (12,24,28). Key hepatic enzymes in lipid metabolism such as hepatic lipase and acetyl-CoA carboxylase are also regulated by pancreatic hormones in vitro in isolated hepatocytes or in vivo in injected fish (13,21). Several in vivo studies suggest that growth hormone (GH) and somatolactin act together, in a complementary way, to regulate fat stores in gilthead seabream (4, 20). Howev...

PKC and ERK mediate GH-stimulated lipolysis

“…Hepatocytes were preincubated for 3 h with [ 32 P]-monopotassium phosphate in the presence or absence of specific inhibitors for the following signaling elements: JAK2 (10 mM HexZJAK-I), PKC (10 mM CCZPKC-I), MEK (10 mM U0126ZMEK-I), and PKA (10 mM H-89 dihydrochlorideZPKA-I); after which the cells were treated with or without (control) GH at 100 ng/ml for another 3 h. HSL was purified from the isolated hepatocytes by ammonium sulfate fractionation, and the phosphorylated enzyme was separated by SDS-PAGE and detected by autoradiography as described in the Materials and methods section. fish (Sheridan 1994, Albalat et al 2005 showing the GH-stimulated hydrolysis of stored TG and the release of glycerol and FAs and with studies in fish showing that GH increased the specific activity of hepatic HSL in vivo and in vitro (O'Connor et al 1993, Sheridan 1994. Such lipolytic action underlies the increase in plasma FAs in mammals and fish following GH injection (Fain 1980, Lafontan & Langin 2009).…”

“…Akt also regulates several lipogenic enzymes, including the activation of the acetyl-CoA carboxylase (Berggreen et al 2009), the ratelimiting enzyme of FA synthesis. Given that Akt is a chief element of INS signaling (Caruso & Sheridan 2011), it is through Akt that INS exerts its lipogenic and antilipolytic actions (Albalat et al 2005, Chaves et al 2011). The present findings suggest that by reducing the activation of Akt, GH shifts the balance of the antilipolysis/lipogenesislipolysis antagonism toward lipolysis by suppressing antilipolytic/lipogenic processes.…”

“…Sheridan (1994)). Subsequently, GH has been shown to directly stimulate lipolysis by us in liver isolated from rainbow trout (O'Connor et al 1993) and others in adipose tissue isolated from sea bream (Albalat et al 2005).…”

Towards Fish Lipid Nutrigenomics: Current State and Prospects for Fin-Fish Aquaculture

“…In vitro, insulin stimulates hepatic de novo lipid synthesis (Cowley and Sheridan, 1993), and inhibits adipocyte and hepatic lipolysis (Albalat et al 2005b;Harmon and Sheridan, 1992a). In sea bream, on the other hand, insulin does not affect adipocyte lipolysis in vitro, except in fish fed a plant protein diet (Albalat et al 2005a). Insulin may also modulate gene transcription as in Atlantic salmon hepatocytes where it upregulates mRNA levels of scavenger receptor class B, type I, which is important in mammalian lipid homeostasis (Kleveland et al 2006b).…”

“…Glucagon is lipolytic and stimulates hepatic TG lipase activity in rainbow trout and coho salmon (Harmon and Sheridan, 1992a;Plisetskaya et al 1989). Glucagon also stimulates TG lipase activity in adipocytes in sea bream and rainbow trout (Albalat et al 2005a;Albalat et al 2005b). However, further studies are needed to understand the molecular mechanisms of glugon action in fish.…”