Regulation of the enzymes of hepatic microsomal triacylglycerol lipolysis and re-esterification by the glucocorticoid dexamethasone

Abstract

Hepatic VLDL (very-low-density lipoprotein) assembly is a complex process that is largely regulated by the provision of lipid for apolipoprotein B assembly. Intracellular stored TAG (triacylglycerol) undergoes an initial lipolysis followed by re-esterification of the lipolytic products to form TAG prior to their incorporation into a VLDL particle. TGH (TAG hydrolase) is a lipase that hydrolyses intracellular TAG within the hepatocyte. We have utilized both dexamethasone-injected mouse and primary hepatocyte models to address whether stimulation of TAG biosynthesis by the synthetic glucocorticoid, dexamethasone, altered hepatic lipolysis and re-esterification and the provision of stored TAG for lipoprotein secretion. Dexamethasone treatment resulted in decreased TGH expression, primarily due to a dexamethasone-induced decrease in TGH mRNA stability. The expression and activities of diacylglycerol acyltransferases 1 and 2 were stimulated by dexamethasone. The combination of reduced intracellular TAG lipolysis and increased TAG biosynthesis contributed to the accumulation of TAG within the livers of dexamethasone-injected mice. The rate of hepatic TAG secretion in dexamethasonetreated mice was maintained at similar levels as in control mice. Our data demonstrate that stimulation of de novo TAG synthesis by dexamethasone increased the proportion of secreted TAG that was derived from de novo sources, while the utilization of stored TAG for secretion was reduced. The results show that, during markedly increased TAG synthesis, some TAGs are diverted from the cytosolic storage pool and are utilized directly for VLDL assembly within the endoplasmic reticulum lumen.

5α-Reductase Type 1 Deficiency or Inhibition Predisposes to Insulin Resistance, Hepatic Steatosis, and Liver Fibrosis in Rodents

“…The abundance of 5aR1 remains unchanged in WT mice after being fed a high-fat diet, similar to that in Wistar rats (32). With high-fat feeding, differences in transcriptional responses in livers of 5aR1-KO mice were consistent with the shunting of fatty acids away from b-oxidation (lack of induction of Cpt1a and its key transcription factor Ppara) and from export as VLDL, in favor of triglyceride synthesis and cytosolic storage (selective upregulation of mGpat [33] and Dgat2 [34]); glucocorticoids are known to increase the expression of Dgat2 in hepatocytes (35). In Zucker obese rats, Dgat2 was downregulated by finasteride, perhaps reflecting differential control of this leptin-regulated transcript (36) in these leptin-resistant rats.…”

Attenuation of Adipocyte Triacylglycerol Hydrolase Activity Decreases Basal Fatty Acid Efflux

“…HSL and ATGL are soluble cytosolic proteins, whereas TGH is localized to particulate (microsomal) fractions (10). Initially, TGH was shown to play a role in hepatic TG metabolism (12)(13)(14)(15)(16). Overexpression of TGH in hepatocytes resulted in increased hydrolysis of preformed TG storage pool and increased delivery of the lipolytic products for re-esterification into very low density lipoprotein TG (12,15).…”

“…First-strand cDNA synthesis from 2 g of total RNA was performed using SuperScript TM II reverse transcriptase (Invitrogen) primed by oligo(dT) [12][13][14][15][16][17][18] primers. The following primers were used: aP2 (adipocyte lipid-binding protein), 5Ј-GAA CCT GGA AGC TTG TCT TCG-3Ј (forward (F)) and 5Ј-ACC AGC TTG TCA CCA TCT CG-3Ј (reverse (R)); cyclophilin, 5Ј-TCC AAA GAC AGC AGA AAA CTT TCG-3Ј (F) and 5Ј-TCT TCT TGC TGG TCT TGC CAT TCC-3Ј (R); ATGL, 5Ј-TGG AAC ATC TCA TTC GCT GG-3Ј (F) and 5Ј-AAT GCC GCC ATC CAC ATA G-3Ј (R); adiponutrin, 5Ј-GTT TGC AGG CTG CGG CTT CC-3Ј (F) and 5Ј-GGC AGA TGT CAT GCT CAC CG-3Ј (R); peroxisome proliferator-activated receptor ␥, 5Ј-ATG TCT CAT AAT GCC ATC-3Ј (F) and 5Ј-CTA GTA CAA GTC CTT GTA-3Ј (R); diacylglycerol acyltransferase 1, 5Ј-ATT CAC GGA TCA TTG AGC G-3Ј (F) and 5Ј-CTG CCA TGT CTG AGC ATA GG-3Ј (R); diacylglycerol acyltransferase 2, 5Ј-CTA CGT TGG CTG GTA ACT TCC-3Ј (F) and 5Ј-AAC CAG ATC AGC TCC ATG G-3Ј (R).…”

“…We have characterized another neutral intracellular TG hydrolase (TGH) in hepatic microsomes (10). TGH (also termed Ces3) associates with hepatic lipid droplets (11) and hydrolyzes stored hepatic TG (12)(13)(14)(15). TGH is also expressed in 3T3-L1 adipocytes (16,17) and adipose tissue (18 -20) where it has been also shown to associate with adipocyte lipid droplets (19).…”

Obesity and corticosteroids: 11β-Hydroxysteroid type 1 as a cause and therapeutic target in metabolic disease

“…ApoE-11 -HSD1 mice showed modest insulin resistance and hypertriglyceridaemia and were hypertensive due to over-expression of hepatic angiotensinogen, echoing the aP2-11 -HSD1 hypertensive mechanism. Curiously, the major effects of hepatic 11 -HSD1 overexpression were on lipid synthesis and transport enzymes rather than glucose homeostasis (Paterson et al, 2004), but do highlight the important role of glucocorticoids in regulation of lipid homeostasis (Brindley, 1995, Dolinsky et al, 2004). An attenuated metabolic syndrome driven by high liver 11 -HSD1 in mice M a n u s c r i p t 13 indicates the enzyme makes an important contribution to metabolic disease in other rodent models , Liu et al, 2005 and indeed indicates a potential therapeutic avenue in patients with a liver selective increase in 11 -HSD1 such as myotonic dystrophy (Johansson et al, 2001).…”