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Nonsense-mediated mRNA decay (NMD) degrades mRNAs carrying a premature termination codon (PTC) in eukaryotes. Cellular stresses, including endoplasmic reticulum (ER) stress, inhibit NMD, and up-regulate PTC-containing mRNA (PTC-mRNA) levels in several cell lines. However, whether similar effects exist under in vivo conditions that involve systemic nutritional status is unclear. Here, we compared the effects of pharmacological induction of ER stress with those of nutritional interventions on hepatic PTC-mRNA levels in mice. In mouse livers, the ER stress inducer tunicamycin increased PTC-mRNA levels of endogenous marker genes. Tunicamycin decreased body weight and perturbed nutrient metabolism in mice. Food restriction or deprivation mimicked the effect of tunicamycin on weight loss and metabolism, but did not increase PTC-mRNA levels. Hyperphagia-induced obesity also had little effect on hepatic PTC-mRNA levels. Meanwhile, in mouse liver phosphorylation of eIF2α, a factor that regulates NMD, was increased by both tunicamycin and nutritional interventions. Hepatic expression of GRP78, a central chaperone in ER stress responses, was increased by tunicamycin but not by the nutritional interventions. In cultured liver cells (Hepa), exogenous overexpression of a phosphomimetic eIF2α failed to increase PTC-mRNA levels. However, GRP78 overexpression in Hepa cells increased PTC-mRNA and PTC-mRNA-derived protein levels. ER stress promoted localization of GRP78 to mitochondria, and exogenous expression of a GRP78 fusion protein targeted to mitochondria mimicked the effect of wild type GRP78. These results indicate that GRP78, but not nutritional status, is a potent up-regulator of hepatic PTC-mRNA levels during induction of ER stress in vivo. J. Cell. Biochem. 118: 3810-3824, 2017. © 2017 Wiley Periodicals, Inc., (© 2017 Wiley Periodicals, Inc.)

Glycerol-3-phosphate acyltransferase 1 (GPAT1) acts as a rate limiting enzyme in triacylglycerol and phospholipid synthesis in mammals. GPAT1 regulates hepatic lipid accumulation associated with metabolic disorders. Here we have identified two transcriptional initiation sites and two promoters (promoter I and II) required for expression of the human GPAT1 (hGPAT1) gene. Promoter I regulates transcription of three alternative hGPAT1 mRNA variants, hGPAT1-V1, V2, and V3, while promoter II induces expression of a fourth variant, hGPAT1-V4. RT-PCR analysis and luciferase reporter assays revealed that promoter II acts in lipogenic tissues like the liver (and liver-derived HepG2 cells), whereas promoter I is differentially regulated and also acts in non-liver HeLa cells. Among liver-enriched transcription factors, HNF4α and C/EBPα slightly activated hGPAT1 promoter I, while factors including HNF1α altered promoter II activity. The lipogenic transcription factor SREBP1c greatly increased promoter II activity in HepG2 cells. The use of various truncated or mutated fragments of promoter II revealed that one sterol regulatory element-like motif and one inverted CCAAT box on promoter II contributed to the SREBP1c response. These cis-acting elements and trans-acting factors can be potential targets for manipulation of hepatic GPAT1 levels in humans., (Copyright © 2012 Elsevier Inc. All rights reserved.)

A number of recent publications have reported an increased frequency prevalence of glucose intolerance with hyperinsulinemia in liver cirrhosis. The aim of this work was to detect, in CCl(4)-induced liver cirrhosis rat, the presence and starting point of muscle and liver insulin resistance. Eighteen rats received intraperitoneal injection of 2 ml of soybean oil containing of CCl(4) twice a week for 20 weeks. We executed standard oral glucose tolerance and clamp study to evaluate systemic insulin resistance. Hepatic glucose uptake was much lower in CCl(4) group than that in control group, but peripheral glucose uptake was not decreased in this study. In contrast, early-phase insulin secretion was enhanced in CCl(4) rat using oral glucose load during clamp methods. These data suggested that increased early insulin secretion compensate adequately for hepatic insulin resistance in rats. However there was a report that peripheral glucose uptake was decreased in the case of human liver cirrhosis, which was formed in the course of time. In a chronic condition, this may be associated with reduced insulin content and developed systemic insulin resistance in liver cirrhosis. Then a long term observation study will be required to examine the presence of muscle insulin resistance in liver cirrhosis.

We identified a novel alternative splicing event that constitutively produces a truncated active form of human sterol regulatory element-binding protein 1 (SREBP-1). A cDNA of this splicing variant (named SREBP-1Delta) contains a translational stop codon-encoding exon sequence between exons 7 and 8. It produces SREBP-1aDelta (470 a.a.) and SREBP-1cDelta (446 a.a.) proteins that lack transmembrane and C-terminal regulatory sequences necessary for localization of SREBP-1 to the endoplasmic reticulum. A luciferase reporter assay showed that SREBP-1aDelta and SREBP-1cDelta transactivated lipogenic gene promoters to the same extent as that induced by N-terminal active fragments of SREBP-1a and SREBP-1c, respectively. SREBP-1Delta mRNA is expressed in human cell lines as well as adipose and liver tissues. Expression levels ranged from 5% to 16% of total SREBP-1 expression. The ratio of SREBP-1Delta expression to total SREBP-1 expression in HepG2 cells was not affected by either insulin or high glucose treatment.

Insulin and nutrients activate hepatic p70 S6 kinase (S6K1) to regulate protein synthesis. Paradoxically, activation of S6K1 also leads to the development of insulin resistance. In this study, we investigated the effect of TRB3, which acts as an endogenous inhibitor of Akt, on S6K1 activity in vitro and in vivo. In cultured cells, overexpression of TRB3 completely inhibited insulin-stimulated S6K1 activation by mammalian target of rapamycin, whereas knockdown of endogenous TRB3 increased both basal and insulin-stimulated activity. In C57BL/6 mice, adenoviral overexpression of TRB3 inhibited insulin-stimulated activation of hepatic S6K1. In contrast, overexpression of TRB3 did not inhibit nutrient-stimulated S6K1 activity. We also investigated the effect of starvation, feeding, or insulin treatment on TRB3 levels and S6K1 activity in the liver of C57BL/6 and db/db mice. Both insulin and feeding activate S6K1 in db/db mice, but only insulin activates in the C57BL/6 strain. TRB3 levels were 3.5-fold higher in db/db mice than C57BL/6 mice and were unresponsive to feeding or insulin, whereas both treatments reduced TRB3 in C57BL/6 mice. Akt was activated by insulin alone in the C57BL/6 strain and but not in db/db mice. Both insulin and feeding activated mammalian target of rapamycin similarly in these mice; however, feeding was unable to activate the downstream target S6K1 in C57BL/6 mice. These results suggest that the nutrient excess in the hyperphagic, hyperinsulinemic db/db mouse primes the hepatocyte to respond to nutrients resulting in elevated S6K1 activity. The combination of elevated TRB3 and constitutive S6K1 activity results in decreased insulin signaling via the IRS-1/phosphatidylinositol 3-kinase/Akt pathway.

Abstract: Glycerol-3-phosphate acyltransferase 1 (GPAT1) is a rate limiting enzyme in de novo glycerophospholipid synthesis. The murine GPAT1 promoter sequence (the “classical” sequence) was reported previously. However, the organization of this DNA sequence does not fully match the mouse genome sequences on NCBI/GenBank. Here we have identified net cis-acting promoter sequences for the mouse GPAT1 gene: promoter 1a which includes part of the classical sequence and the downstream promoter 1b. Promoter 1a facilitates transcription of two alternative GPAT1 transcript variants, GPAT1-V1 and V2, while promoter 1b produces a third transcript variant, GPAT1-V3. Upstream stimulating factor-1 (USF-1) controlled both promoters whereas sterol regulatory element-binding protein-1 (SREBP-1) exclusively regulated promoter 1a activity in vitro. Feeding increased GPAT1-V1 and V2, but not V3 mRNA levels in mouse liver. The obese condition of db/db mice did not alter the hepatic expression levels of any of the three GPAT1 variants. Feeding enhanced hepatic mRNA levels, intranuclear protein levels and promoter 1a-binding levels of SREBP-1, but not of USF-1. Thus, promoter 1a was exclusively activated by routine feeding in vivo. Our results indicate differential roles of the two promoters in the regulation of hepatic GPAT1 gene expression in mice. [Copyright &y& Elsevier]

Endothelin (ET)-1(1–31) is a novel vasoconstrictor peptide produced by human mast cell chymase, which selectively cleaves big ET-1 at the Try31-Gly32 bond. We investigated the localization of ET-1(1–31) in various hamster tissues by immunohistochemistry and compared it to the distribution of ET-1(1–21). We found that the localization and amount of ET-1(1–31) were different from those of ET-1(1–21) in each tissue. ET-1(1–31)-like immunoreactivities (IR) in the heart, lung, and adrenal gland were observed in the same areas as ET-1(1–21) but were significantly weaker, suggesting that ET-1(1–31) might play a role only in mast cell/chymase-related pathological conditions in these tissues. In the liver, ET-1(1–31)-like IR was strongly detected in Kupffer cells where ET-1(1–21)-like IR was seen more weakly. In the kidney, ET-1(1–31)-like IR was slightly higher than ET-1(1–21). These results suggest that ET-1(1–31) might have physiological roles distinct from those of ET-1(1–21) in some hamster tissues. [Copyright &y& Elsevier]