Activating transcription factor 6 plays protective and pathological roles in steatosis due to endoplasmic reticulum stress in zebrafish

Abstract

Many etiologies of fatty liver disease (FLD) are associated with hyper-activation of one of the three pathways that comprise the unfolded protein response (UPR), a harbinger of endoplasmic reticulum (ER) stress. The UPR is mediated by pathways initiated by PERK, IRE1a/XBP1and ATF6, and each of these pathways have been implicated as either protective or pathological in FLD. We use zebrafish with FLD and hepatic ER stress to explore the relationship between Atf6 and steatosis. Mutation of the foie gras (foigr) gene causes FLD and hepatic ER stress. Prolonged treatment of wild-type larvae with a dose of tunicamycin that causes chronic ER stress phenocopies foigr. In contrast, acute exposure to a high dose of tunicamycin robustly activates the UPR but is less effective at inducing steatosis. The Srebp transcription factors are not required for steatosis in any of these models. Instead, depleting larvae of active Atf6 either through mbtps1 mutation or atf6 morpholino injection protects against steatosis caused by chronic ER stress whereas it exacerbates steatosis caused by acute tunicamycin treatment. Conclusion ER stress causes FLD. Loss of Atf6 prevents steatosis caused by chronic ER stress but can also potentiate steatosis caused by acute ER stress. This demonstrates that Atf6 can play both protective and pathological roles in FLD.

Defining Hepatic Dysfunction Parameters in Two Models of Fatty Liver Disease in Zebrafish Larvae

“…45 Zebrafish larvae exposed to EtOH from 4-5.5 dpf 28,30 or to 1 lg/mL Tm from 3-5 dpf 43,44 develop hepatomegaly, indicated as a change from the thin crescent shaped livers in controls to oval shaped livers in EtOH-and Tm-treated larvae (Fig. 1A and previous reports [28][29][30]43 ). Analysis of liver area by ImageJ displayed a mild increase in liver size in ethanol-treated fish versus controls (38711.5 lm 2 in controls vs. 40709.2 lm 2 in ethanol-treated larvae, or a 5% total increase in size).…”

“…Tm blocks Nlinked glycosylation of proteins by inhibition of DPAGT1, 38,39 causing ER stress and, by an as of yet unknown mechanism, results in fatty liver disease in mammals [40][41][42] and zebrafish. 43,44 To determine the effect of EtOH and Tm on liver morphology, we employed a transgenic zebrafish line expressing dsRed specifically in hepatocytes (Tg(fabp10:dsRed)). 45 Zebrafish larvae exposed to EtOH from 4-5.5 dpf 28,30 or to 1 lg/mL Tm from 3-5 dpf 43,44 develop hepatomegaly, indicated as a change from the thin crescent shaped livers in controls to oval shaped livers in EtOH-and Tm-treated larvae (Fig.…”

“…43,44 To determine the effect of EtOH and Tm on liver morphology, we employed a transgenic zebrafish line expressing dsRed specifically in hepatocytes (Tg(fabp10:dsRed)). 45 Zebrafish larvae exposed to EtOH from 4-5.5 dpf 28,30 or to 1 lg/mL Tm from 3-5 dpf 43,44 develop hepatomegaly, indicated as a change from the thin crescent shaped livers in controls to oval shaped livers in EtOH-and Tm-treated larvae (Fig. 1A and previous reports [28][29][30]43 ).…”

Liver immune responses to inflammatory stimuli in a diet-induced obesity model of zebrafish

“…One of the most modulated genes in the zebrafish NAFLD liver is crebrf. This gene acts as a negative regulator of the unfolded protein response, which is hyperactivated in this condition (Cinaroglu et al 2011). Furthermore, ubiquitination was also altered through the modulation of ugt5a3 and uba52.…”

In vivo cell biology in zebrafish – providing insights into vertebrate development and disease

“…TRAPPC11 mutation in humans causes myopathy, intellectual impairment and hyperkinetic movements (Bögershausen et al, 2013). Interstingly, zebrafish foie gras (foigr) mutants carrying a mutagenic viral insertion in the trappc11 gene display a different phenotype from other models of TRAPP complex disruption: they develop fatty liver, hepatomegaly, smaller gut and jaw, and fin defects (Cinaroglu et al, 2011;Sadler et al, 2005). The foigr/trappc11 fatty liver phenotype is partially attributed to activation of the unfolded protein response (Cinaroglu et al, 2011); however, neither mammalian cultured cells that are depleted of TRAPPC11 nor trappc2 morphants induce this response (A.M.V.…”