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Phosphorylation of RXRa mediates the effect of JNK to suppress hepatic FGF21 expression and promote metabolic syndrome

ABSTRACT: The cJun NH2-terminal kinase (JNK) signaling pathway in the liver promotes systemic changes in metabolism by regulating PPARa-dependent expression of the hepatokine FGF21. Hepatocyte-specific gene ablation studies demonstrated that the Mapk9 gene (encodes JNK2) plays a key mechanistic role. Mutually exclusive inclusion of exons 7a and 7b yields expression of the isoforms JNK2a and JNK2b. Here we demonstrate that Fgf21 gene expression and metabolic regulation is primarily regulated by the JNK2a isoform. To identify relevant substrates of JNK2a, we performed a quantitative phosphoproteomic study of livers isolated from control mice, mice with JNK-deficiency in hepatocytes, and mice that express only JNK2a or JNK2b in hepatocytes. We identified the JNK substrate RXRa as a protein that exhibited JNK2a-promoted phosphorylation in vivo. RXRa functions as a heterodimeric partner of PPARa and may therefore mediate the effects of JNK2a signaling on Fgf21 expression. To test this hypothesis, we established mice with hepatocyte-specific expression of wild-type or mutated RXRa proteins. We found that the RXRa phosphorylation site Ser260 was required for suppression of Fgf21 gene expression. Collectively, these data establish a JNK-mediated signaling pathway that regulates hepatic Fgf21 expression.

INSTRUMENT(S): Orbitrap Fusion

ORGANISM(S): Mus Musculus (mouse)

TISSUE(S): Liver

SUBMITTER: Marta Isasa

LAB HEAD: Roger J. Davis

PROVIDER: PXD034183 | Pride | 2022-10-12

REPOSITORIES: Pride

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	144208_m00630_SV_Pro_2.mzIdentML	Mzid
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	144211_m00633_SV_Pro_5.mzIdentML	Mzid

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Project description:Background/aims: Serum concentrations of the hepatokine fibroblast growth factor (FGF) 21 are elevated in obesity, type‐2 diabetes, and the metabolic syndrome. We asked whether FGF21 levels differ between subjects with metabolically healthy vs. unhealthy obesity (MHO vs. MUHO) opening the possibility that FGF21 is a cross‐talker between liver and adipose tissue in MUHO. Furthermore, we studied the effects of chronic FGF21 treatment on adipocyte differentiation, lipid storage, and adipokine secretion. Methods/study design: In 20 morbidly obese donors of abdominal subcutaneous fat biopsies discordant for their whole‐body insulin sensitivity (hereby classified as MHO or MUHO subjects), serum FGF21 was quantified. The impact of chronic FGF21 treatment on differentiation, lipid accumulation, and adipokine release was assessed in isolated preadipocytes differentiated in vitro. Results: Serum FGF21 concentrations were more than two‐fold higher in MUHO as compared to 37 MHO subjects (457 ±378 vs. 211 ±123 pg/mL; p<0.05). FGF21 treatment of human preadipocytes for the entire differentiation period was modestly lipogenic (+15%; p<0.05), reduced the expression of key adipogenic transcription factors (PPARG and CEBPA, ‐15% and ‐40%, respectively; p<0.01 both), reduced adiponectin expression (‐20%; p<0.05), markedly reduced adiponectin release (‐60%; p<0.01), and substantially increased leptin (+60%; p<0.01) and interleukin‐6 (+50%; p<0.001) release. Interpretation/conclusions: The hepatokine FGF21 exerts weak lipogenic and anti‐adipogenic actions and marked adiponectin‐suppressive and leptin and interleukin‐6 release‐promoting effects in human differentiating preadipocytes. Together with the higher serum concentrations in MUHO subjects, our findings reveal FGF21 as a circulating factor promoting the development of metabolically unhealthy adipocytes. We analysed in vitro differentiated preadipocytes from 20 human donors treated with FGF21 or control cultures.

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Phosphorylation of RXRa mediates the effect of JNK to suppress hepatic FGF21 expression and promote metabolic syndrome

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