Sam68 promotes hepatic gluconeogenesis via CRTC2

Kirk M Habegger,Chan Boriboun,Shiyue Xu,Jianxin Deng,Teayoun Kim,Stephane Richard,Liu Yang,Yuhua Song,Eric Zhang,Chunxiang Zhang,Jianyi Zhang,Yongchao C Ma,Hongyu Qiu,Wenxia Ma,Gangjian Qin,Junlan Zhou,Baolong Yan,Aijun Qiao

doi:10.1038/s41467-021-23624-9

Abstract

Hepatic gluconeogenesis is essential for glucose homeostasis and also a therapeutic target for type 2 diabetes, but its mechanism is incompletely understood. Here, we report that Sam68, an RNA-binding adaptor protein and Src kinase substrate, is a novel regulator of hepatic gluconeogenesis. Both global and hepatic deletions of Sam68 significantly reduce blood glucose levels and the glucagon-induced expression of gluconeogenic genes. Protein, but not mRNA, levels of CRTC2, a crucial transcriptional regulator of gluconeogenesis, are >50% lower in Sam68-deficient hepatocytes than in wild-type hepatocytes. Sam68 interacts with CRTC2 and reduces CRTC2 ubiquitination. However, truncated mutants of Sam68 that lack the C- (Sam68ΔC) or N-terminal (Sam68ΔN) domains fails to bind CRTC2 or to stabilize CRTC2 protein, respectively, and transgenic Sam68ΔN mice recapitulate the blood-glucose and gluconeogenesis profile of Sam68-deficient mice. Hepatic Sam68 expression is also upregulated in patients with diabetes and in two diabetic mouse models, while hepatocyte-specific Sam68 deficiencies alleviate diabetic hyperglycemia and improves insulin sensitivity in mice. Thus, our results identify a role for Sam68 in hepatic gluconeogenesis, and Sam68 may represent a therapeutic target for diabetes.

Highlights

Hepatic gluconeogenesis is essential for glucose homeostasis and a therapeutic target for type 2 diabetes, but its mechanism is incompletely understood
We initiated our investigation by comparing blood-glucose levels in Sam68−/− mice and their matched WT littermates; glucose levels were significantly lower in Sam68−/− mice under both feeding and fasting conditions (Fig. 1a), as well as in the pyruvate-tolerance (Fig. 1b) and glucagon-tolerance (Fig. 1c) tests (PTT and glucagon-tolerance test (GcTT), respectively), both of which measure gluconeogenesis
Since gluconeogenesis occurs primarily in the liver, we generated hepatocyte-specific Sam[68] knockout (Sam68LKO) mice (Supplementary Fig. 1a–e), confirmed that Sam[68] protein levels declined in the liver but not in other organs (Supplementary Fig. 1f), and repeated our assessments: blood glucose levels were significantly lower in Sam68LKO mice than in their littermates with normal Sam[68] expression (Sam68f/f mice), whether the animals were fed or fasted (Fig. 1d), and when evaluated in the pyruvate-tolerance test (PTT) (Fig. 1e) and GcTT (Fig. 1f)

Summary

Introduction

Hepatic gluconeogenesis is essential for glucose homeostasis and a therapeutic target for type 2 diabetes, but its mechanism is incompletely understood. We report that Sam[68], an RNA-binding adaptor protein and Src kinase substrate, is a novel regulator of hepatic gluconeogenesis Both global and hepatic deletions of Sam[68] significantly reduce blood glucose levels and the glucagon-induced expression of gluconeogenic genes. We show that Sam[68] deletions, both globally and when restricted to the liver, reduce blood-glucose levels in mice by impeding gluconeogenesis, and that these effects are at least partially mediated by declines in CRTC2 protein stability and CRTC2/CREB-induced activation of gluconeogenic gene transcription. Our results demonstrate that hepatic Sam[68] deficiencies improve insulin sensitivity and reduce hyperglycemia in diabetic mice, which suggests that Sam[68] could be a therapeutic target for the treatment of T2D

Methods

Results

Conclusion