Hepatic PDIA5 improves hyperglycaemia and insulin resistance through stabilisation of ATF6

AIMS/HYPOTHESIS: Dysregulation of hepatic glucose production, primarily driven by aberrantly elevated gluconeogenesis, is a core pathological feature of type 2 diabetes. However, the molecular mechanisms governing this process remain incompletely understood. Protein disulfide isomerase A5 (PDIA5) is ubiquitously expressed, but its specific function in hepatic metabolism is unknown. We hypothesised that PDIA5 plays a critical role in regulating hepatic gluconeogenesis, and that its dysregulation contributes to hyperglycaemia. METHODS: We assessed hepatic PDIA5 expression in liver biopsies from individuals with metabolic dysfunction-associated steatotic liver disease (MASLD) and from mouse models of type 2 diabetes. The function of PDIA5 was investigated using adenoviral-mediated overexpression in the livers of male mice fed a high-fat diet (HFD) and in mouse primary hepatocytes (MPHs). Metabolic phenotypes were characterised through glucose, insulin and pyruvate tolerance tests and hyperinsulinaemic-euglycaemic clamps. Mechanistic investigations in MPHs and HEK293T cells, involving co-immunoprecipitation, ubiquitination assays and chromatin immunoprecipitation, were used to delineate the PDIA5-ATF6 (activating transcription factor 6) signalling axis. The physiological necessity for PDIA5 was confirmed using whole-body Pdia5 knockout male mice and adenovirus-mediated liver-specific Pdia5-deficient female mice. RESULTS: Hepatic PDIA5 expression was significantly reduced in both individuals with MASLD and diabetic mice. Hepatic overexpression of PDIA5 in HFD-fed mice ameliorated hyperglycaemia and improved glucose homeostasis by suppressing expression of gluconeogenic genes (Pck1 and G6pc1) and glucose production. Mechanistically, PDIA5 was found to physically interact with and stabilise the endoplasmic reticulum stress sensor ATF6, which is a known suppressor of gluconeogenesis. PDIA5 shields ATF6 from the E3 ubiquitin ligase synoviolin 1 (SYVN1), thereby inhibiting its ubiquitination and subsequent proteasomal degradation. This stabilisation of ATF6 attenuated recruitment of the co-activator CRTC2 to gluconeogenic promoters and suppressed the PERK-TRB3 signalling axis, enhancing hepatic insulin sensitivity. Importantly, knockdown of ATF6 abolished the protective effects of PDIA5 on glucose metabolism. Conversely, genetic deletion of Pdia5 exacerbated glucose intolerance in HFD-fed mice. CONCLUSIONS/INTERPRETATION: Our findings identify a novel PDIA5-SYVN1-ATF6 regulatory axis that is crucial for maintaining hepatic glucose homeostasis. These results establish PDIA5 as a critical molecular checkpoint linking hepatic lipid overload to glucose dysregulation, and highlight it as a potential therapeutic target for type 2 diabetes.