Aims/hypothesis: Chronic stimulation of β₂-adrenoceptors, opposite to acute treatment, was reported to reduce blood glucose levels, as well as to improve glucose and insulin tolerance in rodent models of diabetes by essentially unknown mechanisms. We recently described a novel pathway that mediates glucose uptake in skeletal muscle cells via stimulation of β₂-adrenoceptors. In the current study we further explored the potential therapeutic relevance of β₂-adrenoceptor stimulation to improve glucose homeostasis and the mechanisms responsible for the effect. Methods: C57Bl/6N mice with diet-induced obesity were treated both acutely and for up to 42 days with a wide range of clenbuterol dosages and treatment durations. Glucose homeostasis was assessed by glucose tolerance test. We also measured in vivo glucose uptake in skeletal muscle, insulin sensitivity by insulin tolerance test, plasma insulin levels, hepatic lipids and glycogen. Results: Consistent with previous findings, acute clenbuterol administration increased blood glucose and insulin levels. However, already after 4 days of treatment, beneficial effects of clenbuterol were manifested in glucose homeostasis (32% improvement of glucose tolerance after 4 days of treatment, p < 0.01) and these effects persisted up to 42 days of treatment. These favourable metabolic effects could be achieved with doses as low as 0.025 mg kg⁻¹ day⁻¹ (40 times lower than previously studied). Mechanistically, these effects were not due to increased insulin levels, but clenbuterol enhanced glucose uptake in skeletal muscle in vivo both acutely in lean mice (by 64%, p < 0.001) as well as during chronic treatment in diet-induced obese mice (by 74%, p < 0.001). Notably, prolonged treatment with low-dose clenbuterol improved whole-body insulin sensitivity (glucose disposal rate after insulin injection increased up to 1.38 ± 0.31%/min in comparison with 0.15 ± 0.36%/min in control mice, p < 0.05) and drastically reduced hepatic steatosis (by 40%, p < 0.01) and glycogen (by 23%, p < 0.05). Conclusions/interpretation: Clenbuterol improved glucose tolerance after 4 days of treatment and these effects were maintained for up to 42 days. Effects were achieved with doses in a clinically relevant microgram range. Mechanistically, prolonged treatment with a low dose of clenbuterol improved glucose homeostasis in insulin resistant mice, most likely by stimulating glucose uptake in skeletal muscle and improving whole-body insulin sensitivity as well as by reducing hepatic lipids and glycogen. We conclude that selective β₂-adrenergic agonists might be an attractive potential treatment for type 2 diabetes. This remains to be confirmed in humans.

Metabolic dysfunction-associated steatotic liver disease (MASLD), closely associated with obesity, can progress to metabolic dysfunction-associated steatohepatitis when the liver undergoes overt inflammatory damage. A-kinase anchoring protein 1 (AKAP1) has been shown to control lipid accumulation in brown adipocytes. However, the role of AKAP1 signaling in hepatic lipid metabolism and MASLD remains poorly understood. Here, we showed that hepatocyte-specific AKAP1 deficiency exacerbated hepatic steatosis and steatohepatitis in male mice subjected to a high-fat diet and fast-food diet, respectively. Mechanistically, AKAP1 directly phosphorylated and inactivated glycerol-3-phosphate acyltransferase 1 (GPAT1) in a PKA-dependent manner, thus suppressing lysophosphatidic acid (LPA) production. Increased endogenous LPA in hepatocytes promoted hepatocellular triglyceride (TG) synthesis and initiated pronounced inflammatory response in Kupffer cells. Restoring hepatic AKAP1 or repressing LPA levels via GPAT1 knockdown alleviated MASLD exacerbation. Overall, AKAP1 plays a protective role against MASLD by inhibiting GPAT1 activity, highlighting the potential of targeting AKAP1/PKA/GPAT1 signalosome for MASLD therapy.

Hyperglycemia often occurs in severe burns; however, the underlying mechanisms and importance of managing postburn hyperglycemia are not well recognized. This study was designed to investigate the dynamic changes of postburn hyperglycemia and the underlying mechanisms and to evaluate whether early glycemic control is beneficial in severe burns. Prospective, randomized experimental study. Animal research laboratory. Sprague-Dawley rats. Anesthetized rats were subjected to a full-thickness burn injury comprising 40% of the total body surface area and were randomized to receive vehicle, insulin, and a soluble form of receptor for advanced glycation endproducts treatments. An in vitro study was performed on cultured H9C2 cells subjected to vehicle or carboxymethyllysine treatment. We found that blood glucose change presented a distinct pattern with two occurrences of hyperglycemia at 0.5- and 3-hour postburn, respectively. Acute insulin resistance evidenced by impaired insulin signaling and glucose uptake occurred at 3-hour postburn, which was associated with the second hyperglycemia and positively correlated with mortality. Mechanistically, we found that serum carboxymethyllysine, a dominant species of advanced glycation endproducts, increased within 1-hour postburn, preceding the occurrence of insulin resistance. More importantly, treatment of animals with soluble form of receptor for advanced glycation endproducts, blockade of advanced glycation endproducts signaling, alleviated severe burn-induced insulin resistance. In addition, early hyperglycemic control with insulin not only reduced serum carboxymethyllysine but also blunted postburn insulin resistance and reduced mortality. These findings suggest that severe burn-induced insulin resistance is partly at least mediated by serum advanced glycation endproducts and positively correlated with mortality. Early glycemic control with insulin or inhibition of advanced glycation endproducts with soluble form of receptor for advanced glycation endproducts ameliorates postburn insulin resistance.

Metabolic-associated steatohepatitis (MASH) is an advanced and progressive liver disease that potentially causes cirrhosis and hepatocellular carcinoma. Exercise is a crucial and effective intervention for ameliorating metabolic dysfunction-associated steatotic liver disease. This study aimed to provide a comprehensive understanding of the underlying mechanisms of MASH, which benefit a broad spectrum of MASH patients, including those who have difficulty engaging in physical activity.