TLR4 mediates lipotoxic β-cell dysfunction by inhibiting the TMEM24/PI3K/AKT pathway

Abstract

Immune imbalance is the core pathophysiological mechanism of the deterioration of β-cell function driven by lipid metabolism disorders. Toll-like receptor 4 (TLR4) inflammatory signaling is a key pathway that mediates lipotoxic injury in β-cells, but the underlying mechanism needs to be further elucidated. Transmembrane protein 24 (TMEM24) is a key transporter that regulates pulsatile insulin secretion, but its pathophysiology in lipotoxicity remains unclear. In this study, we investigate whether TLR4-mediated lipotoxicity is affected by the inhibition of TMEM24 expression. The PPI network shows that TLR4 is associated with both insulin secretion and ER stress proteins in islets from obese rats. Using in vitro lipotoxic β-cell models, we found that TMEM24 is the target signal of palmitic acid (PA)-induced insulin secretion impairment in islet β-cells, and TLR4 plays a mediating role in this process. Mechanistically, TLR4 mediates lipotoxicity by binding to TMEM24 and downregulating its protein expression to suppress PI3K/AKT signaling, leading to β-cell dysfunction. TLR4 knockout ameliorates islet function impairment through TMEM24/PI3K/AKT signaling in HFD-induced obese rats. Taken together, our results show that TLR4 mediates lipotoxicity in islet β-cells by inhibiting the TMEM24/PI3K/AKT pathway, and the mechanism of TLR4-mediated lipotoxicity is elucidated from the perspective of insulin vesicular secretion.

1. Introduction

Diabetes mellitus (DM) is a serious threat to public health [1–3]. It is widely believed that insulin deficiency in pancreatic islet β-cells is necessary for the onset and development of DM [4,5], and long-term lipotoxicity is an important factor in the decline in β-cell function [6,7]. Previous studies have confirmed that metabolic inflammation is an important pathway through which glucolipotoxicity causes β-cell damage [8,9]. Lipotoxicity can trigger a series of inflammatory signaling cascades in vivo and induce internal and external inflammatory responses, leading to β-cell failure and inflammatory apoptosis [10–12]. Furthermore, the inhibition of metabolic inflammation can significantly ameliorate the lipotoxic effects on β-cells [13,14], but specific treatments are still lacking. Further exploration of targeted interventions to delay the decline in islet function is needed.

Toll-like receptor 4 (TLR4) is a key factor in the inflammatory signaling pathway activated by innate immunity, and its role in the pathogenesis of metabolic inflammation in type 2 diabetes has attracted much attention [15,16]. TLR4 is the key signal for lipotoxicity-induced metabolic inflammation in pancreatic β-cells, as confirmed by recent studies [13,17]. We have previously confirmed that lipotoxicity can directly activate the TLR4-JNK signaling cascade in pancreatic β-cells and induce insulin secretion disorders [18]. The main function of JNK is to mediate damage to metabolic inflammatory signals in pancreatic β-cells, such as apoptosis [19]. However, the most prominent manifestation of β-cell dysfunction is impaired insulin pulse secretion, and insulin vesicle secretion is regulated by intricate factors [20,21]. From the perspective of insulin secretion, whether TLR4 directly affects insulin secretion function by regulating the related proteins secreted by β-cell insulin vesicles remains to be determined. PI3K/AKT is an important signal in insulin signaling [22]. Previous studies have shown that TLR4 can also regulate the PI3K/AKT pathway in other diseases [23,24]. However, whether TLR4 regulates the islet signaling pathway through the PI3K/AKT pathway and the mechanism by which it regulates PI3K/AKT signaling requires further investigation.

Transmembrane protein 24 (TMEM24) is an endoplasmic reticulum (ER) protein that is a key lipid transporter involved in the regulation of insulin pulse secretion and can promote the release of the insulin reserve pool [25]. TMEM24 is located in the lipid bilayer at the junction between the ER and the cell membrane and can transport lipids synthesized by the ER to the cell membrane, thereby regulating Ca2+ and PIP2-IP3 signals in