Overweight/obesity continues to rise globally, and obesity rates are not expected to decline by 2030 (Ampofo and Boateng, 2020). With the improvement of living standards, over-nutrition and reduced energy expenditure lead to an increase in obese people (Vekic et al., 2019). However, epidemiological studies reported that some people exhibit resistance to obesity despite consuming a high-fat diet (HFD), a phenomenon known as the “obesity resistance phenotype” (Levine et al., 1999, Forbes et al., 1986). While there is no difference in body weight between obesity-resistant (OR) and obesity-prone (OP) individuals under normal diet, disparities in hormone regulation and energy metabolism emerge when exposed to HFD (Ding et al., 2015). The underlying mechanism of the differences remains unclear.
With nutrient overload, the expression of extracellular matrix (ECM) protein and lipid deposition increased in organs, leading to inflammation, fibrosis, and metabolic disorders (Wu et al., 2010). Collagen, the predominant ECM protein, plays a crucial role in white adipose tissue (WAT) fibrosis, characterized by abnormal collagen deposition (Sun et al., 2011). Jones et al. found that ECM-related gene expression increased in WAT of 60% high-fat fed mice, and the activated transforming growth factor-β (TGFβ) pathway exacerbated fibrosis and inhibited adipogenesis (Jones et al., 2020). TGFβ binds with receptors to promote mothers against DPP homolog (SMAD) protein phosphorylation, which up-regulates ECM protein (Iwayama et al., 2015). Notably, deletion of Smad3, a downstream molecule of TGFβ, protects mice from obesity and enhances beige fat differentiation (Yadav et al., 2011).
Beige adipocytes, originating from PDGFRα+ precursor cells, are capable of differentiating into myofibroblast-like cells (Lin et al., 2018). Browning, the process of up-regulating the number and activity of beige adipocytes, is associated with resistance to obesity and metabolic diseases (Wu et al., 2013). Uncoupling protein-1 (UCP-1) positive cells in WAT exhibit properties much like-brown adipocytes (Kiefer, 2016). Particularly, UCP-1 is a key gene in regulating browning (Hemmeryckx et al., 2019). Umesh et al. demonstrated that TGFβ receptor 1 deficiency in WAT increased UCP-1 expression, promoting WAT browning and resisting to HFD-induced obesity (Wankhade et al., 2018). Clinically, the use of UCP-1 positive cells in combination with precursor cells effectively inhibits muscle fibrosis, treating degenerative diseases of muscle atrophy (Davies et al., 2022). Currently, the studies related to WAT browning and fibrosis in obesity-resistant mice have not been reported.
Iroquois homeobox 3 (IRX3), a member of iroquois homobox gene family (Son et al., 2021), showed a bidirectional regulation with WAT browning. Zou et al. found that the expression of IRX3 was greatly elevated in mice under cold/β-adrenergic agonist stimulation (Zhang et al., 2021). Consistently, during the differentiation of induced preadipocytes to beige cells in vitro, IRX3 knockdown suppressed the expression of brown-like genes (Zou et al., 2017). Zou et al. confirmed that IRX3 directly bound to UCP-1 promoter and enhanced its transcription in vitro, inducing WAT browning (Zou et al., 2017). IRX3 specific binding site ACATGTGTT is located in -3470~-3463 bp of the transcriptional start site in UCP-1, where deletion of the sequence could inhibit UCP-1 transcription (Zou et al., 2017). Consequently, IRX3 may act as a neo-transcriptional factor regulating thermogenesis, which may mediate the onset of obesity resistance by affecting browning. IRX3 level, in human adenomas, correlates inversely with the gene expression signature of response to TGFβ (Martorell et al., 2014), but whether IRX3 indirectly regulates WAT fibrosis has not been reported.
Therefore, we hypothesized that under HFD exposure, IRX3 may enhance metabolism and thermogenesis in peripheral organs to resist high-fat-induced obesity by promoting WAT browning, which also may inhibit WAT fibrosis.