Effects of PM_2.5 and heat/cold exposure on AKT/GLUT4 pathway in mouse skeletal muscle and white adipose tissue

Background It is unclear if there is any combined effect of air pollutants and non-optimal temperature on metabolic syndrome, or any molecular mechanisms of related signaling pathways in the process, which requires urgent systematic research.

Objective To observe the effects of combined exposure to PM_2.5 and non-optimal temperature on metabolic damage at gene and protein levels in mice, and elucidate the role of related signaling pathway in crucial organs.

Methods A total of 60 six-week-old male C57BL/6J mice were randomly divided into six groups: a normal temperature-filter air group (T_N-FA), a normal temperature-concentrated PM_2.5 group (T_N-PM), a heat-filter air group (T_H-FA), a heat-concentrated PM_2.5 group (T_H-PM), a cold-filter air group (T_C-FA), and a cold-concentrated PM_2.5 group (T_C-PM). The Shanghai Meteorological and Environmental Animal Exposure System (Shanghai-METAS) was used to provide combined exposure settings of air types concentrated PM_2.5 and filter air (FA) and temperatures normal (22°C), cold (4°C), and heat (30°C) for 4 weeks. Skeletal muscle and white adipose tissue (WAT) of the mice were sampled at the end of exposure, and transcriptomics and Western blot (WB) assay were adopted to observe selected gene and protein expression levels in the samples respectively.

Results The transcriptomics results indicated that the PM_2.5 exposure enhanced the number of differentially expressed genes. Specifically, 4820 genes were differentially expressed in the T_N-PM mice compared to the T_N-FA mice at normal temperature, and 1143 genes were differentially expressed in the Tc-PM mice compared to the Tc-FA mice in the cold environment. The phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) signaling pathway and the endoplasmic reticulum protein processing pathway were identified as the most significant pathways in metabolic injury resulting from combined exposure to PM_2.5 and non-optimal temperature exposure. The WB results showed that exposure to PM_2.5 in the normal temperature and the cold environments led to a significant increase in the expression of p-AKT in WAT (P<0.01, P<0.05) and a significant decrease in the expression of GLUT4 (P<0.05, P<0.01). In skeletal muscle, exposure to PM_2.5 led to a significant decrease in GLUT4 (P<0.05) in all environments, with a consistent trend of change as observed in WAT.

Conclusion Cold/heat exposure might promote PM_2.5-induced metabolic disorder through suppression of the AKT/GLUT4 pathway, aggravating metabolic damage.