M1 macrophage polarization induces inflammatory response of adipocytes via MAPK/AKT-PPARγ pathway in large yellow croaker (Larimichthys crocea)

Obesity-related chronic inflammation in adipose tissue is closely associated with macrophage infiltration and M1 polarization. Adipose tissue macrophages (ATMs) aggravate metabolic disorders and inflammation in adipocytes. However, the molecular mechanisms underlying this process in fish remain unclear. In this study, we established an in vitro co-culture system comprising M1 macrophages and adipocytes derived from large yellow croaker (Larimichthys crocea). The results demonstrated that M1 macrophages significantly upregulated the mRNA expression of pro-inflammatory genes in co-cultured adipocytes, while downregulating the anti-inflammatory gene expression. M1 macrophages significantly increased the phosphorylation levels of p38 and JNK MAPK signaling pathway. Inhibition of p38 and JNK significantly attenuated the M1-induced upregulation of pro-inflammatory genes in co-cultured adipocytes. Moreover, M1 macrophages significantly reduced Insulin signaling-related genes expression and Akt phosphorylation level, suggesting impaired Insulin signaling. Inhibition of Akt further aggravated the M1-induced inflammatory response in co-cultured adipocytes. Furthermore, M1 macrophages significantly downregulated the expression of Peroxisome Proliferator-activated Receptor γ (PPARγ) in co-cultured adipocytes. Activation of PPARγ with troglitazone significantly alleviated the M1-induced inflammation in co-cultured adipocytes, whereas its inhibition with GW9662 exacerbated the inflammatory response. The M1-induced changes in PPARγ expression were significantly modulated by inhibition of p38, JNK or Akt, indicating that PPARγ acts a downstream of MAPK and Akt pathways. Notably, PPARγ directly bound to the promoters of TNF-α and IL-1β, thereby suppressing their transcription. Collectively, these findings demonstrated that M1 macrophages induced inflammatory responses in co-cultured adipocytes via the MAPK/AKT-PPARγ signaling axis. This study provides novel insights into the molecular regulation of adipose tissue inflammation in fish and may offer potential targets for addressing obesity-related metabolic disorders from an evolutionary perspective.