ABCB1 and obesity due to melanocortin 4 receptor deficiency: Though plenty of evidence has pointed to the potential relationship of insulin and P-glycoprotein and chemoresistance, the direct contacts of insulin with cancer chemoresistance still are paucity, especially in the research of gastric cancer in spite of previous epidemiological findings that gastric cancer was associated with obesity [19,20] and experimental findings that linked PI3K/AKT or MAPK signaling pathways to gastric cancer [21,22].