Hyperinsulinemia and IR are considered the primary basis of the metabolic impairment associated with MS. Insulin is one of the most important anabolic hormones that is able to stimulate cell proliferation. Harmful local and systemic reactions, including intracellular lipid accumulation in adipocytes, mitochondrial and endoplasmic reticulum stress, and IR, can be induced by obesity, accompanied by changes of circulating factors such as adipokines, free fatty acids and a variety of inflammatory mediators.
The insulin-like growth factor (IGF) family and the changes of its components resulting from IR play a crucial role in tumor formation and progression. The IGF family consists of IGF-1, IGF-2 and their receptors, IGF-1R and IGF-2R, as well as six types of IGF binding proteins (IGFBPs), IGFBP-1 to 6. IGF-1 and IGFBP-3 are important members of the IGF family, and have been investigated in many studies to understand their role in RCC carcinogenesis. Rasmuson et al. evaluated the prognostic information of serum IGF-1 in 256 patients with RCC, and reported that IGF-1 did not correlate with tumor stage or grade, whereas tumor stage and IGF-1 levels were independent prognostic factors using a multivariable analysis. They concluded that high serum IGF-1 levels at diagnosis were related to favorable prognosis in RCC [
45]. In the human RCC cell lines Caki-2 (from a primary tumor) and SK-RC-52 (from a metastasis), IGF-1 may enhance transforming growth factor-β (TGF-β) signaling and raise IGFBP-3 levels with TGF-β acting in synergy [
46]. Through activation of mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase (PI3K) signaling pathways, IGF-1 that is bound to IGF-1R may play a role in promotion of mitosis and cell migration, and inhibition of apoptosis [
47]. Additionally, IGF-1 can stimulate tumor angiogenesis by increasing vascular endothelial growth factor (VEGF) levels [
48]. IGFBP-3 may inhibit the activity of IGF-1 through competitive binding to IGF-1, because IGF-1 has a higher affinity for IGFBP-3 than for its receptors. Moreover, IGFBP-3 regulates the biological functions of IGFs, by means of controlling their transport from circulation into tissues, modulating their metabolism and clearance, and establishing their specific location on tissues and cells [
49]. Although no correlation was observed between serum IGFBP-3 levels and RCC [
45,
50], high expression of IGFBP-3 was found in clear cell RCC tissue, and high grade (Fuhrman grades 3 and 4) clear cell RCC showed higher IGFBP-3 staining intensity than low grade clear cell RCC [
51]. Considering the close link between IGFs and RCC, potential therapeutics targeting IGFs might be promising for the treatment of RCC.