Despite its original description as an inhibitor of gastric acid secretion (18
), the primary biological property of GIP appears to be as a mediator of the enteroinsular axis, whereby the peptide stimulates the release of insulin from pancreatic islet β-cells following the ingestion of glucose and fat. The existence of a chemical stimulant of the endocrine pancreas had been suggested by Moore et al.
in 1906 (19
), who coined the term “incretin” as an insulinotropic substance emanating from small intestine released into the circulation following glucose-containing meals. Of the several GI peptides that have been proposed as candidates, only glucagon-like peptide-1 (GLP-1) and GIP have been shown to function as physiological incretins (20
As mentioned above, several previous reports have suggested an etiologic role for GIP in the development of obesity. Postprandial GIP secretion has been shown to be increased in obese subjects compared with age-matched healthy controls (22
). Animal models of obesity, represented by leptin-deficient (ob/ob) mice, have also demonstrated that the concentrations of plasma GIP are enhanced when chronically fed a high-fat diet. In addition, diets high in fat content induced K-cell hyperplasia in these mice (23
). Miyawaki et al.
) demonstrated that while normal mice became obese and developed insulin resistance and T2DM in response to a high-fat diet, GIPR-deficient (GIPR-/-
) mice were protected and remained normal while consuming the identical diet. More recently, Althage et al.
) used regulatory elements for the rat GIP promoter to express an attenuated Diphtheria toxin in transgenic mice. K-cell number, GIP transcripts, and plasma GIP levels were all profoundly diminished, and body weight was reduced by 25% in the transgenic mice.
Although a relationship between obesity and cancer had long been suspected (25
), Calle et al. (6
) conducted a prospective analysis of 900,000 American adults, who were free of cancer at the outset of the study. During the ensuing 16 years, 6.3% (57,145) died from cancer-related illness. From their extensive analysis, they estimated that cancer-related deaths in 14% and 20% of men and women, respectively, were associated with excess weight. The relative risk of mortality due to CRC increased directly with the body-mass index (BMI): 1.20 for overweight male individuals (BMI of 25.0-29.9), 1.47 for men with functional class I obesity (BMI of 30.0-34.9), and 1.84 for men with a BMI of 35.0-39.9. The risk of CRC-related death was somewhat diminished in women compared with men. The relative risk of mortality due to CRC was 1.36 and 1.46 for women with a BMI of 35.0-39.9 and >40, respectively.
The cause of obesity-related CRC is likely multifactorial, and previous studies have suggested that insulin-like peptides likely represent one of the principal biochemical mediators (26
). The precise mechanisms by which insulin and insulin-like growth factors might contribute to the development of CRC have not been elucidated. Upon binding to the receptor, insulin activates its receptor's intrinsic kinase, leading to autophosphorylation and tyrosine phosphorylation of several substrates, including members of the insulin receptor substrate (IRS) family (30
). IRS phosphorylation, in turn, recruits other signaling molecules, including PI3K (30
). One of the downstream targets of PI3K is protein kinase B (Akt), and its activation causes a cascade of cellular responses (31
). Akt appears to contribute to the development of carcinogenesis via several downstream pathways, including attenuation of apoptosis and stimulation of cell proliferation (32
We have previously reported that, like insulin, GIP induces the activation of the Akt in adipocytes via a wortmannin-sensitive pathway, which, in turn, promoted fat cell membrane GLUT-4 accumulation and enhanced [3
H]-2-deoxyglucose uptake (7
). The results of the present studies provide further evidence for the insulin mimetic properties of GIP. After demonstrating the presence of the GIPR in human and mouse CRC cell lines, we found that GIP stimulated the proliferation of the cells in a concentration-dependent manner. We next showed that GIP promoted Akt phosphorylation in a concentration-dependent manner () and activated the substrate-specific for Akt in a concentration-dependent manner ().
In addition to its effects on Akt activation, GIP treatment of CRC cells also promoted the phosphorylation of p70S6K, a principal component of the mTOR pathway, which was abolished in the presence of rapamycin (). Emerging evidence suggests that mTOR plays a key role in several pathways that are involved in human cancer (15
), as well as obesity and its sequelae (33
). Two mTOR complexes, mTORC1 and mTORC2, have been described, each possessing distinct features, including different combinations of interacting components and variable sensitivity to rapamycin; mTORC1, but not mTORC2, is rapamycin sensitive (35
). One of the principal downstream targets of mTORC1 is p70S6K, and the p70S6K through phosphorylation represent important events involved in the regulation of protein synthesis.
The notion that GIP might possess proliferative properties has been reported in studies employing β-cells and surrogate insulin-secreting cell lines. GIP has been shown to activate mitogenic signaling, such as the mitogen-activated protein kinase (MAPK) and PI3K/Akt pathways, in β-cells (15
). Akt is a major mediator of GIP action on pancreatic islets, increasing β-cell mass and function and promoting β-cell survival (36
). Trδmper et al. (37
) reported that GIP and glucose acted synergistically as anti-apoptotic factors in the well-differentiated β-cell line INS-1 and involved the activation of several pathways, including adenylyl cyclase, MAPK, and PI3K/Akt. Similarly, Kim et al. (13
) reported that GIP stimulated the anti-apoptotic Bcl-2
gene through the adenylyl cyclase pathway in INS-1 cells. Using this same cell line, they later showed that GIP regulated β-cell apoptosis by down-regulating the pro-apoptotic bax
gene and up-regulating Bcl-2 via the activation of PI3K (38
). Down-regulation of the bax
gene via PI3K appeared to be mediated by GIP-induced phosphorylation of the Forkhead/winged helix family member Foxo1 (38
As stated above, Akt activation has been shown enhance cell growth both through the attenuation of apoptosis and the stimulation of cell proliferation (32
). Although the effects of GIP apoptosis in CRC were not examined in the present study, we have demonstrated that the proliferation of CRC cells appears to involve several known mitogenic pathways, including the downstream target target p70S6K, as well as adenylyl cyclase and MAPK. Moreover, because GIP is overexpressed in obesity, these studies provide another potential link between obesity and CRC and possibly other malignancies. Additional studies will be necessary to determine the effects of GIP on apoptosis, as well as to elucidate more completely the cellular mechanisms involved in mediating the mitogenic properties of GIP in CRC. Finally, in vivo
experiments and epidemiological studies in obese humans will be required to provide further evidence linking obesity and enhanced GIP expression with CRC.