Role of Gastrointestinal Hormones in the Proliferation of Normal and ...

More documents

Recommendations

Info

586 Endocrine Reviews, October 2003, 24(5):571–599 Thomas et al. • Gastrointestinal Hormones and ProliferationFIG. 6. Gastrin receptor (GR) content by stage of disease. A, Numberof patients with tumors highly positive for gastrin receptors (10fmol/mg protein) and either negative or poorly positive (10 fmol/mgprotein) for receptors, by stage of disease. B, Percentage of patientswith tumors demonstrating more than 10 fmol gastrin receptors (GRper milligram protein) by stage of disease. *, P 0.05. [Reproducedwith permission from J. R. Upp Jr. et al.: Cancer Res 49:488–492,1989 (254).]bition of cellular proliferation in serum-free media. Next,proliferation of the colon cancer cell lines HCT-116 and CBSwas assessed using dibutyryl cyclic GMP (cGMP), a gastrinreceptor antagonist (256). Dibutyryl cGMP inhibited proliferationand induced morphological change in both cell lines.In binding studies, dibutyryl cGMP competed with 125 I-labeled gastrin for binding to HCT-116 cells (IC 50 , 1.8 mm).RIA of conditioned media from both cell lines demonstratedthe presence of gastrin. Additionally, Northern blot analysisdetected expression of gastrin mRNA.Additional studies provide support for the potential autocrinerole of gastrin in colorectal cancer growth. Smith andWatson (257) found no expression of gastrin or gastrin/CCK-B receptors in normal colonic mucosa although demonstrating78 and 81% expression, respectively, in colonicpolyps. Immunohistochemical analysis identified progastrinin 91% of polyps, Gly-G in 80%, and G-17 in only 47%. Thisstudy was the first to demonstrate widespread expression ofboth gastrin and its receptor in preneoplastic colorectal polyps,suggesting that gastrin may promote tumor progressionthrough the adenoma-carcinoma sequence in the colon. Becausea majority of human colon cancers express the gastringene (258), it was suggested that expression of gastrin maybe critically linked to gastrin-controlled growth of cancercells. This supposition was further supported by the demonstrationthat antisense RNA to the gastrin gene inhibitedthe growth of Colo-320 and HCT-116 colon cancer cells,which express gastrin, whereas the growth of Colo-205Acells, which do not express gastrin, was not affected by antisensetreatment (66).Although the proliferative effects of exogenous gastrin ongastrin-receptor positive colorectal cancers have been demonstratedin numerous in vitro and in vivo models, the rolesof endogenous hypergastrinemia and colon carcinogenesisare controversial. McGregor et al. (259) found that antralexclusion, which produces a profound hypergastrinemia,increased DNA synthesis and carcinogen-induced colon tumorsbut did not alter the incidence of tumor formation. Incontrast, Oscarson et al. (260) demonstrated that endogenoushypergastrinemia, induced by fundectomy, did not potentiatedimethylhydrazine (DNH)-induced colon carcinogenesis.The discrepancies between these studies may be relatedto the timing and administration of the carcinogen as well asthe method of producing hypergastrinemia.In addition, there remains considerable confusion regardingthe forms of gastrin that are responsible for the regulationof human colorectal cancer growth. For example, Kameyamaet al. (261) reported that the incidence of liver metastasis incolorectal cancer was significantly higher in patients withelevated serum gastrin levels. Thorburn et al. (262) conducteda case control study among 128,992 subscribers to a healthmaintenance program and found that median gastrin levelswere similar in patients with colorectal cancers and matchedcontrols (41.7 vs. 40.7 pg/ml). However, elevated gastrinlevels were associated with increased risk of colorectal malignancy(odds ratio, 3.9). These authors concluded that thisrelationship was causal, with 8.6% of colorectal cancers thatcould be attributed to hypergastrinemia. Other studies havedemonstrated increased circulating levels of gastrin in somepatients with colorectal cancer but little or no change inothers (263–265).There is increasing evidence that Gly-G and the lessprocessedprogastrin peptides, the so-called nonamidatedgastrins, may be the critical regulator of colorectal cancergrowth. Siddheshwar et al. (266) found elevated levels ofGly-G, but not G-17, in patients with colorectal cancer anddysplastic polyps. Stepan et al. (267) found that G-17 failedto bind or stimulate the growth of LoVo, HT29, HCT-116,Colo 320DM, or T84 cells. In contrast, LoVo and HT29 cellsdisplayed saturable binding of Gly-G. Treatment with Gly-Gincreased proliferation of LoVo and HT29 cells as measuredby [ 3 H]thymidine uptake; these effects were not blocked byG-17 or the gastrin/CCK-B receptor antagonist, PD134308.These data suggest that Gly-G, binding to a novel receptor,plays a critical role in the growth of certain colon cancer cells.Similarly, Litvak et al. (268) found that Gly-G stimulatedgrowth of DLD-1 cancer xenografts in nude mice. Moreover,a novel gastrin-receptor antagonist, JMV1155, blocked thestimulatory effects of Gly-G.Additional support for a role of gastrins, other than the amidatedG-17 and G-34 gastrins, in colorectal cancer growth isprovided by findings in transgenic mice. Singh et al. (269, 270)evaluated the mitogenic induction and adenoma/adenocarcinomaformation after azoxymethane treatment in miceDownloaded from edrv.endojournals.org by on July 16, 2007
Thomas et al. • Gastrointestinal Hormones and Proliferation Endocrine Reviews, October 2003, 24(5):571–599 587overexpressing progastrin compared with wild-type mice andmice overexpressing G-17. Progastrin-overexpressing mice displayedincreased tumor formation and mitogenic indices comparedwith both wild-type and G-17-overexpressing mice.Recently, Siddheshwar et al. (266) found that progastrin, and notG-17, was elevated in patients with colorectal cancer and dysplasticpolyps, thus providing clinical support for the importanceof nonamidated gastrins.Most recently, the identification of a constitutively activevariant of the gastrin/CCK-B receptor is providing novelinsight into further effects of gastrin in the proliferation ofcolorectal cancers. Hellmich et al. (271) identified a CCK-Breceptor splice variant that is expressed only in colorectalcancers but not in normal colonic mucosa adjacent to thecancer. This splice variant exhibits constitutive (ligandindependent)activation of pathways regulating intracellular-freecalcium and cell growth. Primary cultures of cellsisolated from resected colorectal cancers exhibited spontaneous,ligand-independent oscillatory increases in intracellularcalcium, with an increase in intracellular calcium notedwith the addition of G-17. Selective CCK-B receptor antagonistsblocked the G-17-stimulated calcium stimulation butnot the spontaneous intracellular calcium oscillations. Cellsexpressing the CCK-B receptor splice variant exhibited anincreased growth rate in the absence of G-17 compared withcells stably transfected with the wild-type CCK-B receptor.The selective pattern of expression, the constitutive activityand trophic action associated with this CCK-B receptor splicevariant suggested that this novel variant may regulate colorectalcancer cell proliferation through a gastrin-independentmechanism. This splice variant has recently been identifiedin pancreatic cancers, suggesting that expression may not belimited to colorectal cancer (272).In addition to gastrin, NT stimulates growth of certainhuman colon cancer cell lines both in vitro and in vivo (273,274). Yoshinaga et al. (273) demonstrated that administrationof NT (either 300 or 600 g/kg, three times daily for 21 d)significantly stimulated mean tumor area, weight, and DNA,RNA, and protein content of the murine colon cancer, MC-26.In addition, the survival rate of mice bearing MC-26 tumorsand treated with NT was significantly decreased comparedwith the control group. Similarly, NT stimulated growth ofthe human colon cancer, LoVo. We have shown that approximately25% of colon cancers analyzed express NT mRNA(274). An autocrine effect for NT in certain colorectal cancersis postulated on the basis of NT expression in four colorectalcancer cell lines (LoVo, HT29, HCT-116, and CBS), one ofnine cancer xenografts, and two of six freshly resected coloncancers; NT expression was not identified in the adjacentnormal colonic tissue. NT peptide was detected in LoVo,HT29, and HCT-116 extracts; and NTR expression was demonstratedin HT29 and HCT-116 cells. In a study by Rovereet al. (275), all 13 human colon cancer cell lines displayed lowto moderate levels of pro-NT/neuromedin N protein expression.Only six of these 13 tumors, however, processed theprecursor to produce NT and larger precursor fragmentsending with the NT or neuromedin N sequence. Similar tothe signaling pathways mediating the effects of NT in pancreaticcancers, Ehlers et al. (276) have shown that NT treatmentof the human colon cancer cell line KM20, whichexpresses high levels of the NTR, results in calcium mobilizationas well as activation of the ERK pathway. Thesefindings suggest that the proliferative effect of NT may bethrough the ERK stimulatory pathway and that, similar togastrin, NT may be an autocrine growth factor in certaincolon cancers. The fact that intraluminal fats are the mostpotent stimulus for NT release suggests an association betweenthe known stimulatory effects of fats on colon carcinogenesisand hormones, such as NT, which increase coloncancer proliferation.Additionally, NT appears to promote carcinogenesis in thecolon of rats. Tatsuta et al. (184) reported that rats given10 weekly injections of azoxymethane (7.4 mg/kg), as well asNT (200 g/kg every other day for 40 wk), displayed significantincreases in the number and size of colon tumors,with a higher degree of submucosal penetration. This wascontrasted by the fact that NT alone did not increase theoverall incidence of colonic tumors in rats. NT caused asignificant increase in tumor cell modification above that ofnormal colonic mucosa.Similar to findings in gastric and pancreatic cancers, somatostatinhas been shown to inhibit growth of certain humancolon cancers (65–67). Our laboratory has found that thesomatostatin analog, MK-678, significantly inhibited growthof human colon cancer xenografts (RIP and DRUM) (277).One mechanism may be due to inhibition of gastrin release,which is further supported by the demonstration that administrationof octreotide blocked gastrin-induced stimulationof the MC-26 colon cancer cell line (65). Melen-Muchaet al. (278) have demonstrated that octreotide inhibits proliferation,as measured by BrdU, and increases apoptosis, asassessed by increased nuclear DNA fragmentation. Octreotidedecreases the proliferation/apoptosis ratio in favor ofcell death in Colon 38 transplanted tumors in mice. Inhibitionof colorectal cancer growth with somatostatin analogs hasnot been demonstrated in all cancer cell lines. di Paolo et al.(67) found that the somatostatin analog, SMS 201-995, inhibitedgrowth of SW480 colon cancer cells, but failed to alterthe growth of SW620 cells. However, SW620 cells were sensitiveto SMS 201-995 when used in combination with IL-1 orinterferon-. These results suggest the existence of differentsomatostatin receptor subtypes, which may account for differentialeffects of somatostatin analogs in colorectal cancercells. A recent study has found that linking toxic chemotherapeuticagents to somatostatin analogs that selectively bindto somatostatin receptor subtypes can direct chemotherapeuticagents and provide more efficient delivery of cytotoxicagents to colorectal tumors (279).D. Other cancersGI hormones have been shown to play trophic roles in neoplasmsoutside the GI tract as well. Notable examples includebreast, lung, and prostate cancers and neuroblastomas.1. Breast cancer. In 2002, of the 647,000 new cases of canceraffecting women in the United States, almost one third werebreast cancers, making breast cancer the most prevalent canceramong women (249). Hormonal therapy, with antiestrogenagents such as tamoxifen, is an important adjuvant agentDownloaded from edrv.endojournals.org by on July 16, 2007
Page 1 and 2: 0163-769X/03/$20.00/0 Endocrine Rev
Page 3 and 4: Thomas et al. • Gastrointestinal
Page 15: Thomas et al. • Gastrointestinal
Page 29: Thomas et al. • Gastrointestinal

Role of Gastrointestinal Hormones in the Proliferation of Normal and ...

Create successful ePaper yourself

Delete template?

Save as template?