The promoting effect of sodium lithocholate, cholesterol, cholesterol-5α,6α-epoxide (cholesterol epoxide), and cholestane-3β,5α,6β-triol (triol) on colon carcinogenesis was studied in female Fischer (F-344) conventional and germ-free rats. Evaluation of these compounds for their colon tumor-promoting activity in a germ-free animal system would indicate whether further modification of these compounds by gut microflora is required for tumor promotion. At 7 weeks of age, groups of germ-free and conventional rats were given intrarectal instillations of N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) at a dose rate of 2.5 mg/rat, twice a week for 2 weeks, and then were given intrarectal doses of sodium lithocholate, cholesterol, cholesterol epoxide, or triol at a dose rate of 20 mg/rat 3 times a week for 46 weeks; other groups received MNNG for 2 weeks and vehicle thereafter for 46 weeks or lithocholate or cholesterol metabolites alone for 48 weeks. No tumors were detected in rats given sodium lithocholate, cholesterol, cholesterol epoxide, or triol alone, which suggested that these compounds, or their microbially modified metabolites, were not carcinogenic to colon mucosa under our experimental conditions. Sodium lithocholate increased MNNG-induced colon tumor incidence in both germ-free and conventional rats. The colon tumor incidence was similar in the MNNG, MNNG-plus-cholesterol, MNNG-plus-cholesterol epoxide and MNNG-plus-triol groups. The results obtained in germ-free rats also indicate that lithocholate without being further modified by the gut microflora acts as a colon tumor promoter. It is concluded that sodium lithocholate, but not cholesterol, cholesterol epoxide, triol, or their microbial products, had a promoting effect in MNNG-induced colon carcinogenesis in rats.
Supported by USPHS Contract CP-33208 and Grant CA-12376 from the National Cancer Institute.