Early induction of rat colonic epithelial ornithine and S-adenosyl-L-methionine decarboxylase activities by N-methyl-N'-nitro-N-nitrosoguanidine or bile salts.

The responses of male noninbred rat colonic epithelial ornithine decarboxylase (EC 4.1.1.17) (ODC) and S-adenosyl-L-methionine decarboxylase (EC 4.1.1.50) (SAMD) activities following topical administration of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) or bile salts were studied. A single intrarectal installation of 13 mumol of MNNG resulted in a significant (p < 0.001) 20-fold peak ODC activity after 4 hr, with a prompt return to control levels by 12 hr. Stimulation of SAMD activity was less pronounced but significant (p < 0.01), with a broad 2-fold peak over controls. No significant responses of colonic epithelial enzyme activities were detected following a single intrarectal instillation of N-methyl-N'-nitroguanidine, a noncarcinogenic and nonmutagenic metabolite of MNNG, at a dose equimolar to that of MNNG. Bile salts significantly (p < 0.001) induced ODC with almost the same kinetic pattern as that observed after MNNG administration in the following order: sodium deoxycholate > sodium chenodeoxycholate > sodium cholate. Activations of SAMD were similar for these 3 bile salts. Glycine- or taurine-conjugated deoxycholate showed ODC and SAMD enzyme activations similar to that of nonconjugated deoxycholate. No significant enzyme response was seen after sodium dehydrocholate treatment. Stimulation of activities of both enzymes was directly dependent on bile salt dose. Induced ODC and SAMD activities were principally localized in colonic epithelium. Deoxycholate-stimulated enzyme activities were significantly inhibited by cycloheximide. Enzyme stimulations by active compounds were accompanied by morphological changes such as mucosal cell degeneration, mucus depletion, submucosal congestion, and punctate hemorrhage, followed by submucosal leukocytic cellular infiltration. These data support the concept that initiating and promoting events may be involved in colon carcinogenesis.