Stimulation of ornithine decarboxylase activity and DNA synthesis by phorbol esters or bile acids in rat colon.

The changes of colonic epithelial ornithine decarboxylase (ODC) activity and DNA synthesis following intrarectal administration of a tumor-promoting agent, 12-O-tetradecanoylphorbol-13-acetate (TPA), or various bile acids to male noninbred rats were studied. A single instillation of TPA, at a dose as low as 16 nmol, led to a significant (about 10-fold) increase in colonic ODC activity. Peak ODC activity was observed at 4 hr, and the enzyme activity returned to the control level about 24 hr after intrarectal TPA. This pattern was almost the same as that observed after sodium deoxycholate treatment. TPA showed more potent induction of ODC activity than deoxycholate, although the maximal induction was greater in the case of deoxycholate treatment. Both TPA and deoxycholate stimulated DNA synthesis at 2 days after intrarectal instillation, after an initial depression at 4-12 hr. A structure-activity study of 26 bile acids revealed that 5 beta-cholanoic acid with alpha-hydroxy groups in two of the 3 alpha, 7 alpha, 12 alpha positions and 5 beta-cholanoic acid with a 3 alpha-hydroxy group induced colonic ODC activity significantly, while the 3 alpha, 6 alpha-dihydroxy acid did not. Replacement of hydroxy groups by keto groups or a change from alpha to beta configuration decreased the ODC-inducing activities. Tri-substituted 5 beta-cholanoic acid derivatives, whether hydroxy or keto, did not stimulate ODC. These data indicate that a specific bile acid structure with a definite spatial relationship of the hydroxy groups is required for induction of colonic ODC activity.