BUTYRATE ALTERS ACTIVITY OF SPECIFIC CAMP-RECEPTOR PROTEINS IN A TRANSGENIC MOUSE COLONIC CELL-LINE

There is great interest in utilizing butyrate as a chemotherapeutic ag ent. To elucidate its mechanism of action, the effect of butyrate on c AMP receptor protein kinase (PKA) activity in young adult mouse colon (YAMC) cells isolated from transgenic mice bearing a temperature sensi tive mutation of the SV40 large T antigen gene was investigated. Condi tionally immortalized cultures were plated at the permissive temperatu re (33 degrees C) or growth arrested by incubation at the nonpermissiv e temperature (39 degrees C). In addition, cells were incubated at 33 degrees C with or without 1 mmol/L butyrate for 24 h. Butyrate treatme nt reduced cell proliferation by 28% and enhanced apoptosis by 350% co mpared with cultures not exposed to butyrate. The PKA type I/II isozym e activity ratio was lower (P < 0.05) in cells incubated with butyrate . The relative level of PKA I isozyme was higher in proliferating cell s at 33 degrees C (63% of total PKA), while the relative level of PKA II was higher in nonproliferating cells undergoing apoptosis at 39 deg rees C (59% of total PKA). Neither incubation conditions (33 vs. 39 de grees C) nor butyrate treatment altered total PKA activity. When YAMC cells were incubated with 8-Cl-cAMP, an activator of PKA II, growth wa s markedly inhibited in cells at both temperatures. Consistent with in vitro data, increased PKA I isozyme levels were associated with dysre gulated growth in vivo. Specifically, the relative level of PKA I isoz yme was three- to fivefold higher in rat colonic tumors compared with normal nontransformed colonic mucosa. These data indicate that the bio logical effects of butyrate on colonocyte proliferation and apoptosis are associated with changes in PKA isozyme-dependent signal transducti on, and the YAMC cell line is a relevant model to examine the molecula r mechanisms by which dietary-derived factors affect relative cancer r isk.