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Manuscript received 22 July 1996. Initial reviews completed 21 August 1996. Revision accepted 9 September 1996.
Faculty of Nutrition, Molecular and Cell Biology Group, Texas A & M University, College Station, TX, 77843-2471 and * Department of Pathology, Texas Tech University Health Science Center, Lubbock, TX, 79430
There is great interest in utilizing butyrate as a chemotherapeutic agent. To elucidate its mechanism of action, the effect of butyrate on cAMP receptor protein kinase (PKA) activity in young adult mouse colon (YAMC) cells isolated from transgenic mice bearing a temperature sensitive mutation of the SV40 large T antigen gene was investigated. Conditionally immortalized cultures were plated at the permissive temperature (33°C) or growth arrested by incubation at the nonpermissive temperature (39°C). In addition, cells were incubated at 33°C with or without 1 mmol/L butyrate for 24 h. Butyrate treatment reduced cell proliferation by 28% and enhanced apoptosis by 350% compared with cultures not exposed to butyrate. The PKA type I/II isozyme activity ratio was lower (P < 0.05) in cells incubated with butyrate. The relative level of PKA I isozyme was higher in proliferating cells at 33°C (63% of total PKA), while the relative level of PKA II was higher in nonproliferating cells undergoing apoptosis at 39°C (59% of total PKA). Neither incubation conditions (33 vs. 39°C) nor butyrate treatment altered total PKA activity. When YAMC cells were incubated with 8-Cl-cAMP, an activator of PKA II, growth was markedly inhibited in cells at both temperatures. Consistent with in vitro data, increased PKA I isozyme levels were associated with dysregulated growth in vivo. Specifically, the relative level of PKA I isozyme was three- to fivefold higher in rat colonic tumors compared with normal nontransformed colonic mucosa. These data indicate that the biological effects of butyrate on colonocyte proliferation and apoptosis are associated with changes in PKA isozyme-dependent signal transduction, and the YAMC cell line is a relevant model to examine the molecular mechanisms by which dietary-derived factors affect relative cancer risk.
Key words: apoptosis, butyrate, YAMC cells, large T-antigen, protein kinase A.
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