Butyrate metabolism upstream and downstream acetyl-CoA synthesis and growth control of human colon carcinoma cells

Butyrate is a short chain fatty acid (SCFA) produced by bacterial fermentat ion of dietary fibers in the colon lumen which severely affects the prolife ration of colon cancer cells in in vitro experiments. Although butyrate is able to interfere with numerous cellular targets including cell cycle regul ator expression, little is known about butyrate metabolism and its possible involvement in its effect upon colon carcinoma cell growth. In this study, we found that HT-29 Glc(-/+) cells strongly accumulated and oxidized sodiu m butyrate without producing ketone bodies, nor modifying oxygen consumptio n nor mitochondrial ATP synthesis. HT-29 cells accumulated and oxidized sod ium acetate at a higher level than butyrate. However, sodium butyrate, but not sodium acetate, reduced cell growth and increased the expression of the cell cycle effector cyclin D3 and the inhibitor of the G1/S cdk-cyclin com plexes p21/WAF1/Cip1, demonstrating that butyrate metabolism downstream of acetyl-CoA synthesis is not required for the growth-restraining effect of t his SCFA. Furthermore, HT-29 cells modestly incorporated the C-14-labelled carbon from sodium butyrate into cellular triacylglycerols and phospholipid s. This incorporation was greatly increased when d-glucose was present in t he incubation medium, corresponding to the capacity of hexose to circulate in the pentose phosphate pathway allowing NADPH synthesis required for lipo genesis. Interestingly, when HT-29 cells were cultured in the presence of s odium butyrate, their capacity to incorporate C-14-labelled sodium butyrate into triacylglycerols and phospholipids was increased more than twofold. I n such experimental conditions, HT-29 cells when observed under an electron ic microscope, were found to be characterized by an accumulation of lipid d roplets in the cytosol. Our data strongly suggest that butyrate acts upon c olon carcinoma cells upstream of acetyl-CoA synthesis. In contrast, the met abolism downstream of acetyl-CoA [i.e. oxidation in the tricarboxylic acid (TCA) cycle and lipid synthesis] likely acts as a regulator of butyrate int racellular concentration.