The differentiation inducers phenylacetate and phenylbutyrate modulate camptothecin sensitivity in colon carcinoma cells in vitro by intracellular acidification

Aromatic fatty acids such as phenylbutyrate (PB) and its metabolite phenyla cetate (PA) induce growth arrest, differentiation and apoptosis in solid tu mor cells. Despite their antiproliferative action they were reported to exh ibit a synergistic effect in combination with cytotoxic drugs like topoteca n, and others. Since the activity of the camptothecines (CPTs) depends on l ocal pH conditions, we investigated, whether PB/PA modulate CPT effects ind irectly by affecting intracellular pH in SW620 and SW480 colon cancer cells . The results for the colon carcinoma cells show an antagonistic interactio n for the combination of CPT and 0.25-5 mM PA in viability assays, resultin g in an approximately 3-fold increase in IC50 (control: 20 +/-7 nM). A syne rgistic effect with significantly increased numbers of late apoptotic/necro tic cancer cells (difference +21 +/-4%) and 1.4-fold sensitization were det ected upon inclusion of 2.5 mM PA during a 4-h CPT (10 muM) loading phase. In response to 0.25-1 mM PA/PB the cells exhibit a reversible decrease of p Hi (0.1-0.31 pH units) in HEPES- or bicarbonate-buffered media. Dose-depend ent acidification and pHi-recovery occurred following addition of PA and PB after an acid load and inhibition of the Na+/H+-antiporter and bicarbonate exchangers, pointing to a possible intracellular mechanism of cytoplasmic acidification. It is concluded that the synergistic modulation of CPT toxic ity by short-term PA/PB treatment in colon carcinoma cells is caused by cha nges in intracellular pH, possibly affecting, quantity and localization of the active closed lactone form of this drug.