n-Butyrate inhibits the growth of
colon cancer cell lines. In the HCT 116 cell line,
butyrate-induced growth inhibition is almost fully reversible, whereas in the VACO 5 cell line, a subpopulation undergoes apoptosis within 30 hr of treatment with
butyrate. Concurrent treatment of VACO 5 cells with
butyrate and the
phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) accelerates and increases the incidence of cell death to nearly 100% of the population, whereas HCT 116 cells largely remain alive during treatment with this combination. The action of
butyrate as an inhibitor of
histone deacetylase was assessed in these cell lines by examining extracted core
histones for their electrophoretic mobility in Triton/
acid/
urea gels. The concentrations of
butyrate that were effective for inducing apoptosis were similar to the concentrations that caused hyperacetylation of core
histones in the VACO 5 cell line. Furthermore, an examination of other
carboxylic acids for induction of apoptosis revealed a rank order that corresponded to the order of potency in causing hyperacetylation of core
histones. Specifically, the active
acids were 3-5 carbons in length and lacked substitution at the 2-position. Isovaleric and
propionic acids, in particular, proved to be effective inducers of both hyperacetylation and apoptosis at 5 mM concentrations, a finding of potential relevance to the unusual
pancytopenia occurring after acidotic episodes in isovaleric and
propionic acidemias. The duration of
butyrate treatment required for
chromatin fragmentation (10-20 hr) corresponded to the time required for
histone H4 to become predominantly tetraacetylated. Furthermore,
trichostatin A, a structurally dissimilar inhibitor of
histone deacetylase, mimicked
butyrate-induced apoptosis of VACO 5 cells and growth inhibition of HCT 116 cells. The dramatic enhancement of VACO 5 cell death by TPA, and the high level resistance of HCT 116 cells to
butyrate were not evident from
histone acetylation determinations. Thus, applications of
butyrate for cytoreduction
therapy will benefit from pharmacodynamic assessment of
histone acetylation, but will require additional work to predict susceptibility to
butyrate-induced death.