PHORBOL ESTER AUGMENTS BUTYRATE-INDUCED APOPTOSIS OF COLON-CANCER CELLS

Butyrate is a potentially selective therapeutic agent for many adenoca rcinomas. Butyrate causes reversible growth arrest as well as some dea th of VACO 5 colon cancer cells. Combined treatment with butyrate and the phorbol ester TPA leads instead only to cell death, while TPA caus es little death on its own. Cells dying during treatment with TPA and butyrate, as well as those dying in the presence of butyrate alone, ex hibit features typical of apoptosis, including detachment, shrinkage a nd internucleosomal DNA cleavage. Pre-treating VACO 5 cell cultures wi th TPA for as little as 6 hr prior to butyrate addition led to a marke dly diminished enhancement of butyrate-induced apoptosis. Treatment wi th a distinct PKC activator, bryostatin I, was ineffective in enhancin g butyrate-induced death and, furthermore, counteracted the death-enha ncing actions of TPA. Such antagonism was apparent when bryostatin was added after 12 hr of TPA/butyrate treatment but was much less effecti ve thereafter. The duration of TPA/butyrate treatment required for dep ressing cell survival by >95% was thereby estimated to be 24 hr. Other colon cancer cell lines were examined for the extent of cell death fo llowing treatment with TPA/butyrate. In each of these lines, butyrate inhibited cell replication in a reversible manner, similar to that see n in VACO 5. However, the combination of butyrate and TPA led to high levels of cell death in only a subset of these lines. TPA/butyrate-tre ated cultures of COLO 201 exhibited extensive apoptosis, similar in ti ming and magnitude to the response by VACO 5, whereas HCT 116 was reve rsibly growth-arrested. Our findings indicate that the PKC system play s a critical role in maintaining cell survival during butyrate-induced growth arrest. (C) 1996 Wiley-Liss, Inc.