Apple polyphenols affect protein kinase C activity and the onset of apoptosis in human colon carcinoma cells

Polyphenol-rich apple extracts have been reported to suppress human colon cancer cell growth in vitro. The protein kinase C (PKC) is among the signaling elements known to play an important role in colon carcinogenesis. In the present study, we investigated whether apple polyphenols affect PKC activity and induce apoptosis in the human colon carcinoma cell line HT29. A polyphenol-rich apple juice extract (AE02) was shown to inhibit cytosolic PKC activity in a cell-free system. In contrast, incubation of HT29 cells for 1 or 3 h with AE02 up to 2 mg/mL did not affect the cytosolic PKC activity. After prolonged incubation (24 h), cytosolic PKC activity was modulated, albeit a u-shaped curve of effectiveness was observed, with an initial inhibitory effect followed by the recurrence and even induction of enzyme activity. Concomitantly, in the cytosol, a significant decrease of the protein levels of PKCalpha, PKCbetaII, and PKCgamma together with a significant increase of a proapoptotic PKCdelta fragment was observed. However, the effects on the protein levels of these PKC isoforms in the cytosol were not associated with translocation between the different cellular compartments but might instead result from the onset of apoptosis. Indeed, the treatment with AE02 was shown to induce apoptosis by the activation of caspase-3, DNA fragmentation, and cleavage of poly(ADP ribose) polymerase. So far, identified and available constituents of the apple extract did not contribute substantially to the observed effects on PKC and apoptosis induction. In summary, apple polyphenols were found to inhibit PKC activity in a cell-free system. However, our results indicate that within intact cells PKC does not represent the primary target of apple polyphenols but appears to be affected in the course of apoptosis induction.