Carnitine-dependent
fatty acid import into mitochondria and beta-oxidation seem to be impaired in
tumor cells. In the present study we show that a supply of
palmitoylcarnitine together with
L-carnitine potently induces apoptosis in HT-29 human
colon cancer cells as a consequence of accelerated
fatty acid oxidation. Caspase-3-like activities, measured by the cleavage rate of a fluorogenic tetrapeptide substrate and nuclear fragmentation determined after
DNA labeling in fixed cells by fluorescence microscopy, served as indicators of apoptosis. Neither
L-carnitine nor
palmitoylcarnitine alone were able to increase caspase-3-like activities and DNA fragmentation, but when provided together, apoptosis occurred. That exogenous
carnitine was indeed able to enhance
fatty acid uptake into mitochondria was demonstrated by an increased influx of a fluorescent
palmitic acid analog. Enhanced
fatty acid availability in mitochondria led to an increased generation of O*2-, as detected by a O*2- -sensitive fluorogenic
dye, indicating oxidation of delivered substrates.
Benzoquinone, an O*2- scavenger, blocked O*2- generation and prevented apoptosis as initiated by the combination of
palmitoylcarnitine and
carnitine. The lack of effect of the
ceramide synthesis inhibitor
fumonisin on
palmitoylcarnitine/
carnitine-induced apoptosis further supports the notion that apoptotic cell death is specifically due to
fatty acid oxidation. In contrast to HT-29 cells, nontransformed human colonocytes did not respond to exogenous
palmitoylcarnitine/
carnitine and no apoptosis was observed. In conclusion, our studies provide evidence that a limited mitochondrial
fatty acid import in human
colon cancer cells prevents high rates of mitochondrial O*2- production and protects
colon cancer cells from apoptosis that can be overcome by an exogenous
carnitine supply.