Increased carnitine-dependent fatty acid uptake into mitochondria of human colon cancer cells induces apoptosis

Carnitine-dependent fatty acid import into mitochondria and β-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 O₂^-., as detected by a O₂^-.-sensitive fluorogenic dye, indicating oxidation of delivered substrates. Benzoquinone, an O₂^-. scavenger, blocked O₂^-. 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 O₂ ^-. production and protects colon cancer cells from apoptosis that can be overcome by an exogenous carnitine supply.