Role of mitochondrial function in the invasiveness of human colon cancer cells

We investigated the role of mitochondrial function in the invasiveness of human colorectal cancer (CRC) cell lines, using paired primary SW480 and metastatic SW620 cells, and appraised the clinical relevance of the alteration of mtDNA copy number in 33 pairs of CRC specimens after surgical resection. Suppression of mitochondrial function was achieved by the exposure of cells to oligomycin A (OA) or by knockdown of mitochondrial transcriptional factor A (TFAM) to evaluate their effects on energy metabolism, reactive oxygen species, protein expression levels of epithelial-mesenchymal transition (EMT) markers and invasive activity of CRC cells. We found that SW620 cells expressed higher levels of TFAM and mitochondrial DNA (mtDNA)-encoded NADH dehydrogenase subunit 6 (ND6) and cytochrome c oxidase subunit II (COX-II) and nuclear DNA-encoded NADH ubiquinone oxidoreductase subunit A9 (NDUFA9), iron-sulfur protein subunit B of succinate dehydrogenase (SDHB), ubiquinol-cytochrome c reductase core protein I/II (UQCRC1/2) and cytochrome c oxidase subunit IV (COX-IV) when compared with the SW480 cells. The mtDNA copy number, ADP-triggered oxygen consumption rate (OCR) and respiratory control ratio (RCR) of succinate-supported respiration in the SW620 cells were higher than those noted in the SW480 cells. The intracellular levels of H2O2 and O-2(-center dot) in the SW620 cells were lower than levels noted in the SW480 cells. Moreover, SW620 cells displayed lower protein levels of hexokinase II (HK-II), glucose 6-phosphate isomerase (GPI) and lactate dehydrogenase (LDH), and lower lactate production rate, and expressed higher levels of EMT markers N-cadherin, vimentin and Snail, and showed higher Transwell migration and invasion activities as compared with the SW480 cells. After OA treatment, SW620 cells exhibited a decrease in OCR and RCR of succinate-supported respiration, an increase in lactate production rate and intracellular levels of H2O2 and O-2(-center dot). Moreover, the level of vimentin and Transwell migration activity of the SW620 cells were decreased. After TFAM knockdown, the protein levels of TFAM, ND6 and COX-II, and mtDNA copy number, OCR and RCR of succinate-supported respiration in the SW620-KD#4 and SW620-KD#5 cells were all lower than those noted in the SW620-Control cells. By contrast, the protein level of HK-II, lactate production rate, the intracellular levels of H2O2 and O-2(-center dot) in the SW620-KD#4 and SW620-KD#5 cells were all higher than those noted in the SW620-Control cells. Subsequently, both SW620-KD#4 and SW620-KD#5 cells had lower Transwell invasion activity than did the SW620-Control cells. Furthermore, we found that deeper invasion (P=0.025) and longer tumor length (P=0.069) were associated with higher mtDNA copy ratios in the 33 pairs of CRC specimens obtained from surgical resection. Taken together, we conclude that higher mtDNA copy number and mitochondrial function may confer an invasive advantage to CRCs.