DIMINISHED TUMORIGENIC PHENOTYPE AFTER DEPLETION OF MITOCHONDRIAL-DNA

Modulation of tumorigenicity has been considered to be a reflection of the (nuclear) genetic and cellular aberrations present in tumor cells . Recent studies have suggested that cytoplasmic elements can also con tribute to the malignant phenotype of cancer, and that mitochondria ma y be important in this process. We, therefore, undertook a study to ev aluate the effects of depletion of functional mitochondria on the tumo rigenic phenotype. Brain and breast tumor cells were depleted of mitoc hondrial DNA [rho(-)] by treatment with ethidium bromide. These rho(-) respiratory-deficient cells showed a distinct change in the tumorigen ic phenotype, including loss of ability to grow in an anchorage-indepe ndent fashion and, interestingly, a substantial increase in sensitivit y to cytotoxic drugs (1,3-bis-chloroethyl-1-nitrosourea and cis-diammi nedichloroplatinum(II)). Reversion to the tumorigenic phenotype was ac complished with transfer of normal mitochondria into the diminished tu morigenic rho(-) cells. No changes in expression of the apoptosis gene s bcl-2 and bax, nor the drug resistance genes mdr1, mrp, or O-6-alkyl transferase was found in any of the cell types (de novo, rho(-), or cy brid). Further, the type of cell death remained the same, i.e., cells with and without mitochondria underwent apoptosis in response to expos ure to cytotoxic agents. Our results indicate that mitochondria/mitoch ondrial DNA play a direct role in modulating aspects of the tumorigeni c phenotype, although they are not necessarily a sine qua non for apop totic cell death. This is particularly interesting because most tumor tissues are more dependent upon glycolysis for energy production, rath er than mitochondrially mediated oxidative phosphorylation. Creation o f rho(-) cells will be useful to study the mitochondrial processes inv olved in tumorigenesis.