Mitochondrial deficiency impairs hypoxic induction of HIF-1 transcriptional activity and retards tumor growth

Mitochondria can be involved in regulating cellular stress response to hypoxia and tumor growth, but little is known about that mechanistic relationship. Here, we show that mitochondrial deficiency severely retards tumor xenograft growth with impairing hypoxic induction of HIF-1 transcriptional activity. Using mtDNA-deficient ρ(0) cells, we found that HIF-1 pathway activation was comparable in slow-growing ρ(0) xenografts and rapid-growing parental xenografts. Interestingly, we found that ex vivo ρ(0) cells derived from ρ(0) xenografts exhibited slightly increased HIF-1α expression and modest HIF-1 pathway activation regardless of oxygen concentration. Surprisingly, ρ(0) cells, as well as parental cells treated with oxidative phosphorylation inhibitors, were unable to boost HIF-1 transcriptional activity during hypoxia, although HIF-1α protein levels were ordinarily increased in these cells under hypoxic conditions. These findings indicate that mitochondrial deficiency causes loss of hypoxia-induced HIF-1 transcriptional activity and thereby might lead to a constitutive HIF-1 pathway activation as a cellular adaptation mechanism in tumor microenvironment.