It has been suggested that some nuclear transcription factors may participate in the regulation of mitochondrial functions through transcriptional control of mitochondrial DNA. Very little is known about the response of transcription factors within mitochondria to the activation of death receptors. Recent publications indicate that nuclear factor-kappaB (NF-kappaB) is localized in mitochondria of mammalian cells. Because of the critical role of mitochondria in the execution of many apoptotic pathways, we suggest that NF-kappaB-dependent mechanisms operating at the level of mitochondria contribute to its role in regulating death receptor signaling. We have found NF-kappaB p65 and p50 subunits with DNA binding activity in the mitochondria of prostatic carcinoma cell lines. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) affects DNA binding activity of mitochondria-associated NF-kappaB but does not change the amount of p65 in mitochondria, which suggests activation of mitochondrial NF-kappaB without additional translocation of NF-kappaB subunits to mitochondria. We have also shown that TRAIL decreases mitochondrial genome encoded mRNA levels and inhibition of NF-kappaB prevents this decrease. TRAIL effects on mitochondrial NF-kappaB-DNA binding and mitochondrial genome encoded mRNA levels also depend on Bcl-2 overexpression. In addition, transcription factor activator protein-1 with DNA binding activity is also found in mitochondria of prostatic carcinoma cells and TRAIL treatment affects this binding. In summary, NF-kappaB is found in mitochondria of prostatic carcinoma cells, where it is thought to regulate mitochondria genome encoded mRNA levels in response to TRAIL treatment.