Mitochondria can govern local concentrations of second messengers, such as reactive oxygen species (ROS), and mitochondrial translocation to discrete subcellular regions may contribute to this signaling function. Here, we report that exposure of pulmonary artery endothelial cells to hypoxia triggered a retrograde mitochondrial movement that required microtubules and the microtubule motor protein dynein and resulted in the perinuclear clustering of mitochondria. This subcellular redistribution of mitochondria was accompanied by the accumulation of ROS in the nucleus, which was attenuated by suppressing perinuclear clustering of mitochondria with nocodazole to destabilize microtubules or with small interfering RNA–mediated knockdown of dynein. Although suppression of perinuclear mitochondrial clustering did not affect the hypoxia-induced increase in the nuclear abundance of hypoxia-inducible factor 1α (HIF-1α) or the binding of HIF-1α to an oligonucleotide corresponding to a hypoxia response element (HRE), it eliminated oxidative modifications of the VEGF (vascular endothelial growth factor) promoter. Furthermore, suppression of perinuclear mitochondrial clustering reduced HIF-1α binding to the VEGF promoter and decreased VEGF mRNA accumulation. These findings support a model for hypoxia-induced transcriptional regulation in which perinuclear mitochondrial clustering results in ROS accumulation in the nucleus and causes oxidative base modifications in the VEGF HRE that are important for transcriptional complex assembly and VEGF mRNA expression.

Objective Our objective was to execute a prospective cohort study to determine relationships between plasma mtDNA DAMP levels and the occurrence of systemic inflammatory response syndrome (SIRS), multiple organ dysfunction syndrome (MODS), and mortality. Background Mitochondrial DNA damage-associated molecular patterns (DAMPs) accumulate in the circulation after severe injury. Observations in animal models demonstrate that mtDNA DAMPs contribute to organ dys-function; however, the link between plasma mtDNA DAMPs and outcome in severely injured human subjects has not been established. Methods DNA was isolated from plasma samples taken from severely injured patients at hospital days 0, 1, and 2. Real-time PCR was used to quantify selected ≈200 base pair sequences of mtDNA within the COX1, ND1, and ND6 genes, as well as from the D-Loop transcriptional regulatory region. MODS was defined as a Denver Multiple Organ Failure score of 4 or greater. Results MtDNA DAMPs were quantified as PCR threshold cycle number. Lower threshold cycles indicate increased mtDNA DAMP content. Patients with SIRS had significantly increased mtDNA DAMP levels in all 4 sequences examined (32.14 ± 0.90 vs 29.00 ± 1.15 for COX1, 31.90 ± 0.47 vs 30.16 ± 1.42 for ND1, 32.40 ± 0.61 vs 28.94 ± 1.13 for ND6, and 33.12 ± 0.83 vs 28.30 ± 1.14 for D-Loop). Patients who developed MODS also had elevated mtDNA DAMP levels compared with those who did not (32.57 ± 0.74 vs 27.12 ± 0.66 for COX1, 32.45 ± 0.65 vs 28.20 ± 0.73 for ND1, 32.52 ± 0.56 vs 27.60 ± 0.79 for ND6, and 32.85 ± 0.75 vs 27.86 ± 1.27 for D-Loop). Patients with above-median mtDNA DAMP levels had a significantly elevated relative risk for mortality. Four patients died secondary to severe MODS. Conclusions These findings comprise the first observational evidence that plasma mtDNA DAMPs is associated with the evolution of SIRS, MODS, and mortality in severely injured human subjects.

In hypoxia, mitochondria-generated reactive oxygen species not only stimulate accumulation of the transcriptional regulator of hypoxic gene expression, hypoxia inducible factor-1 (Hif-1), but also cause oxidative base modifications in hypoxic response elements (HREs) of hypoxia-inducible genes. When the hypoxia-induced base modifications are suppressed, Hif-1 fails to associate with the HRE of the VEGF promoter, and VEGF mRNA accumulation is blunted. The mechanism linking base modifications to transcription is unknown. Here we determined whether recruitment of base excision DNA repair (BER) enzymes in response to hypoxia-induced promoter modifications was required for transcription complex assembly and VEGF mRNA expression. Using chromatin immunoprecipitation analyses in pulmonary artery endothelial cells, we found that hypoxia-mediated formation of the base oxidation product 8-oxoguanine (8-oxoG) in VEGF HREs was temporally associated with binding of Hif-1α and the BER enzymes 8-oxoguanine glycosylase 1 (Ogg1) and redox effector factor-1 (Ref-1)/apurinic/apyrimidinic endonuclease 1 (Ape1) and introduction of DNA strand breaks. Hif-1α colocalized with HRE sequences harboring Ref-1/Ape1, but not Ogg1. Inhibition of BER by small interfering RNA-mediated reduction in Ogg1 augmented hypoxia-induced 8-oxoG accumulation and attenuated Hif-1α and Ref-1/Ape1 binding to VEGF HRE sequences and blunted VEGF mRNA expression. Chromatin immunoprecipitation-sequence analysis of 8-oxoG distribution in hypoxic pulmonary artery endothelial cells showed that most of the oxidized base was localized to promoters with virtually no overlap between normoxic and hypoxic data sets. Transcription of genes whose promoters lost 8-oxoG during hypoxia was reduced, while those gaining 8-oxoG was elevated. Collectively, these findings suggest that the BER pathway links hypoxia-induced introduction of oxidative DNA modifications in promoters of hypoxia-inducible genes to transcriptional activation.

An explosion of new information about mitochondria reveals that their importance extends well beyond their time-honored function as the “powerhouse of the cell.” In this Perspectives article, we summarize new evidence showing that mitochondria are at the center of a reactive oxygen species (ROS)-dependent pathway governing the response to hypoxia and to mitochondrial quality control. The potential role of the mitochondrial genome as a sentinel molecule governing cytotoxic responses of lung cells to ROS stress also is highlighted. Additional attention is devoted to the fate of damaged mitochondrial DNA relative to its involvement as a damage-associated molecular pattern driving adverse lung and systemic cell responses in severe illness or trauma. Finally, emerging strategies for replenishing normal populations of mitochondria after damage, either through promotion of mitochondrial biogenesis or via mitochondrial transfer, are discussed.

Background: The modulation of gap junctional communication between tumor cells and between tumor and vascular endothelial cells during tumorigenesis and metastasis is complex. The notion of a role for loss of gap junctional intercellular communication in tumorigenesis and metastasis has been controversial. While some of the stages of tumorigenesis and metastasis, such as uncontrolled cell division and cellular detachment, would necessitate the loss of intercellular junctions, other stages, such as intravasation, endothelial attachment, and vascularization, likely require increased cell-cell contact. We hypothesized that, in this multi-stage scheme, connexin-43 is centrally involved as a cell adhesion molecule mediating metastatic tumor attachment to the pulmonary endothelium.