Combined Analyses of the VHL and Hypoxia Signaling Axes in an Isogenic Pairing of Renal Clear Cell Carcinoma Cells.

The loss of function of the Von Hippel-Lindau (VHL) tumor suppressor leads to the development of hypervascular tumors, exemplified by clear-cell-type renal cell carcinoma (RCC). VHL governs the adaptive responses to fluctuation of oxygen levels largely through the regulated suppression of hypoxia inducible factors (HIFs). Here, we combine proteome and phospho-proteomic analysis of isogenic 786-O RCC (±VHL) cells to compare signatures that reflect hypoxia and/or loss of VHL. VHL-independent hypoxic responses, notably include up-regulation of phosphorylation at Ser232 on the pyruvate dehydrogenase α subunit that is known to promote glycolysis. Hypoxic responses governed by VHL include up-regulation of known biomarkers of RCC (e.g., GLUT1, NDRG1) and the signaling adaptor molecule IRS-2. Notably, we also observe down-regulation of linked-components associated with the Jacobs-Stewart cycle, including the intracellular carbonic anhydrase II (CA2), which governs cellular response to CO2 fluctuations that often accompany hypoxia in tumors. Further studies indicate an unusual mechanism of control for CA2 expression that, at least in part, reflects enhanced activity of the NFκB pathway, which is associated with loss of VHL.

HIF-1 alpha signaling is augmented during intermittent hypoxia by induction of the Nrf2 pathway in NOX1-expressing adenocarcinoma A549 cells.

Fluctuations in cellular oxygenation causing intermittent hypoxia and oxidative stress affect the regulation of hypoxia-inducible factor (HIF-1) and the nuclear factor erythroid 2-related factor 2 (Nrf2). HIF-1 is primarily induced in hypoxia, whereas Nrf2 is induced in response to oxidative stress. Whereas HIF-1 regulates the expression of genes important for the adaptation of cells to hypoxia, Nrf2 induces antioxidative enzymes such as thioredoxin 1 (Trx1), exerting a cytoprotective role. Here, we investigated the regulation and cross talk of HIF-1 alpha and Nrf2 in intermittent hypoxia in lung adenocarcinoma A549 cells expressing high levels of the NADPH oxidase subunit NOX1. Whereas continuous hypoxia induced only HIF-1 alpha, intermittent hypoxia induced both HIF-1 alpha and Nrf2, including its target Trx1. NOX1 was determined to be crucial for enhanced ROS production in intermittent hypoxia that in turn mediated induction of Nrf2 and Trx1. The regulation of Nrf2 and Trx1 by NOX1 was confirmed by both inhibition of endogenous NOX1 and overexpression of recombinant NOX1 protein. By using a proteasomal inhibitor, NOX1 was demonstrated to activate Nrf2 by protein stabilization. Subsequently, Nrf2-dependent Trx1 induction turned out to enhance HIF-1 alpha signaling in intermittent hypoxia.

TIAR and TIA-1 mRNA-binding proteins co-aggregate under conditions of rapid oxygen decline and extreme hypoxia and suppress the HIF-1α pathway.

T-cell intracellular antigen (TIA)-1 and TIA-1-related protein (TIAR) are mRNA-binding proteins that can aggregate within granules under specific stress conditions. In this study, we analyzed TIAR/TIA-1 aggregation under different hypoxic conditions, and studied the effects on the hypoxia-inducible factor (HIF)-1α in different cancer cell lines. Under acute and pronounced hypoxic conditions TIAR/TIA-1 co-aggregated to granules and positive co-staining with eIF3η marker suggested these to represent stress granules. In parallel, HIF-1α expression was blocked in cells displaying TIAR/TIA-1 granules. Silencing of TIAR and TIA-1 caused upregulation of HIF-1α expression, as demonstrated by western blot, immunocytochemistry and HIF-1-dependent reporter gene expression. Additionally, a critical region of the 3' end of the untranslated HIF-1α mRNA with possible adenosine-uridine-rich elements (AREs) was coupled to the luciferase reporter gene, causing downregulation of expression. Employing this reporter construct, inhibition of TIAR by siRNA attenuated the inhibitory cis-effect of this ARE-sequence. Furthermore, immunohistochemical analysis of A549 cell tumor xenografts revealed a nearly complementary expression of HIF-1α and TIAR reflecting the control of HIF-1α expression by TIAR as revealed in the cell culture studies. In sum, rapid and severe hypoxia caused co-aggregation of TIAR/TIA-1 and these proteins suppressed HIF-1α expression.

NOX4 regulates ROS levels under normoxic and hypoxic conditions, triggers proliferation, and inhibits apoptosis in pulmonary artery adventitial fibroblasts.

The NADPH oxidases are involved in vascular remodeling processes and oxygen sensing. Hypoxia-induced pulmonary arterial remodeling results in thickening of the vessel wall and reduction of the area of vessel lumen, leading to pulmonary hypertension and cor pulmonale. The proliferation of pulmonary artery adventitial fibroblasts (PAFB) is critically involved in this process. In this study, we analyzed the role of the non-phagocytic NADPH oxidase subunits NOX1 and NOX4 in PAFB. NOX4 was predominantly expressed in comparison to NOX1 at mRNA levels. Under hypoxic conditions, NOX4 was significantly upregulated at mRNA and protein levels. Silencing of NOX4 by siRNA caused reduction of ROS levels under both normoxic and hypoxic (24 h) conditions and suppressed the significant hypoxic-induced ROS increase. PAFB proliferation was significantly decreased in cells transfected with NOX4 siRNA, whereas apoptosis was enhanced. Also, the expression of NOX4 was studied in PAFB isolated from the lungs of patients with idiopathic pulmonary arterial hypertension (IPAH). Interestingly, a significant increase of NOX4 mRNA expression was observed under hypoxic conditions in PAFB from the lungs with IPAH compared to healthy donors. In conclusion, NOX4 maintains ROS levels under normoxic and hypoxic conditions and enhances proliferation and inhibits apoptosis of PAFB.