Hypoxia-inducible factor 1α (HIF-1α) is rapidly degraded by the ubiquitin-proteasome pathway under normoxic conditions. Ubiquitination of HIF-1α is mediated by interaction with von Hippel-Lindau tumor suppressor protein (pVHL). In our previous report, we found that hypoxia-induced active signal transducer and activator of transcription3 (STAT3) accelerated the accumulation of HIF-1α protein and prolonged its half-life in solid tumor cells. However, its specific mechanisms are not fully understood. Thus, we examined the role of STAT3 in the mechanism of pVHL-mediated HIF-1α stability. We found that STAT3 interacts with C-terminal domain of HIF-1α and stabilizes HIF-1α by inhibition of pVHL binding to HIF-1α. The binding between HIF-1α and pVHL, negative regulator of HIF-1α stability, was interfered dose-dependently by overexpressed constitutive active STAT3. Moreover, we found that the enhanced HIF-1α protein levels by active STAT3 are due to decrease of poly-ubiquitination of HIF-1α protein via inhibition of interaction between pVHL and HIF-1α. Taken together, our results suggest that STAT3 decreases the pVHL-mediated ubiquitination of HIF-1α through competition with pVHL for binding to HIF-1α, and then stabilizes HIF-1α protein levels.

We examined the effects of acute hypoxia on Ba²⁺-sensitive inward rectifier K⁺(K_IR) current in rabbit coronary arterial smooth muscle cells. The amplitudes of K_IRcurrent was definitely higher in the cells from small-diameter (200 μm, LCASMC, −1.5 ± 0.1 pA pF⁻¹). Western blot analysis confirmed that Kir2.1 protein was expressed in SCASMC but not LCASMC. Hypoxia activated much more K_IRcurrents in symmetrical 140 K⁺. This effect was blocked by the adenylyl cyclase inhibitor SQ-22536 (10 μM) and mimicked by forskolin (10 μM) and dibutyryl-cAMP (500 μM). The production of cAMP in SCASMC increased 5.7-fold after 6 min of hypoxia. Hypoxia-induced increase in K_IRcurrents was abolished by the PKA inhibitors, Rp-8-(4-chlorophenylthio)-cAMPs (10 μM) and KT-5720 (1 μM). The inhibition of G protein with GDPβS (1 mM) partially reduced (∼50%) the hypoxia-induced increase in K_IRcurrents. In Langendorff-perfused rabbit hearts, hypoxia increased coronary blood flow, an effect that was inhibited by Ba²⁺. In summary, hypoxia augments the K_IRcurrents in SCASMC via cAMP- and PKA-dependent signaling cascades, which might, at least partly, explain the hypoxia-induced coronary vasodilation.

The endoplasmic reticulum (ER) is not only a home for folding and posttranslational modifications of secretory proteins but also a reservoir for intracellular Ca²⁺. Perturbation of ER homeostasis contributes to the pathogenesis of various neurodegenerative diseases, such as Alzheimer’s and Parkinson diseases. One key regulator that underlies cell survival and Ca²⁺ homeostasis during ER stress responses is inositol-requiring enzyme 1α (IRE1α). Despite extensive studies on this ER membrane-associated protein, little is known about the molecular mechanisms by which excessive ER stress triggers cell death and Ca²⁺ dysregulation via the IRE1α-dependent signaling pathway. In this study, we show that inactivation of IRE1α by RNA interference increases cytosolic Ca²⁺ concentration in SH-SY5Y cells, leading to cell death. This dysregulation is caused by an accelerated ER-to-cytosolic efflux of Ca²⁺ through the InsP3 receptor (InsP3R). The Ca²⁺ efflux in IRE1α-deficient cells correlates with dissociation of the Ca²⁺-binding InsP3R inhibitor CIB1 and increased complex formation of CIB1 with the pro-apoptotic kinase ASK1, which otherwise remains inactivated in the IRE1α–TRAF2–ASK1 complex. The increased cytosolic concentration of Ca²⁺ induces mitochondrial production of reactive oxygen species (ROS), in particular superoxide, resulting in severe mitochondrial abnormalities, such as fragmentation and depolarization of membrane potential. These Ca²⁺ dysregulation-induced mitochondrial abnormalities and cell death in IRE1α-deficient cells can be blocked by depleting ROS or inhibiting Ca²⁺ influx into the mitochondria. These results demonstrate the importance of IRE1α in Ca²⁺ homeostasis and cell survival during ER stress and reveal a previously unknown Ca²⁺-mediated cell death signaling between the IRE1α–InsP3R pathway in the ER and the redox-dependent apoptotic pathway in the mitochondrion.

Aberrant membrane transport caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene is associated with a wide spectrum of respiratory and digestive diseases as well as cystic fibrosis. Using a gene scanning method, we found 11 polymorphisms and mutations of the CFTR gene in the Korean population. Individual variants at these sites were analyzed by conventional DNA screening in 117 control and 75 patients having bronchiectasis or chronic pancreatitis. In a haplotype determination based on a Bayesian algorithm, 15 haplotypes were assembled in the 192 individuals tested. Several haplotypes, especially with Q1352H, IVS8 T ₅ , and E217G, were found to have disease associations in a case–control study. Notably, a common polymorphism of M470V appears to affect the intensity of the disease association. Among the two haplotypes having IVS8 T ₅ , the T ₅ -V470 haplotype showed higher disease association than the T ₅ -M470 haplotype. In addition, a Q1352H mutation found in a V470 background showed the strongest disease association. The physiological significances of the identified mutations were rigorously analyzed. Non-synonymous E217G and Q1352H mutations in the M470 background caused a 60–80% reduction in CFTR-dependent Cl ⁻ currents and HCO ₃⁻ -transport activities. Surprisingly, the additional M470V polymorphic variant with the Q1352H mutation completely abolished CFTR-dependent anion transport activities. These findings provide the first evidence on the importance of CFTR mutations in the Asian population. Importantly, the results also reveal that interactions between multiple genetic variants in cis affect the final function of the gene products.