Regulation and destabilization of HIF-1alpha by ARD1-mediated acetylation

Hypoxia-inducible factor 1 (HIF-1) plays a central role in cellular adaptation to changes in oxygen availability. Recently, prolyl hydroxylation was identified as a key regulatory event that targets the HIF-1alpha subunit for proteasomal degradation via the pVHL ubiquitination complex. In this report, we reveal an important function for ARD1 in mammalian cells as a protein acetyltransferase by direct binding to HIF-1alpha to regulate its stability. We present further evidence showing that ARD1-mediated acetylation enhances interaction of HIF-1alpha with pVHL and HIF-1alpha ubiquitination, suggesting that the acetylation of HIF-1alpha by ARD1 is critical to proteasomal degradation. Therefore, we have concluded that the role of ARD1 in the acetylation of HIF-1alpha provides a key regulatory mechanism underlying HIF-1alpha stability.     

HIF-1 and tumor progression: pathophysiology and therapeutics

Hypoxia-inducible factor 1 (HIF-1) controls oxygen delivery (via angiogenesis) and metabolic adaptation to hypoxia (via glycolysis). HIF-1 consists of a constitutively expressed HIF-1β subunit and an oxygen- and growth-factor-regulated HIF-1α subunit. In xenografts, tumor growth and angiogenesis are correlated with HIF-1 expression. In human cancers, HIF-1α is overexpressed as a result of intratumoral hypoxia and genetic alterations affecting key oncogenes and tumor suppressor genes. HIF-1α overexpression in biopsies of brain, breast, cervical, esophageal, oropharyngeal and ovarian cancers is correlated with treatment failure and mortality. Increased HIF-1 activity promotes tumor progression, and inhibition of HIF-1 could represent a novel approach to cancer therapy.     

Thioredoxin 1 and thioredoxin 2 have opposed regulatory functions on hypoxia-inducible factor-1alpha

Hypoxia inducible factor 1 (HIF-1), a key regulator for adaptation to hypoxia, is composed of HIF-1alpha and HIF-1beta. In this study, we present evidence that overexpression of mitochondria-located thioredoxin 2 (Trx2) attenuated hypoxia-evoked HIF-1alpha accumulation, whereas cytosolic thioredoxin 1 (Trx1) enhanced HIF-1alpha protein amount. Transactivation of HIF-1 is decreased by overexpression of Trx2 but stimulated by Trx1. Inhibition of proteasomal degradation of HIF-1alpha in Trx2-overexpressing cells did not fully restore HIF-1alpha protein levels, while HIF-1alpha accumulation was enhanced in Trx1-overexpressing cells. Reporter assays showed that cap-dependent translation is increased by Trx1 and decreased by Trx2, whereas HIF-1alpha mRNA levels remained unaltered. These data suggest that thioredoxins affect the synthesis of HIF-1alpha. Trx1 facilitated synthesis of HIF-1alpha by activating Akt, p70S6K, and eIF-4E, known to control cap-dependent translation. In contrast, Trx2 attenuated activities of Akt, p70S6K, and eIF-4E and provoked an increase in mitochondrial reactive oxygen species production. MitoQ, a mitochondria specific antioxidant, reversed HIF-1alpha accumulation as well as Akt activation under hypoxia in Trx2 cells, supporting the notion of translation control mechanisms in affecting HIF-1alpha protein accumulation.