Differential Dependence of Hypoxia-inducible Factors 1α and 2α on mTORC1 and mTORC2

Abstract

Constitutive expression of hypoxia-inducible factor (HIF) has been implicated in several proliferative disorders. Constitutive expression of HIF1␣ and HIF2␣ has been linked to a number of human cancers, especially renal cell carcinoma (RCC), in which HIF2␣ expression is the more important contributor. Expression of HIF1␣ is dependent on the mammalian target of rapamycin (mTOR) and is sensitive to rapamycin. In contrast, there have been no reports linking HIF2␣ expression with mTOR. mTOR exists in two complexes, mTORC1 and mTORC2, which are differentially sensitive to rapamycin. We report here that although there are clear differences in the sensitivity of HIF1␣ and HIF2␣ to rapamycin, both HIF1␣ and HIF2␣ expression is dependent on mTOR. HIF1␣ expression was dependent on both Raptor (a constituent of mTORC1) and Rictor (a constitutive of mTORC2). In contrast, HIF2␣ was dependent only on the mTORC2 constituent Rictor. These data indicate that although HIF1␣ is dependent on both mTORC1 and mTORC2, HIF2␣ is dependent only on mTORC2. We also examined the dependence of HIF␣ expression on the mTORC2 substrate Akt, which exists as three different isoforms, Akt1, Akt2, and Akt3. Interestingly, the expression of HIF2␣ was dependent on Akt2, whereas that of HIF1␣ was dependent on Akt3. Because HIF2␣ is apparently more critical in RCC, this study underscores the importance of targeting mTORC2 and perhaps Akt2 signaling in RCC and other proliferative disorders in which HIF2␣ has been implicated. Hypoxia-inducible factor (HIF)2 is a critical transcriptional regulator of cellular responses to a variety of stressful conditions (1, 2). Under non-stressful conditions, HIF␣ is ubiquitinated by the von Hippel-Lindau (VHL) gene product pVHL, a substrate-conferring component of a ubiquitin-protein isopeptide ligase that targets HIF␣ for degradation by the proteasome (3). Loss of the VHL gene results in a variety of pathologies, most significantly renal cell carcinoma (RCC) (4 -6). In the absence of pVHL, there is an up-regulation of HIF␣, and elevated expression of HIF␣ has been strongly implicated in VHL disease and RCC (4 -6). HIF␣ dimerizes with HIF␤ to form a transcription factor HIF that stimulates the transcription of genes that regulate angiogenesis and other factors important for responding to hypoxic and other stressful conditions such as vascular endothelial growth factor and glycolytic enzymes (2,7,8). There are several distinct ␣-subunits, but it is the expression of HIF1␣ and HIF2␣ that is most frequently elevated in human cancers (4, 9). Whereas HIF1␣ has both pro-and anti-proliferative properties, HIF2␣ lacks the anti-proliferative properties and is more strongly implicated in tumorigenesis (10). The somewhat overlapping and antagonistic effects of HIF1␣ and HIF2␣ are poorly understood, but it is clear that in RCC, HIF2␣ is a critical factor in that suppression of HIF2␣ blocks tumor formation by renal cancer cells (11,12). It is believed that the elevated expression of HIF2␣ contributes to the survival signals in rena...

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