Novel Regulation of ATF4 and HIF-1alpha (alpha) Transcription Factors in Response to Hypoxia

Hypoxic regions of tumors are resistant to treatment with radiotherapies and chemotherapies. The hypoxic resistance is due, in part, to changes in gene expression. Two distinct pathways have been identified that promote cellular survival during periods of hypoxia. The first involves the induction of the hypoxia inducible factors (HIFs), and the second is the activation of the unfolded protein response (UPR). There are two hypoxia induced transcription factors (HIF-1 and HIF-2) which activate common as well as unique target genes; however, the role of the HIFs is gene regulation is not fully understood. During acute hypoxia, HIF-1alpha and HIF-2alpha are induced and HIF-1alpha is the dominant transcription factor; however, during prolonged hypoxia, HIF-1alpha protein is lost and HIF-2alpha protein is maintained. The reduction in HIF-1alpha is a result of the loss of its mRNA through destabilization of the message which occurs in part by the induction of the antisense HIF RNA (aHIF). aHIF is part of a negative feedback loop which destabilizes the HIF-1alpha mRNA reducing the expression of the HIF-1alpha protein. These findings indicate that during prolonged hypoxia HIF-2alpha is key in maintaining the malignant progression of solid tumors. In addition, the UPR is preferentially activated during prolonged hypoxia. The UPR promotes global translational repression to prevent protein overload in the ER while simultaneously promoting the induction of key targets such as Activating Transcription Factor 4 (ATF4). The UPR has dueling functions in that it promotes cell survival by inducing chaperones and other proteins that restore ER homeostasis, while prolonged activation results in apoptosis through the induction of the pro-apoptotic protein CHOP. We found that prolonged hypoxia does not translationally activate the ATF4 mRNA, but there is an induction of the ATF4 mRNA that results in a slight induction of the protein. The inability to translationally activate the ATF4 mRNA prevents robust ATF4 induction thereby inhibiting the detrimental effects of prolonged UPR activation. This data strongly suggests targeting both HIF-1alpha and HIF-2alpha in combination with the UPR will he most effective in killing hypoxic tumor cells, since these pathways work in conjunction to provide growth advantages to hypoxic tumors.