The unfolded protein response protects human tumor cells during hypoxia through regulation of the autophagy genes MAP1LC3B and ATG5

TLDR

Tumor hypoxia impairs phenotype and therapy response, and cells adapt through mechanisms such as the unfolded protein response, which regulates autophagy‑related genes like MAP1LC3B and ATG5 that mediate autophagosome formation. The study investigates whether the unfolded protein response enhances autophagy in hypoxic tumor cells to promote their survival. The authors show that hypoxia activates PERK, which induces ATF4 and CHOP to upregulate MAP1LC3B and ATG5 transcription in cancer cell lines. The data demonstrate that UPR‑mediated induction of MAP1LC3B sustains autophagy during hypoxia, that PERK‑deficient cells cannot maintain MAP1LC3B and are depleted during hypoxia, that autophagy and MAP1LC3B are enriched in hypoxic tumor regions, and that inhibiting autophagy sensitizes tumors to hypoxia and radiation, indicating UPR‑driven autophagy confers treatment resistance.

Abstract

Tumor hypoxia is a common microenvironmental factor that adversely influences tumor phenotype and treatment response. Cellular adaptation to hypoxia occurs through multiple mechanisms, including activation of the unfolded protein response (UPR). Recent reports have indicated that hypoxia activates a lysosomal degradation pathway known as autophagy, and here we show that the UPR enhances the capacity of hypoxic tumor cells to carry out autophagy, and that this promotes their survival. In several human cancer cell lines, hypoxia increased transcription of the essential autophagy genes microtubule-associated protein 1 light chain 3β (MAP1LC3B) and autophagy-related gene 5 (ATG5) through the transcription factors ATF4 and CHOP, respectively, which are regulated by PKR-like ER kinase (PERK, also known as EIF2AK3). MAP1LC3B and ATG5 are not required for initiation of autophagy but mediate phagophore expansion and autophagosome formation. We observed that transcriptional induction of MAP1LC3B replenished MAP1LC3B protein that was turned over during extensive hypoxia-induced autophagy. Correspondingly, cells deficient in PERK signaling failed to transcriptionally induce MAP1LC3B and became rapidly depleted of MAP1LC3B protein during hypoxia. Consistent with these data, autophagy and MAP1LC3B induction occurred preferentially in hypoxic regions of human tumor xenografts. Furthermore, pharmacological inhibition of autophagy sensitized human tumor cells to hypoxia, reduced the fraction of viable hypoxic tumor cells, and sensitized xenografted human tumors to irradiation. Our data therefore demonstrate that the UPR is an important mediator of the hypoxic tumor microenvironment and that it contributes to resistance to treatment through its ability to facilitate autophagy.