The unfolded protein response protects human tumor cells during hypoxia through regulation of the autophagy genes MAP1LC3B and ATG5.
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 3beta (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.
Rouschop
Kasper M A
KM
Department of Radiation Oncology (Maastro Lab), GROW School for Oncology and Developmental Biology, Maastricht University,Maastricht, The Netherlands.
van den Beucken
Twan
T
Dubois
Ludwig
L
Niessen
Hanneke
H
Bussink
Johan
J
Savelkouls
Kim
K
Keulers
Tom
T
Mujcic
Hilda
H
Landuyt
Willy
W
Voncken
Jan Willem
JW
Lambin
Philippe
P
van der Kogel
Albert J
AJ
Koritzinsky
Marianne
M
Wouters
Bradly G
BG
D051701:Activating Transcription Factor 4 / Q000502:physiology; D000818:Animals; D001343:Autophagy; D000071187:Autophagy-Related Protein 5; D015687:Cell Hypoxia; D045744:Cell Line, Tumor; D002738:Chloroquine / Q000494:pharmacology; D005260:Female; D006801:Humans; D051379:Mice; D008807:Mice, Inbred BALB C; D008869:Microtubule-Associated Proteins / Q000235:genetics*; D009369:Neoplasms / Q000378:metabolism*; D012333:RNA, Messenger / Q000032:analysis; D051742:Transcription Factor CHOP / Q000502:physiology; D056811:Unfolded Protein Response; D019892:eIF-2 Kinase / Q000502:physiology
D051701:Activating Transcription Factor 4; D000071187:Autophagy-Related Protein 5; D002738:Chloroquine; D008869:Microtubule-Associated Proteins; D012333:RNA, Messenger; D051742:Transcription Factor CHOP; D019892:eIF-2 Kinase; C497109:ATF4 protein, human; C502778:ATG5 protein, human; C497249:DDIT3 protein, human; C502070:MAP1LC3B protein, human