Yu-Long Hu, Arman Jahangiri, Michael De Lay and Manish K. Aghi*Department of Neurosurgery; Diller Cancer Research Building; University of California at San Francisco (UCSF); San Francisco, CA USA
http://dx.doi.org/10.4161/auto.20232*Correspondence to: Manish K. Aghi;Email: [email protected]
Punctum to: Hu YL, Delay M, Jahangiri A,Molinaro AM, Rose SD, Carbonell WS, et al.Hypoxia-Induced Autophagy Promotes TumorCell Survival and Adaptation to AntiangiogenicTreatment in Glioblastoma. Cancer Res 2012;72:1773–83; PMID:22447568; http://dx.doi.org/10.1158/0008-5472.CAN-11-3831
While anti-angiogenic therapy wasinitially greeted enthusiastically by
the cancer community, initial successeswith this therapeutic modality weretempered by the failure of angiogenesisinhibitors to produce sustained clinicalresponses in most patients, with resis-tance to the inhibitors frequentlydeveloping. We recently reported thathypoxia increases after the devasculariza-tion caused by anti-angiogenic therapy,consistent with the goals of these ther-apies, but that some tumor cells becomeresistant and survive the hypoxic insultelicited by anti-angiogenic therapythrough autophagy by activating bothAMPK and HIF1A pathways. Thesefindings suggest that modulating theautophagy pathway may someday allowanti-angiogenic therapy to fulfill itstherapeutic potential. However, furtherwork will clearly be needed to developmore potent and specific autophagyinhibitors and to better understand theregulators of autophagy in malignant cells.
The hypothesis that tumor progression canbe curbed by anti-angiogenic agents tar-geting abnormal tumor blood vessels hasbeen confirmed by preclinical evidenceand evidence from clinical trials over thepast three decades. However, these initialsuccesses were tempered by the failure ofangiogenesis inhibitors to produce endur-ing clinical responses in most patients. Forexample, in clinical trials of the vascularendothelial growth factor (VEGF)neutralizing antibody bevacizumab inglioblastoma treatment, there were 40–60% rates of radiographic progression afterinitially successful treatment, consistent
with the development of acquired resis-tance to anti-angiogenic therapy, a statethat we have found exhibits a poorprognosis and poor response to availabletreatments. The molecular basis of theadaptive responses of tumors to anti-angiogenic treatments causing the lack ofsustained responses seen to date remainsundefined. We hypothesized that theregression of blood vessels caused byanti-angiogenic therapy increases tumorhypoxia, and that this hypoxia mediatesthe adaptive response to anti-angiogenictherapy.
Cellular stressors have long been knownto activate autophagy, a pathway in whichdouble-membrane vesicles form andengulf protein aggregates, cytoplasm andorganelles that are then delivered tolysosomes for degradation. Several cancertherapies, including DNA-damagingchemotherapeutics such as temozolomide,and radiation induce autophagy in cultureand animal models, and the autophagicresponse to many of these treatments iscytoprotective. Radiation therapy pro-motes autophagy by upregulating tran-scription of several autophagy essentialgenes such as BECN1, ATG3, ATG4,ATG5 and ATG12, with a survival-promoting effect confirmed by autophagyinhibition. Other studies have shown thatsome chemotherapy agents like histonedeacetylase (HDAC) inhibitors and cispla-tin induce autophagy by increasing pro-duction of reactive oxygen species (ROS)in mitochondria.
These observations reflecting autophagyas an adaptive response to radiationtherapy and conventional DNA damagingchemotherapy have been augmented byour recent finding that autophagy is an
AUTOPHAGIC PUNCTUM
Autophagy 8:6, 979–981; June 2012; G 2012 Landes Bioscience
Do not distribute.adaptive response to anti-angiogenic treat-ment. We found that the devascularizationcaused by anti-angiogenic therapyincreases tumor hypoxia, consistent withthe goals of these therapies, but that sometumor cells survive the hypoxic insultthrough autophagy by activating bothAMPK and HIF1A pathways (Fig. 1).
Our finding of hypoxia-induced auto-phagy in tumor cells as an adaptiveresponse to the hypoxia caused by anti-angiogenic therapy can be expanded todetermine the effect of hypoxia on cellsin the tumor microenvironment. Forexample, we have found hypoxia doesnot induce autophagy in endothelial cellsisolated from glioblastoma multiforme(GBM; unpublished data), consistent withour finding that the vessel density inGBMs resistant to anti-angiogenic therapywas suppressed and suggesting that tumorsgrow during anti-angiogenic therapywithout increased endothelial survival.
Furthermore, because hypoxia increasesthe cancer stem cell (CSC) population,one could hypothesize that hypoxia pro-motes autophagy in CSCs. Confirming thishypothesis would provide additional ratio-nale for autophagy inhibition to preventresistance to anti-angiogenic treatment.
The adaptive response of tumors toanti-angiogenic therapy may involveincreased tumor cell invasiveness. Furtherstudies will be needed to determinewhether cells surviving anti-angiogenictherapy through autophagy exhibitincreased invasiveness, as occurs in cellstreated with a chemical that inducesautophagy. Demonstration that tumorcells surviving anti-angiogenic therapythrough autophagy exhibit increased inva-siveness would suggest that autophagyinhibition could inhibit the invasionoccurring after anti-angiogenic therapy bydisrupting it at an earlier stage, which maybe more effective than targeting invasion
directly, as the numerous mediators ofinvasion make invasion difficult to phar-macologically disrupt.
Based on the preclinical evidence above,autophagy inhibition is currently beinginvestigated as a way of modulating theresponse to cancer therapies in patients.Currently, the only FDA-approved agentsable to inhibit autophagy are chloroquine,an antimalaria drug, and its derivativehydroxychloroquine, which block auto-phagy by disrupting lysosome/autolyso-some acidification. One notable completedstudy was a randomized trial combiningchloroquine with conventional treatmentfor glioblastoma with a benefit not quitesignificant. In addition, there are currently22 phase I/II cancer clinical trials involv-ing chloroquine or hydroxychloroquineopen nationwide (www.clinicaltrials.gov),including two combining hydroxychloro-quine with bevacizumab and conventionalDNA-damaging chemotherapy, results of
Figure 1. Simplified scheme of nonselective vs. selective autophagy, and how they might be affected in cancer cells by oncogenic pathways andstressors in the microenvironment such as the hypoxia triggered by anti-angiogenic therapy. Shown are regulators of nonselective vs. selectiveautophagy in tumor cells. Hypoxia, as caused by anti-angiogenic therapy, influences both nonselective and selective autophagy, with mechanisms moreclearly identified for the former. Abbreviations used: ROS, reactive oxygen species; HIF1A, hypoxia-inducible factor-1a; AMPK, AMP-activated proteinkinase; and EGLN1/PHD2, prolyl hydroxylase domain-containing protein 2.
which could support the preclinical datawe obtained showing a role for autophagyin resistance to anti-angiogenic therapy.
Despite these ongoing clinical efforts,the use of autophagy inhibition as atherapeutic strategy in cancer may needfurther preclinical evaluation to optimizethe chances of success. Challenges in usingautophagy inhibition as a therapeuticstrategy include: (1) recognizing the dualroles for autophagy in tumors—cytopro-tective or cytocidal depending on whetherthe tumor is in early or late stages ofoncogenesis and the type of tumor; and (2)recognizing functional autophagy statusin tumors, as some tumors may possessautophagy pathway defects, while otherswill have preserved autophagy capacity.Furthermore, based on the hypothesisthat tumor cells exhibit minimal basalsurvival-promoting autophagy, and thatautophagy may be most significant as an
adaptive response to anticancer therapies,autophagy inhibition will likely be ofminimal utility as a monotherapy.Therefore, the clinical trials of chloroquineand hydroxychloroquine to date have allcombined these agents with treatments,which induce autophagy as an adaptiveresponsive.
Additional preclinical work will also beneeded to develop autophagy inhibitorsother than chloroquine or hydroxychloro-quine. While preclinical studies like ourshave suggested that these agents disruptautophagy in animal models, other studieshave shown that the ability of chloroquineto potentiate the effects of autophagy-inducing chemotherapies may occurindependent of autophagy disruption.Furthermore, it has yet to be proven thatchloroquine or hydroxychloroquine effec-tively block autophagy in human tumors,or how the genetic makeup of these
tumors influences their susceptibility tothese agents. Should chloroquine orhydroxychloroquine ultimately prove tobe too nonspecific for clinical use asautophagy inhibitors, the development ofmore specific autophagy inhibitors willrequire focusing on kinases like ULK1/ATG1 and PIK3C3/VPS34, or proteaseslike ATG4 that specifically regulate theactivation of autophagy and autophago-some formation, with minimal intra-cellular roles outside of autophagy.
Acknowledgments
Work was supported by funding toM.K.A. from the American Brain TumorAssociation, the American Cancer Society,the James S. McDonnell Foundation,the NIH (5K02NS64167-2) and theUCSF Brain Tumor SPORE. A.J. isa Howard Hughes Medical InstituteResearch Fellow.
