HER-2, Notch, and Breast Cancer Stem Cells:Targeting anAxis of Evil55Commentary onMagnifico et al., p. 2010

Hasan Korkaya and Max S.Wicha

Increasing evidence indicates that tumor-initiating (cancer stem) cells may contribute to treatmentresistance and relapse, suggesting that improved clinical outcome will require effective targetingof this cell population. Recent studies suggest that the remarkable clinical efficacy of trastuzumabmay relate to its ability to target cancer stem cell populations.

Several recent studies, including that of Magnifico andcoworkers (1) in this issue of Clinical Cancer Research , suggestthat the remarkable clinical efficacy of trastuzumab may relateto its ability to target breast cancer stem cells. These studies haveimportant clinical implications for patient selection as well asfor the development of new therapeutic strategies to target thecancer stem cell population.

The HER2 gene is amplified in approximately 20% of humanbreast cancers in which it is associated with aggressive diseaseand early development of metastasis (2). One of the greatestadvancements in the treatment of breast cancer has been thedevelopment of trastuzumab to target HER2-positive disease.When added to chemotherapy, trastuzumab significantlyincreases both disease-free survival as well as overall survivalin women with metastatic breast cancer. The clinical utility oftrastuzumab has been even more remarkable in the adjuvantsetting. The addition of trastuzumab to cytotoxic chemotherapyhas reduced the recurrence rate by f50% in women whosebreast cancers display HER2 amplification. Furthermore, thenatural history of HER2-amplified breast cancer as well as theflattening survival curves suggest that a considerable proportionof these patients may be cured of their disease. Although itis clear that HER2 plays an important role in breast tumori-genesis, the molecular mechanisms which account for theclinical benefit of HER2 inhibition remain unknown.

There is emerging evidence that many tumors, includingbreast cancer, may be initiated and maintained by asubpopulation of cells that maintain or acquire stem cellcharacteristics (3). These tumor stem cells may also contrib-ute to tumor metastasis as well as treatment resistance.Indeed, both in vitro and mouse models have providedevidence for the relative resistance of breast cancer stem cellsto chemotherapy and radiation therapy (4, 5). Recent

neoadjuvant chemotherapy trials showing an increase in cellsexpressing stem cell markers following neoadjuvant chemo-therapy support this concept (6).

Magnifico and colleagues used several HER2-overexpressingbreast cancer cell lines to show an important role for HER2 inmaintaining the cancer stem cell population. They show thatwithin each cell line, cells displaying stem cell properties suchas sphere formation (1) or increased aldehyde dehydrogenaseexpression also have increased HER2 expression compared withthe bulk cell population (1). This suggests that in addition togenetic alterations such as amplification, expression of HER2is also regulated by epigenetic factors independent of geneamplification. Furthermore, they show that the HER2 inhibitortrastuzumab or the combined HER2 and epidermal growthfactor inhibitor lapatinib are able to specifically target thisHER2-overexpressing cancer stem cell population (1). Thiswas shown in vitro as well as by transplantation into nudemice. These studies are consistent with and extend our recentreport that HER2 amplification increases the cancer stem cellpopulation driving tumorigenesis as well as invasion andmetastasis (7).

It has previously been shown that the Notch pathway playsan important role in the function of normal and malignantbreast cancer stem cells (8). A relationship between Notchand HER2 signaling has been suggested by the demonstrationthat the HER2 promoter contains Notch-binding sequences(9). Magnifico and coworkers (1) further demonstrate arelationship between Notch and HER2 expression by showingthat HER2-overexpressing cells display activated Notchsignaling. In contrast, inhibition of Notch signaling using asmall interfering RNA or a g-secretase inhibitor results indown-regulation of HER2 expression resulting in decreasedsphere formation. These studies show important interactionsbetween the Notch and HER2 pathways, both of which areinvolved in the regulation of cancer stem cells. Theinteraction of these pathways is illustrated in Fig. 1.

The study of Magnifico and colleagues (1) and othersrelating to the regulation of breast cancer stem cells haveimportant clinical implications. These studies provide abiological explanation for the observation that lapatinibwas able to reduce the cancer stem cell populationfollowing neoadjuvant chemotherapy (6). An importantquestion raised by these studies is the clinical significance

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Authors’Affiliation: University of Michigan Comprehensive Cancer Center, AnnArbor, MichiganReceived 12/9/08; accepted 12/22/08; published OnlineFirst 3/10/09.Requests for reprints: Max S.Wicha, Department of Internal Medicine/Oncology,Comprehensive Cancer Center, University of Michigan, 1500 East Medical CenterDrive, Ann Arbor, MI 48109-0015. Phone: 734-647-9923; Fax: 734-647-9480;E-mail: mwicha@med.umich.edu.

F2009 American Association for Cancer Research.doi:10.1158/1078-0432.CCR-08-3087

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of HER2 overexpression in the absence of gene amplifica-tion. Tumors with this phenotype may be heterogeneousfor HER2 protein expression as detected by immunochem-istry (10). More importantly, the ability of HER2 targetingagents such as transtuzumab to target cancer stem cells intumors without HER2 amplification suggests that suchagents could potentially have clinical benefit in patientswith breast cancer with non–HER2-amplified tumors.Indeed, clinical discrepancies in the utility of HER2inhibitors when used in the metastatic or adjuvant settingmay reflect targeting of the cancer stem cell versus bulk cellpopulations. There is evidence in a number of studies thatclinical benefit from trastuzumab or lapatinib is directlyrelated to the levels of HER2 expression and these benefitsare found primarily in women with HER2-amplified tumors(11). These studies, which combine chemotherapy withHER2 inhibition, usually measure tumor shrinkage asassessed by Response Evaluation Criteria in Solid Tumors(RECIST) guidelines or relapse-free survival. These resultshave led to the widespread use of HER2 testing with theuse of immunohistochemistry and fluorescence in situhybridization to select patients for anti-HER2 therapy. Inthe adjuvant setting, the use of trastuzumab and lapatinibhas largely been limited to women whose tumors displayHER2 amplification. Recent reports, however, suggest thatwomen with HER2-negative breast cancer who did notmeet established criteria for HER2-positive disease oncentral review received as much clinical benefit fromadjuvant transtuzumab as those women with HER2-ampli-fied tumors (6). Although these discrepancies have been

attributed to HER2 testing errors (11), consideration of therole of HER2 in regulating breast cancer stem cells providesan alternative biological explanation. In metastatic disease,the clinical end points of tumor regression or time totumor progression may reflect changes in bulk cellpopulations. The efficacy of transtuzumab or lapatinib inthis setting may reflect the overexpression of HER2 in bothcancer stem cells and bulk cell populations. In contrast, inthe adjuvant setting, tumor recurrence may be driven bythe cancer stem cell compartment. This compartment inturn may be driven by pathways such as Notch that donot depend on HER2 amplification. This could explain thebenefit of HER2 inhibition in the adjuvant setting inpatients whose tumors do not display HER2 amplification.It would be interesting to determine whether these tumorsdisplay Notch activation, which has been reported to occurin as many as 40% of human breast cancers (12). In thesepatients, inhibition of Notch signaling in addition to HER2blockade represents a rational therapeutic strategy. Theseconcepts may be tested in future trials as g-secretaseinhibitors that inhibit Notch signaling are currently inclinical development. These and other strategies designed totarget cancer stem cells will determine whether theelimination of these cells improves patient outcomes.

Disclosure of Potential Conflicts of Interest

M.S.Wicha holds equity in and is a consultant for OncoMed Pharmaceuticals.

Fig. 1. Notch and HER2 signalingpathways driving self-renewal. Activationand subsequent nuclear localization ofNotch intracellular domain induces thetranscription of target genes includingHER2. Increased levels of HER2, in turn,activate the PI3K/Akt pathway that drivesstem cell self-renewal.

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Targeting an Axis of Evil

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