Inhibition of the autocrine IL-6–JAK2–STAT3–calprotectin axis as targeted therapy for HR−/HER2+ breast cancers

HER2/ERBB2 is a receptor tyrosine kinase found overexpressed in 15%–20% of breast tumors (HER2+ tumors) (Hynes and Lane 2005). It belongs to a family of four receptors (EGFR/HER1, HER2, HER3, and HER4) involved in cell signaling networks regulating cell growth, survival, and differentiation (Hynes and Lane 2005; Lemmon and Schlessinger 2010). In general, EGFR/HER receptors are activated through dimerization, which is promoted by the binding of cognate growth factors. However, HER2 is a unique member of the family and does not rely on direct ligand binding for activation; instead, it is activated through heterodimerization with other EGFR/HER family members that are ligand-bound (Citri and Yarden 2006). When aberrantly overexpressed, HER2 is able to homodimerize and initiate proliferation and prosurvival signal transduction in a ligand-independent fashion (Citri and Yarden 2006). Despite the existence of several HER2-specific targeted therapies that have drastically improved the treatment of HER2+ patients (trastuzumab, lapatinib, TDM1, and pertuzumab), tumor progression remains transient (Hynes and Lane 2005; Tagliabue et al. 2010). Thus, there is a need to find complementary, therapeutic targets that hold the potential for more effective treatments. Although, HER2+ tumors are commonly considered as a single entity, there is increasing evidence indicating that important intrinsic differences associated with hormone receptor (HR) status exist. Each of the two groups, HR+ and HR−, represents about half of all HER2+ breast cancers. Compared with HR+/HER2+, HR−/HER2+ tumors present worse histopathological characteristics (larger size, lymph node involvement, higher American Joint Committee on Cancer [AJCC] stage, and higher histological grade) (Vaz-Luis et al. 2012). HR+/HER2+ tumors preferentially recur in bones, while there is a strong trend for more visceral metastases in the HR−/HER2+ cancers (Paluch-Shimon et al. 2009; Vaz-Luis et al. 2012). Moreover, despite a higher rate of pathologic complete responses (pCRs) to neoadjuvant chemotherapy (Hurley et al. 2006; Vaz-Luis et al. 2012), HR−/HER2+ patients still have an increased risk of death within 5 years of diagnosis (Vaz-Luis et al. 2012). Intrinsic differences between HR−/HER2+ and HR+/HER2+ breast cancers are also found at the molecular level, as highlighted by unsupervised cluster analysis of gene expression profiles. The latter clearly identifies two distinct HER2+ subtypes. Most tumors clinically classified as HR+/HER2+ fall in the luminal B subtype, while most HR−/HER2+ tumors are part of the HER2-enriched subtype (Perou et al. 2000; Sotiriou and Pusztai 2009). Indeed, while HR+/HER2+ patients benefit from anti-hormonal and HER2 targeted therapies (Kaufman et al. 2009), the outcome for HR−/HER2+ patients strongly depends on their response to chemotherapy as well as anti-HER2 therapy. Thus, to identify genes that represent novel mechanistic dependencies in HR−/HER2+ breast cancer cells, we designed an integrative approach that combines functional genomic (RNAi screens) (Luo et al. 2008; Silva et al. 2008) and computational (Basso et al. 2005; Margolin et al. 2006; Carro et al. 2010; Lefebvre et al. 2010; Piovan et al. 2013) algorithms. Our integrative analysis identified STAT3 as a de novo master regulator (MR) gene associated with HER2-mediated transformation in HR− breast cancer cells. Importantly, we demonstrate that aberrant STAT3 activity is necessary to maintain the HR−/HER2+ tumor state, thus representing a nononcogene dependency in these tumors. Mechanistically, we found that HR−/HER2+ breast tumors secrete high levels of interleukin-6 (IL-6). This autocrine mechanism induces the activation of STAT3 via the canonical Janus kinase 2 (JAK2)/STAT3 pathway. Aberrant STAT3 activity induces up-regulation and secretion of the S100A8/9 complex (calprotectin), thus triggering a second autocrine stimulus that enhances proliferation and survival. As a result, disruption of the “IL-6–JAK2–STAT3–S100A8/9 cascade” compromises HR−/HER2+ cell viability, suggesting that the components of this pathway represent putative therapeutic targets in HR−/HER2 tumors. Importantly, small molecule inhibitors and blocking antibodies for components of this double autocrine loop are already FDA-approved or in clinical trials. Here, we demonstrate that blocking the IL-6 receptor (IL-6R) with the humanized monoclonal antibody tocilizumab (Patel and Moreland 2010; Navarro-Millan et al. 2012), STAT3 inactivation with the dual JAK1/2 inhibitor ruxolitinib (Mascarenhas and Hoffman 2012; Mesa et al. 2012), or calprotectin inhibition with the small molecule inhibitor tasquinimod (Dalrymple et al. 2007, 2012), alone or in combination with anti-HER2 therapies, compromises the viability of HR−/HER2+ breast cancer cells. The availability of FDA-approved inhibitors to target this novel mechanism represents an exciting opportunity for rapid translation of these findings to the clinics.