Halting ErbB-2 isoforms retrograde transport to the nucleus as a new theragnostic approach for triple-negative breast cancer

Triple-negative breast cancer (TNBC) is clinically defined by the absence of estrogen and progesterone receptors and the lack of membrane overexpression or gene amplification of receptor tyrosine kinase ErbB-2/HER2. Due to TNBC heterogeneity, clinical biomarkers and targeted therapies for this disease remain elusive. We demonstrated that ErbB-2 is localized in the nucleus (NErbB-2) of TNBC cells and primary tumors, from where it drives growth. We also discovered that TNBC expresses both wild-type ErbB-2 (WTErbB-2) and alternative ErbB-2 isoform c (ErbB-2c). Here, we revealed that the inhibitors of the retrograde transport Retro-2 and its cyclic derivative Retro-2.1 evict both WTErbB-2 and ErbB-2c from the nucleus of BC cells and tumors. Using BC cells from several molecular subtypes, as well as normal breast cells, we demonstrated that Retro-2 specifically blocks proliferation of BC cells expressing NErbB-2. Importantly, Retro-2 eviction of both ErbB-2 isoforms from the nucleus resulted in a striking growth abrogation in multiple TNBC preclinical models, including tumor explants and xenografts. Our mechanistic studies in TNBC cells revealed that Retro-2 induces a differential accumulation of WTErbB-2 at the early endosomes and the plasma membrane, and of ErbB-2c at the Golgi, shedding new light both on Retro-2 action on endogenous protein cargoes undergoing retrograde transport, and on the biology of ErbB-2 splicing variants. In addition, we revealed that the presence of a functional signal peptide and a nuclear export signal (NES), both located at the N-terminus of WTErbB-2, and absent in ErbB-2c, accounts for the differential subcellular distribution of ErbB-2 isoforms upon Retro-2 treatment. Our present discoveries provide evidence for the rational repurposing of Retro-2 as a novel therapeutic agent for TNBC.

Nuclear ErbB-2: a Novel Therapeutic Target in ErbB-2-Positive Breast Cancer?

Membrane overexpression of ErbB-2 (MErbB-2), a member of the ErbB family of receptor tyrosine kinases, occurs in 15-20% of breast cancers (BC) and constitutes a therapeutic target in this BC subtype (ErbB-2-positive). Although MErbB-2-targeted therapies have significantly improved patients' clinical outcome, resistance to available drugs is still a major issue in the clinic. Lack of accurate biomarkers for predicting responses to anti-ErbB-2 drugs at the time of diagnosis is also an important unresolved issue. Hence, a better understanding of the ErbB-2 signaling pathway constitutes a critical task in the battle against BC. In its canonical mechanism of action, MErbB-2 activates downstream signaling pathways, which transduce its proliferative effects in BC. The dogma of ErbB-2 mechanism of action has been challenged by the demonstration that MErbB-2 migrates to the nucleus, where it acts as a transcriptional regulator. Accumulating findings demonstrate that nuclear ErbB-2 (NErbB-2) is involved in BC growth and metastasis. Emerging evidence also reveal a role of NErbB-2 in the response to available anti-MErbB-2 agents. Here, we will review NErbB-2 function in BC and will particularly discuss the role of NErbB-2 as a novel target for therapy in ErbB-2-positive BC.

ErbB-2 nuclear function in breast cancer growth, metastasis and resistance to therapy

Approximately 15-20% of breast cancers (BC) show either membrane overexpression of ErbB-2 (MErbB-2), a member of the ErbBs family of receptor tyrosine kinases, or ERBB2 gene amplification. Until the development of MErbB-2-targeted therapies, this BC subtype, called ErbB-2-positive, was associated with increased metastatic potential and poor prognosis. Although these therapies have significantly improved overall survival and cure rates, resistance to available drugs is still a major clinical issue. In its classical mechanism, MErbB-2 activates downstream signaling cascades, which transduce its effects in BC. The fact that ErbB-2 is also present in the nucleus of BC cells was discovered over twenty years ago. Also, compelling evidence revealed a non-canonical function of nuclear ErbB-2 as a transcriptional regulator. As a deeper understanding of nuclear ErbB-2 actions would be crucial to the disclosure of its role as a biomarker and a target of therapy in BC, we will here review its function in BC, in particular, its role in growth, metastatic spreading and response to currently available MErbB-2-positive BC therapies.

Abstract 2265: Stat3 and ErbB-2 interaction in breast cancer metastasis

Metastasis is a complex multistep process, responsible for as much as 90% of cancer-related deaths, yet obtaining successful treatment for these patients remains an elusive challenge. It has long been recognized that the tyrosine kinase receptor ErbB-2 and the signal transducer and activator of transcription 3 (Stat3), two major players in the breast cancer (BC) scenario, are involved in BC metastatic dissemination through a mechanism where Stat3 acts as a downstream effector of ErbB-2 action. In addition, we and others have also disclosed the role of nuclear ErbB-2 (NErbB-2) in BC, whose presence we identified as a poor prognostic factor in MErbB-2-positive tumors. On the other hand, microRNAs are short non-coding endogenous RNAs with regulatory functions. In particular, high levels of microRNA-21 (miR-21), a well-known oncomiR, have been reported to actively promote invasion and metastasis in BC cell lines and tissues. Here, we describe a novel hierarchical interaction between Stat3, ErbB-2 and miR-21, underlying the metastatic phenotype of ErbB-2-positive BC. We disclosed that Stat3 acts as an upstream regulator of ErbB-2 expression and function. In a panel of cell lines corresponding to different BC subtypes we found that Stat3 induced ErbB-2 expression at the transcriptional level through its recruitment to response elements (called GAS) at the ErbB-2 promoter. Furthermore, we demonstrated that Stat3 co-opted NErbB-2 function, recruiting it as a coactivator, to assemble a transcriptional complex at the GAS sites of the miR-21 promoter, leading to miR-21 up-regulation. We showed that the increase in miR-21 levels resulted in the downregulation of the metastasis suppressor protein PDCD4, a well known miR-21 target. In order to assess the physiological relevance of our molecular findings, we developed an in vivo model of ErbB-2-overexpressing metastatic BC, in which Stat3 activation was inhibited by transfection with a Stat3 dominant negative variant (Stat3Y705F) or its expression was silenced by siRNAs. We demonstrated through reconstitution assays that ErbB-2 and miR-21 were necessary downstream mediators of Stat3-induced metastases development. Furthermore, we explored the clinical significance of our findings in a cohort of ErbB-2-positive primary invasive BC patients and demonstrated that Stat3 and ErbB-2 nuclear co-expression was associated with low PDCD4 expression levels, and that this correlated with the presence of nodal metastases. Our present results in experimental models and in the clinic shed light on the molecular mechanisms underlying BC metastasis and highlight targeting either Stat3 or NErbB-2 as novel therapeutic strategies for ErbB-2-positive BC patients.

#: 15-A-2849-AACR

Citation Format: Leandro Venturutti, Lucía V. Romero, Alejandro J. Urtreger, María F. Chervo, María F. Mercogliano, Rosalía I. Cordo Russo, Matías G. Pereyra, Gloria Inurrigarro, María C. Díaz Flaqué, Victoria Sunblad, Juan C. Roa, Pablo Guzmán, Elisa Bal de Kier-Joffe, Eduardo H. Charreau, Roxana Schillaci, Patricia V. Elizalde. Stat3 and ErbB-2 interaction in breast cancer metastasis. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2265. doi:10.1158/1538-7445.AM2015-2265

PI3K/Akt inhibition modulates multidrug resistance and activates NF-kappaB in murine lymphoma cell lines

Upregulation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway has been described in some tumors related to multidrug resistance (MDR). The aim of this work was to analyze the relationship between PI3K/Akt, MDR and NF-kappaB in murine lymphoma cell lines resistant to vincristine (LBR-V160) and doxorubicin (LBR-D160) as well as in the sensitive line (LBR-). PI3K/Akt activity, analyzed by phosphatidylinositol trisphosphate production and phosphorylated Akt (p-Akt) expression, was higher in the resistant cell lines than in the sensitive one and inhibition with wortmannin or LY294002 improved apoptosis in the resistant cell lines. Vincristine but not doxorubicin increased p-Akt expression whereas co-treatment with PI3K inhibitors and vincristine increased apoptosis in the three cell lines. Wortmannin and LY294002 inhibited P-glycoprotein (Pgp) function and also increased NF-kappaB activity. We concluded that the PI3K/Akt pathway is involved in MDR in lymphoma cell lines and PI3K/Akt inhibition correlates down-regulation of NF-kappaB activity and inhibition Pgp function.

Hyaluronan oligosaccharides sensitize lymphoma resistant cell lines to vincristine by modulating P-glycoprotein activity and PI3K/Akt pathway

Multidrug resistance (MDR) is one of the main reasons for failure of cancer therapy. It may be mediated by overexpression of ATP-dependent efflux pumps or by alterations in survival or apoptotic pathways. Fragments generated by enzymatic degradation of hyaluronan (oHA) were able to modulate growth and cell survival and sensitize MDR breast cancer cells to cytotoxic drugs. In this work the relationship between oHA and MDR in lymphoid malignancies was analyzed using murine lymphoma cell lines resistant to doxorubicin (LBR-D160) or vincristine (LBR-V160) and a sensitive line (LBR-). After oHA treatment, higher apoptosis levels were observed in the resistant cell lines than in the sensitive one. Besides, oHA sensitized LBR-D160 and LBR-V160 to vincristine showing increased apoptosis induction when used in combination with vincristine. Native hyaluronan failed to increase apoptosis levels. As different survival factors could be modulated by hyaluronan, we investigated the PI3K/Akt pathway through PIP3 production and phosphorylated Akt (p-Akt) and survivin expression was also evaluated. Our results showed that oHA decreased p-Akt in the 3 cell lines while anti-CD44 treatment abolished this effect. Besides, survivin was downregulated only in LBR-V160 by oHA. When Pgp function was evaluated, we observed that oHA were able to inhibit Pgp efflux in murine and human resistant cell lines in a CD44-dependent way. In summary, we report for the first time that oHA per se modulate MDR in lymphoma cells by decreasing p-Akt as well as Pgp activity, thus suggesting that oHA could be useful in combination with classical chemotherapy in MDR hematological malignancies.