AXL – a new player in resistance to HER2 blockade

Cancer; HER2 disease; ResistanceCàncer; Malaltia HER2; ResistènciaCáncer; Enfermedad HER2; ResistenciaHER2 is a driver in solid tumors, mainly breast, oesophageal and gastric cancer, through activation of oncogenic signaling pathways such as PI3K or MAPK. HER2 overexpression associates with aggressive disease and poor prognosis. Despite targeted anti-HER2 therapy has improved outcomes and is the current standard of care, resistance emerge in some patients, requiring additional therapeutic strategies. Several mechanisms, including the upregulation of receptors tyrosine kinases such as AXL, are involved in resistance. AXL signaling leads to cancer cell proliferation, survival, migration, invasion and angiogenesis and correlates with poor prognosis. In addition, AXL overexpression accompanied by a mesenchymal phenotype result in resistance to chemotherapy and targeted therapies. Preclinical studies show that AXL drives anti-HER2 resistance and metastasis through dimerization with HER2 and activation of downstream pathways in breast cancer. Moreover, AXL inhibition restores response to HER2 blockade in vitro and in vivo. Limited data in gastric and oesophageal cancer also support these evidences. Furthermore, AXL shows a strong value as a prognostic and predictive biomarker in HER2+ breast cancer patients, adding a remarkable translational relevance. Therefore, current studies enforce the potential of co-targeting AXL and HER2 to overcome resistance and supports the use of AXL inhibitors in the clinic

Protocol to generate a patient derived xenograft model of acquired resistance to immunotherapy in humanized mice

Cancer; Cell isolation; Stem cellsCàncer; Aïllament cel·lular; Cèl·lules mareCáncer; Aislamiento celular; Células madreImmunotherapy has revolutionized cancer treatment, but preclinical models are required to understand immunotherapy resistance mechanisms underlying patient relapse. This protocol describes how to generate an acquired resistance humanized in vivo model to immunotherapies in patient-derived xenografts (PDX). We detail steps to inject human CD34+ cells into NSG mice, followed by generation of immunoresistant PDX in humanized mice. This approach recapitulates the human immune system, allowing investigators to generate preclinical resistance models to different immunotherapies for identifying the resistant phenotype. For complete details on the use and execution of this protocol, please refer to Martínez-Sabadell et al., 2022 and Arenas et al. (2021).This work was supported by Asociación Española Contra el Cancer (GCAEC19017ARRI), Breast Cancer Research Foundation (BCRF-21-008), and Instituto de Salud Carlos III (PI19/01181). A.M.S. was funded by the Spanish Government (PFIS FI20/00188). P.O.R. was funded by the BBVA. E.J.A. was funded by the AECC (POSTD211413AREN). VHIO would like to acknowledge the Cellex Foundation for providing research facilities and equipment and the Centro de Investigación Biomédica en Red de Cáncer (CIBERONC) from the Institute of Health Carlos III (ISCIII) for their support on this research. Authors acknowledge financial support for the Cancer Immunology and Immunotherapy (CAIMI-2) program funded by BBVA Foundation. The Graphical abstract was created with BioRender.com

Targeting HER2-AXL heterodimerization to overcome resistance to HER2 blockade in breast cancer

Breast cancer; HeterodimerizationCáncer de mama; HeterodimerizaciónCàncer de mama; HeterodimeritzacióAnti-HER2 therapies have markedly improved prognosis of HER2-positive breast cancer. However, different mechanisms play a role in treatment resistance. Here, we identified AXL overexpression as an essential mechanism of trastuzumab resistance. AXL orchestrates epithelial-to-mesenchymal transition and heterodimerizes with HER2, leading to activation of PI3K/AKT and MAPK pathways in a ligand-independent manner. Genetic depletion and pharmacological inhibition of AXL restored trastuzumab response in vitro and in vivo. AXL inhibitor plus trastuzumab achieved complete regression in trastuzumab-resistant patient-derived xenograft models. Moreover, AXL expression in HER2-positive primary tumors was able to predict prognosis. Data from the PAMELA trial showed a change in AXL expression during neoadjuvant dual HER2 blockade, supporting its role in resistance. Therefore, our study highlights the importance of targeting AXL in combination with anti-HER2 drugs across HER2-amplified breast cancer patients with high AXL expression. Furthermore, it unveils the potential value of AXL as a druggable prognostic biomarker in HER2-positive breast cancer.A.A.-A., E.J.A., and F.B.-M. were supported by Asociación Española contra el Cáncer AECC (PRDVA18013LLUC to A.A.-A., POSTD211413AREN to E.J.A., and AECC_Postdoctoral17-1062 to F.B.-M.). A.M.-S. was funded by the Spanish Government (PFIS FI20/00188). J.Ar. is supported by Breast Cancer Research Foundation (BCRF-20-08), Instituto de Salud Carlos III Project reference number AC15/00062, and the EC under the framework of the ERA-NET TRANSCAN-2 initiative cofinanced by FEDER, Instituto de Salud Carlos III (CB16/12/00449 and PI19/01181), and Asociación Española Contra el Cáncer (AECC). A.P. was supported by Instituto de Salud Carlos III—PI19/01846, Breast Cancer Now—2018NOVPCC1294. P.E. and A.L. were funded by Instituto de Salud Carlos III and cofinanced by FEDER (PI18/01219 to P.E. and CB16/12/00481 to A.L.). J.M.C. was funded by Sociedad Española de Oncología Médica (Rio Hortega-SEOM) and Compromiso ADAMED

Evaluation of triple negative breast cancer with heterogeneous immune infiltration

Intratumor heterogeneity; Transcriptomics; Tumor-infiltrating lymphocytesHeterogeneïtat intratumoral; Transcriptòmica; Limfòcits infiltrants de tumorsHeterogeneidad intratumoral; Transcriptómica; Linfocitos infiltrantes de tumoresIntroduction: Tumor infiltrating lymphocytes (TILs) are known to be a prognostic and predictive biomarker in breast cancer, particularly in triple negative breast cancer (TNBC) patients. International guidelines have been proposed to evaluate them in the clinical setting as a continuous variable, without a clear defined cut-off. However, there are scenarios where the immune infiltration is heterogeneous that some areas of the patient’s tumour have high numbers of TILs while other areas completely lack them. This spontaneous presentation of a heterogeneous immune infiltration could be a great opportunity to study why some tumours present TILs at diagnosis but others do not, while eliminating inter patient’s differences. Methods: In this study, we have identified five TNBC patients that showed great TIL heterogeneity, with areas of low (≤5%) and high (≥50%) numbers of TILs in their surgical specimens. To evaluate immune infiltration heterogeneity, we performed and analyzed bulk RNA-sequencing in three independent triplicates from the high and low TIL areas of each patient. Results: Gene expression was homogeneous within the triplicates in each area but was remarkable different between TILs regions. These differences were not only due to the presence of TILs as there were other non-inflammatory genes and pathways differentially expressed between the two areas. Discussion: This highlights the importance of intratumour heterogeneity driving the immune infiltration, and not patient’s characteristics like the HLA phenotype, germline DNA or immune repertoire.This research has received funding from “Contigo contra el cancer de la mujer” Foundation and FERO Foundation

The target antigen determines the mechanism of acquired resistance to T cell-based therapies

Cancer; Antigen; ResistanceCáncer; Antígeno; ResistenciaCàncer; Antigen; ResistènciaDespite the revolution of immunotherapy in cancer treatment, patients eventually progress due to the emergence of resistance. In this scenario, the selection of the tumor antigen can be decisive in the success of the clinical response. T cell bispecific antibodies (TCBs) are engineered molecules that include binding sites to the T cell receptor and to a tumor antigen. Using gastric CEA+/HER2+ MKN45 cells and TCBs directed against CEA or HER2, we show that the mechanism of resistance to a TCB is dependent on the tumor antigen. Acquired resistant models to a high-affinity-CEA-targeted TCB exhibit a reduction of CEA levels due to transcriptional silencing, which is reversible upon 5-AZA treatment. In contrast, a HER2-TCB resistant model maintains HER2 levels and exhibit a disruption of the interferon-gamma signaling. These results will help in the design of combinatorial strategies to increase the efficacy of cancer immunotherapies and to anticipate and overcome resistances.This work was supported by Asociación Española Contra el Cancer (AECC), Breast Cancer Research Foundation (BCRF-21-008), and Instituto de Salud Carlos III (PI19/01181). A.M.S. was funded by the Spanish Government (PFIS FI20/00188). B.M. was funded by a fellowship from PERIS (Departament de Salut, Generalitat de Catalunya). M.R.A. was funded by Agency for Management of University and Research Grants (AGAUR, 2022 FI_B2 00080). P.O.R. was funded by the BBVA. E.J.A. was funded by the AECC (POSTD211413AREN). VHIO acknowledges the Cellex Foundation for providing research facilities and equipment, the Centro de Investigación Biomédica en Red de Cáncer (CIBERONC) from the Institute of Health Carlos III (ISCIII), and the Department of Health (Generalitat de Catalunya, SLT008/18/00198 SLT008/18/00205) for their support on this research. The authors acknowledge financial support from the State Agency for Research (Agencia Estatal de Investigación) (CEX2020-001024-S/AEI/10.13039/501100011033) and for the Cancer Immunology and Immunotherapy (CAIMI-2) program funded by BBVA Foundation. We would like to remark the funding from B.M PERIS (Spain). The authors thank Dr. Anne Freimoser-Grundschober and Roche for helping provide the TCBs. The graphical abstract was created with BioRender.com

Vitamin D analogues exhibit antineoplastic activity in breast cancer patient-derived xenograft cells

Despite advances in breast cancer (BC) treatment, its mortality remains high due to intrinsic or acquired resistance to therapy. Several ongoing efforts are being made to develop novel drugs to treat this pathology with the aim to overcome resistance, prolong patient survival and improve their quality of life. We have previously shown that the non-hypercalcemic vitamin D analogues EM1 and UVB1 display antitumor effects in preclinical studies employing conventional cell lines and animal models developed from these cells. In this work, we explored the antitumor effects of EM1 and UVB1 employing BC cells derived from patient-derived xenografts (PDXs), which are a powerful preclinical tool for testing new drugs. We demonstrated that the analogues reduced the viability of HER2-positive and Triple Negative BC-PDXs. Moreover, using an in vitro model of acquired resistance to Trastuzumab-emtansine, UVB1 displayed anti-proliferative actions under 2D and 3D culture conditions. It inhibited both formation and growth of established organoids. In addition, a direct correlation between UVB1 antitumor effects and VDR expression in PDXs was found. In conclusion, all the results reinforce the potential use of these vitamin D analogues as antitumor agents to treat HER2-positive and Triple Negative BC.Fil: Ferronato, María Julia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Investigaciones Bioquímicas de Bahía Blanca. Universidad Nacional del Sur. Instituto de Investigaciones Bioquímicas de Bahía Blanca; Argentina. Universidad Nacional del Sur. Departamento de Biología, Bioquímica y Farmacia; ArgentinaFil: Nadal, Serrano Mercedes. Universidad Autonoma de Barcelona. Hospital Vall D' Hebron. Instituto de Investigación Vall D'hebron; EspañaFil: Arenas Lahuerta, Enrique Javier. Universidad Autonoma de Barcelona. Hospital Vall D' Hebron. Instituto de Investigación Vall D'hebron; EspañaFil: Morales, Cristina Bernadó. Universidad Autónoma de Barcelona. Hospital Vall D' Hebron; EspañaFil: Paolillo, Giuliana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Investigaciones Bioquímicas de Bahía Blanca. Universidad Nacional del Sur. Instituto de Investigaciones Bioquímicas de Bahía Blanca; Argentina. Universidad Nacional del Sur. Departamento de Biología, Bioquímica y Farmacia; ArgentinaFil: Martinez Sabadell, Aliguer Alex. Universidad Autónoma de Barcelona. Hospital Vall D' Hebron; EspañaFil: Mascaró, Marilina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Investigaciones Bioquímicas de Bahía Blanca. Universidad Nacional del Sur. Instituto de Investigaciones Bioquímicas de Bahía Blanca; Argentina. Universidad Nacional del Sur. Departamento de Biología, Bioquímica y Farmacia; ArgentinaFil: Vitale, Cristian Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Química del Sur. Universidad Nacional del Sur. Departamento de Química. Instituto de Química del Sur; ArgentinaFil: Fall, Yagamare. Universidad de Vigo; EspañaFil: Arribas, Joaquín. Universidad Autónoma de Barcelona. Hospital Vall D' Hebron; España. Universitat Autònoma de Barcelona; España. Centro de Investigación Biomédica en Red de Cáncer; España. Institució Catalana de Recerca i Estudis Avancats; EspañaFil: Facchinetti, María Marta. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Investigaciones Bioquímicas de Bahía Blanca. Universidad Nacional del Sur. Instituto de Investigaciones Bioquímicas de Bahía Blanca; Argentina. Universidad Nacional del Sur. Departamento de Biología, Bioquímica y Farmacia; ArgentinaFil: Curino, Alejandro Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Investigaciones Bioquímicas de Bahía Blanca. Universidad Nacional del Sur. Instituto de Investigaciones Bioquímicas de Bahía Blanca; Argentina. Universidad Nacional del Sur. Departamento de Biología, Bioquímica y Farmacia; Argentin

Therapy-Induced Senescence Enhances the Efficacy of HER2-Targeted Antibody–Drug Conjugates in Breast Cancer

Eficàcia; Conjugats de fàrmacs; Càncer de mamaEficacia; Conjugados de medicamentos; Cáncer de mamaEfficacy; Drug conjugates; Breast cancerAntibody–drug conjugates (ADC) are antineoplastic agents recently introduced into the antitumor arsenal. T-DM1, a trastuzumab-based ADC that relies on lysosomal processing to release the payload, is approved for HER2-positive breast cancer. Next-generation ADCs targeting HER2, such as [vic-]trastuzumab duocarmazine (SYD985), bear linkers cleavable by lysosomal proteases and membrane-permeable drugs, mediating a bystander effect by which neighboring antigen-negative cells are eliminated. Many antitumor therapies, like DNA-damaging agents or CDK4/6 inhibitors, can induce senescence, a cellular state characterized by stable cell-cycle arrest. Another hallmark of cellular senescence is the enlargement of the lysosomal compartment. Given the relevance of the lysosome to the mechanism of action of ADCs, we hypothesized that therapies that induce senescence would potentiate the efficacy of HER2-targeting ADCs. Treatment with the DNA-damaging agent doxorubicin and CDK4/6 inhibitor induced lysosomal enlargement and senescence in several breast cancer cell lines. While senescence-inducing drugs did not increase the cytotoxic effect of ADCs on target cells, the bystander effect was enhanced when HER2-negative cells were cocultured with HER2-low cells. Knockdown experiments demonstrated the importance of cathepsin B in the enhanced bystander effect, suggesting that cathepsin B mediates linker cleavage. In breast cancer patient-derived xenografts, a combination treatment of CDK4/6 inhibitor and SYD985 showed improved antitumor effects over either treatment alone. These data support the strategy of combining next-generation ADCs targeting HER2 with senescence-inducing therapies for tumors with heterogenous and low HER2 expression. Significance: Combining ADCs against HER2-positive breast cancers with therapies that induce cellular senescence may improve their therapeutic efficacy by facilitating a bystander effect against antigen-negative tumor cells.This work was supported by Breast Cancer Research Foundation (BCRF-20-008), Instituto de Salud Carlos III (project reference numbers AC15/00062, CB16/12/00449 and PI19/01181), the EC under the framework of the ERA-NET TRANSCAN-2 initiative co-financed by FEDER, Fundación Mutua Madrileña and Asociación Española Contra el Cáncer. S. Duro-Sánchez is supported by the Spanish Ministerio de Universidades by the grant Formación de Profesorado Universitario (FPU20/05388). A. Esteve-Codina is funded by ISCIII /MINECO (PT17/0009/0019) and co-funded by FEDER. The authors acknowledge Alyson MacInnes for reviewing and editing the article

Human Metastatic Cholangiocarcinoma Patient-Derived Xenografts and Tumoroids for Preclinical Drug Evaluation

Human metastatic cholangiocarcinoma; Xenografts; TumoroidsColangiocarcinoma metastàtic humà; Xenoempelts; TumoroidesColangiocarcinoma metastásico humano; Xenoinjertos; TumoroidesPurpose: Cholangiocarcinoma (CCA) is usually diagnosed at advanced stages, with limited therapeutic options. Preclinical models focused on unresectable metastatic CCA are necessary to develop rational treatments. Pathogenic mutations in IDH1/2, ARID1A/B, BAP1, and BRCA1/2 have been identified in 30%–50% of patients with CCA. Several types of tumor cells harboring these mutations exhibit homologous recombination deficiency (HRD) phenotype with enhanced sensitivity to PARP inhibitors (PARPi). However, PARPi treatment has not yet been tested for effectiveness in patient-derived models of advanced CCA. Experimental Design: We have established a collection of patient-derived xenografts from patients with unresectable metastatic CCA (CCA_PDX). The CCA_PDXs were characterized at both histopathologic and genomic levels. We optimized a protocol to generate CCA tumoroids from CCA_PDXs. We tested the effects of PARPis in both CCA tumoroids and CCA_PDXs. Finally, we used the RAD51 assay to evaluate the HRD status of CCA tissues. Results: This collection of CCA_PDXs recapitulates the histopathologic and molecular features of their original tumors. PARPi treatments inhibited the growth of CCA tumoroids and CCA_PDXs with pathogenic mutations of BRCA2, but not those with mutations of IDH1, ARID1A, or BAP1. In line with these findings, only CCA_PDX and CCA patient biopsy samples with mutations of BRCA2 showed RAD51 scores compatible with HRD. Conclusions: Our results suggest that patients with advanced CCA with pathogenic mutations of BRCA2, but not those with mutations of IDH1, ARID1A, or BAP1, are likely to benefit from PARPi therapy. This collection of CCA_PDXs provides new opportunities for evaluating drug response and prioritizing clinical trials.This work was supported by grants from the Fundació Marató TV3 awarded to T. Macarulla, M. Melé, and S. Peiró; BeiGene research grant awarded to T. Macarulla and S. Peiró; AECC (INVES20036TIAN), Ramón y Cajal investigator program (RYC2020-029098-I), Proyecto de I+D+i (PID2019-108008RJ-I00), and FERO Foundation grant awarded to T.V. Tian; Proyecto de Investigación en Salud from the Instituto de Salud Carlos III (ISCIII) (PI20/00898) awarded to T. Macarulla; FIS/FEDER from the Instituto de Salud Carlos III (ISCIII) (PI12/01250; CP08/00223; PI16/00253 and CB16/12/00449) awarded to S. Peiró; and Ramón y Cajal investigator program (RYC-2017-22249) awarded to M. Melé. Q. Serra-Camprubí is a recipient of the Ph.D. fellowship from La Caixa Foundation (LCF/PR/PR12/51070001). A. Llop-Guevara was supported by the AECC (INVES20095LLOP) and V. Serra by the ISCIII (CPII19/00033). E.J. Arenas was funded by the AECC (POSTD211413AREN). J. Arribas is funded by the Instituto de Salud Carlos III (AC15/00062, CB16/12/00449, and PI22/00001). This publication is based upon the work of COST Action CA18122, European Cholangiocarcinoma Network, supported by the COST (European Cooperation in Science and Technology, www.cost.eu), a funding agency for research and innovation networks. The authors would like to thank Dr. V.A. Raker for manuscript editing and Drs. N. Herranz and J. Mateo for scientific discussions. The authors acknowledge the infrastructure and support of the FERO Foundation, La Caixa Foundation, and the Cellex Foundation

Early-Stage Breast Cancer Detection in Breast Milk

Breast cancer; Breast milkCáncer de mama; Leche maternaCàncer de mama; Llet maternaBreast cancer occurring during pregnancy (PrBC) and postpartum (PPBC) is usually diagnosed at more advanced stages compared with other breast cancer, worsening its prognosis. PPBC is particularly aggressive, with increased metastatic risk and mortality. Thus, effective screening methods to detect early PrBC and PPBC are needed. We report for the first time that cell-free tumor DNA (ctDNA) is present in breast milk (BM) collected from patients with breast cancer. Analysis of ctDNA from BM detects tumor variants in 87% of the cases by droplet digital PCR, while variants remain undetected in 92% of matched plasma samples. Retrospective next-generation sequencing analysis in BM ctDNA recapitulates tumor variants, with an overall clinical sensitivity of 71.4% and specificity of 100%. In two cases, ctDNA was detectable in BM collected 18 and 6 months prior to standard diagnosis. Our results open up the potential use of BM as a new source for liquid biopsy for PPBC detection. Significance: For the first time, we show that BM obtained from patients with breast cancer carries ctDNA, surpassing plasma-based liquid biopsy for detection and molecular profiling of early-stage breast cancer, even prior to diagnosis by image.We thank the patients who participated in the study and donated samples for analysis for their generous contribution, with particular thanks to the first patient, Maite, and her daughter Àneu, who inspired us to initiate this study (oral consent to name the patient and her daughter was provided by the patient, and her legal partner provided written consent after patient's exitus). We are grateful to Javier Carmona for his valuable contributions and support in the manuscript's conceptualization, preparation, and revision. VHIO would like to acknowledge the Cellex Foundation for providing research facilities and equipment and the CERCA Programme from the Generalitat de Catalunya for their support of this research. The authors from VHIO acknowledge the State Agency for Research (Agencia Estatal de Investigación) for the financial support as a Center of Excellence Severo Ochoa (CEX2020-001024-S/AEI/10.13039/501100011033). This research is financially supported by the “El paseíco de la mama” Foundation. C. Saura was the recipient of a II FERO-GHD grant from the FERO Foundation (FERO/5086), a Junior Clinical award from the Spanish Association Against Cancer Foundation (FAECC; CLJUN212026ORTI), and a SEOM-Daiichi Sankyo grant for its support on the Breast Cancer Research Projects 2021 (SEOM/FECMA2022) and received funding from the Department of Health (Generalitat de Catalunya SLT008/18/00198) and from the Instituto de Salud Carlos III (ISCIII) and Fondo Europeo de Desarrollo Regional (FEDER), cofunded by the European Union (PI21/01020). C. Ortiz was the recipient of a Junior Clinician award from the FAECC (CLJUN212026ORTI) and a SEOM-Daiichi Sankyo grant for its support on the Breast Cancer Research Projects 2021 (SEOM/FECMA2022), and received funding from the Department of Health (Generalitat de Catalunya SLT008/18/00198). N. Bayó-Puxan received funding from the Department of Health (Generalitat de Catalunya SLT008/18/00205), MCIN/AEI/10.13039/501100011033 (GPE2022-001029) and MCIN/AEI/10.130.39/501100011033, and the European Union “Next GenerationEU/PRTR” (ECT2020-000827). J.M. Miquel received funding from the Department of Health (Generalitat de Catalunya SLT008/18/00205), MCIN/AEI/10.130.39/501100011033, and the European Union “Next GenerationEU/PRTR” (ECT2020-000827). J. Arribas is funded by the Breast Cancer Research Foundation (BCRF-23-008), Instituto de Salud Carlos III (project reference numbers AC15/00062, CB16/12/00449, and PI22/00001), and the European Commission under the framework of the ERA-NET TRANSCAN-2 initiative cofinanced by FEDER and Asociación Española Contra el Cáncer. A. Vivancos was the recipient of a project award from the FAECC (AVP/18/AECC/3219) and received funding from the Advanced Molecular Diagnostic (DIAMAV) program from the FERO Foundation (8361) and from ISDIN for supporting the development of liquid biopsy applications at the Cancer Genomics Lab (1848). M. Sansó was the recipient of a II FERO-GHD grant from the FERO Foundation (FERO/5086) and an investigator award from the FAECC (INVES19056SANS), and received funding from the Health Research Institute of the Balearic Islands (IdISBa), the RADIX-Janssen program (RADIX/JANSSEN21/01), and the Miguel Servet Program funded by the ISCIII (CP22/00131)

Identification of novel mechanisms in human breast cancer lung metastasis and chemoresistance

[eng] Breast Cancer (BC) is one of the major causes of cancer deaths in women. BC is a heterogeneous disease, and within the heterogeneity of the tumor, Tumor-Initiating Cells (TICs) have been reported as important drivers in tumor initiation and progression. On the basis of these observations, we hypothesized that TICs contribute to BC metastasis, as well to chemoresistance. Through genetic, transcriptomic, molecular and therapeutic analyses of tumor xenografts, BC cell lines, and/or human tumors, in this thesis we first demonstrate that RARRES3, a lung metastatic suppressor gene, prevents adhesion to the lung parenchyma and the initiation of metastatic lesions by enforcing the retention of differentiation features and that this retention is driven by its PLA1/2 catalytic activity. These results indicate that RARRES3 genetic activity blocks both tissue-specific metastasis to the lung and metastatic initiation properties. Next, we dissected functional interplay between stem cell (SC)-like master regulator genes (EVI1 and SOX9) and resistance to mTOR inhibitors, which leads to an aggressive metastatic cancer phenotype. Collectively, the data shown in this thesis depict novel evidence regarding the role of tumor initiation properties in: I) BC progression and metastasis; II) how cancer progression is functionally linked to resistance to mTOR; and III) how resistance to therapy driven by tumor initiation properties may eventually produce an aggressive cancer phenotype.[spa] El cáncer de mama (BC, de sus siglas en inglés) es una de las mayores causas de muerte por cáncer en mujeres. El cáncer de mama es una enfermedad heterogénea, y en la heterogeneidad del tumor, las células iniciadoras tumorales (TICs, del inglés) han sido asociadas como importantes responsables en la iniciación y progresión tumoral. Basándonos en estas observaciones, nuestra hipótesis es si estas células contribuyen en la metástasis y quimioresistancia en el cáncer de mama. A través de análisis genéticos, transcriptómicos, moleculares y terapéuticos en xenoinjertos tumorales, líneas celulares y tumores humanos, en esta tesis hemos revelado en primer lugar que RARRES3, es un gen de supresión metastática en pulmón, que previene la adhesión al parénquima pulmonar y la iniciación de lesiones metastáticas, imponiendo las características de diferenciación, y cuya retención es causada por la actividad catalítica PLA1/2. Estos resultados indican que la actividad genética de RARRES3 bloquea específicamente la metástasis a pulmón y las propiedades de iniciación metastática. Además, hemos descrito una relación entre las los genes con características de autorenovación (EVI1 y SOX9) y resistencia a inhibidores de mTOR, que termina con una fenotipo más agresivo y metastático. Conjuntamente, los datos mostrados en esta tesis demuestras evidencias novedosas relacionadas con las propiedades de iniciación tumoral en distintos contextos: I) progresión y metástasis en el cáncer de mama; II) como la progresión tumoral está funcionalmente relacionada con la resistencia a inhibición de mTOR; y III) como la resistencia a terapia que está desencadenada por estas propiedades de iniciación tumoral pueden al final producir un fenotipo más agresivo y metastátic