title: High CDK6 Protects Cells from Fulvestrant-Mediated Apoptosis and is a Predictor of Resistance to Fulvestrant in Estrogen Receptor-Positive Metastatic Breast Cancer.
abstract: OBJECTIVE
  Resistance to endocrine therapy in estrogen receptor-positive (ER+) breast cancer remains a major clinical problem. Recently, the CDK4/6 inhibitor palbociclib combined with letrozole or fulvestrant was approved for treatment of ER+advanced breast cancer. However, the role of CDK4/6 in endocrine resistance and their potential as predictive biomarkers of endocrine treatment response remains undefined.
  
  
  METHODS
  We investigated the specific role of increased CDK6 expression in fulvestrant-resistant cells by gene knockdown and treatment with palbociclib, and evaluated the effect in cell proliferation, apoptosis, and kinase activity. Furthermore, we evaluated CDK6 expression in metastatic samples from breast cancer patients treated or not with fulvestrant.
  
  
  RESULTS
  We found increased expression of CDK6 in two fulvestrant-resistant cell models versus sensitive cells. Reduction of CDK6 expression impaired fulvestrant-resistant cell growth and induced apoptosis. Treatment with palbociclib resensitized fulvestrant-resistant cells to fulvestrant through alteration of retinoblastoma protein phosphorylation. High CDK6 levels in metastatic samples from two independent cohorts of breast cancer patients treated with fulvestrant (N = 45 and 46) correlated significantly with shorter progression-free survival (PFS) on fulvestrant treatment (P = 0.0006 and 0.018), whereas no association was observed in patients receiving other first- or second-/third-line endocrine treatments (N = 68, P = 0.135 and 0.511, respectively).
  
  
  CONCLUSIONS
  Our results indicate that upregulation of CDK6 may be an important mechanism in overcoming fulvestrant-mediated growth inhibition in breast cancer cells. Patients with advanced ER+breast cancer exhibiting high CDK6 expression in the metastatic lesions show shorter PFS upon fulvestrant treatment and thus may benefit from the addition of CDK4/6 inhibitors in their therapeutic regimens. Clin Cancer Res; 22(22); 5514-26. ©2016 AACR.
authors.affiliation: Department of Cancer and Inflammation Research, Institute of Molecular Medicine, University of Southern Denmark, Odense, Denmark. calves@health.sdu.dk hditzel@health.sdu.dk.
authors.forename: Carla L
authors.initials: CL
authors.lastname: Alves
authors.affiliation: Department of Cancer and Inflammation Research, Institute of Molecular Medicine, University of Southern Denmark, Odense, Denmark.
authors.forename: Daniel
authors.initials: D
authors.lastname: Elias
authors.affiliation: Department of Cancer and Inflammation Research, Institute of Molecular Medicine, University of Southern Denmark, Odense, Denmark.
authors.forename: Maria
authors.initials: M
authors.lastname: Lyng
authors.affiliation: Department of Pathology, Odense University Hospital, Odense, Denmark.
authors.forename: Martin
authors.initials: M
authors.lastname: Bak
authors.affiliation: Breast Cancer Group, Cell Death and Metabolism, Danish Cancer Society Research Center, Copenhagen, Denmark.|Department of Surgery, Koege Hospital, Koege, Denmark.
authors.forename: Tove
authors.initials: T
authors.lastname: Kirkegaard
authors.affiliation: Breast Cancer Group, Cell Death and Metabolism, Danish Cancer Society Research Center, Copenhagen, Denmark.
authors.forename: Anne E
authors.initials: AE
authors.lastname: Lykkesfeldt
authors.affiliation: Department of Cancer and Inflammation Research, Institute of Molecular Medicine, University of Southern Denmark, Odense, Denmark. calves@health.sdu.dk hditzel@health.sdu.dk.|Department of Oncology, Odense University Hospital, Odense, Denmark.
authors.forename: Henrik J
authors.initials: HJ
authors.lastname: Ditzel
mesh_terms: D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects*; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D051361:Cyclin-Dependent Kinase 6 / Q000378:metabolism*; D018572:Disease-Free Survival; D019008:Drug Resistance, Neoplasm / Q000187:drug effects*; D004958:Estradiol / Q000031:analogs & derivatives* / Q000494:pharmacology; D005260:Female; D000077267:Fulvestrant; D006801:Humans; D000077289:Letrozole; D061986:MCF-7 Cells; D009570:Nitriles / Q000494:pharmacology; D010879:Piperazines / Q000494:pharmacology; D011725:Pyridines / Q000494:pharmacology; D011960:Receptors, Estrogen / Q000378:metabolism*; D016160:Retinoblastoma Protein / Q000378:metabolism; D014230:Triazoles / Q000494:pharmacology
chemical_list: D018931:Antineoplastic Agents, Hormonal; D009570:Nitriles; D010879:Piperazines; D011725:Pyridines; D011960:Receptors, Estrogen; D016160:Retinoblastoma Protein; D014230:Triazoles; D000077267:Fulvestrant; D004958:Estradiol; D000077289:Letrozole; C495911:CDK6 protein, human; D051361:Cyclin-Dependent Kinase 6; C500026:palbociclib