title: RANKL enhances the effect of an antagonist of inhibitor of apoptosis proteins (cIAPs) in RANK-positive breast cancer cells.
abstract: OBJECTIVE
  Between 65% and 75% of patients with metastatic breast cancer will have decreased 5-year survival and increased morbidity due to cancer relapse in bone. At this stage of disease treatment is palliative, but tumor-targeted compounds could add to the benefits of anti-resorptive agents, improving clinical outcome. Inhibitor-of-apoptosis proteins (IAPs) are overexpressed in many tumors and second mitochondria-derived activator of caspases (Smac) mimetics have been designed to antagonize IAPs. In this work we explored the use of AT-406, a Smac mimetic, to target the tumor compartment of bone metastases.
  
  
  METHODS
  Effect of AT-406 on cancer cells apoptosis, expression of IAPs and osteogenic potential was addressed in vitro using the RANK-positive MDA-MB-231 breast cancer cell line. Effect of AT-406 on osteoclastogenesis was determined by inducing the differentiation of the RAW 264.7 mouse monocytic cell line. Osteoclastogenesis was measured by TRAP staining and TRACP 5b quantification.
  
  
  RESULTS
  AT-406 increased apoptosis in MDA-MB-231 breast cancer cells in vitro, and activation of RANK-pathway improved this effect. RANKL stimuli induced a strong increase in c-IAP2. AT-406 increased osteoclast differentiation and activity, by up-regulating the osteogenic transcription factor Nfatc1, but also increased the apoptosis of mature osteoclasts in the absence of RANKL.
  
  
  CONCLUSIONS
  Our results indicate that despite the anti-tumoral effect of AT-406, its use in the context of bone metastatic disease needs to be carefully monitored for the induction of increased bone resorption. We also hypothesize that the combination of AT-406 with anti-RANKL directed therapies could have a beneficial effect, especially in RANK-positive tumors.
authors.affiliation: Clinical and Translational Oncology Research Unit, Institute of Molecular Medicine, Lisbon Medical School, Lisbon, Portugal.
authors.forename: S
authors.initials: S
authors.lastname: Casimiro
authors.affiliation: Clinical and Translational Oncology Research Unit, Institute of Molecular Medicine, Lisbon Medical School, Lisbon, Portugal.
authors.forename: I
authors.initials: I
authors.lastname: Alho
authors.affiliation: Clinical and Translational Oncology Research Unit, Institute of Molecular Medicine, Lisbon Medical School, Lisbon, Portugal.
authors.forename: M
authors.initials: M
authors.lastname: Bettencourt
authors.affiliation: Clinical and Translational Oncology Research Unit, Institute of Molecular Medicine, Lisbon Medical School, Lisbon, Portugal.
authors.forename: R
authors.initials: R
authors.lastname: Pires
authors.affiliation: Penn State Hershey Medical Center, Hershey, PA, USA.
authors.forename: A
authors.initials: A
authors.lastname: Lipton
authors.affiliation: Clinical and Translational Oncology Research Unit, Institute of Molecular Medicine, Lisbon Medical School, Lisbon, Portugal; Oncology Department, Hospital de Santa Maria-CHLN, Lisbon, Portugal.
authors.forename: L
authors.initials: L
authors.lastname: Costa
keywords: AT-406; Bone metastasis; Inhibitors of apoptosis protein (cIAP); RANK-positive breast cancer; Smac mimetics