Abstract 2110: The role of VAV-interacting Kruppel-like factor (VIK) in resistance to palbociclib in estrogen receptor-positive breast cancer
Authors: Lenihan Catherine , Cavicchioli Francesca , O’Leary Karen , Shia Alice , Schmid Peter 
Published: July 2016

Abstract
Background: The cyclin D-CDK4/6-Rb pathway, regulating the G1/S phase transition of the cell cycle, is frequently dysregulated in estrogen receptor (ER) positive breast cancer, and has been connected with endocrine resistance. Combined treatment of palbociclib, a highly selective CDK4/6 inhibitor, with endocrine therapy has led to a substantial improvement in outcome of patients with ER positive metastatic breast cancer. Increasing clinical use means acquired resistance to palbociclib is emerging as a new major clinical challenge, understanding of resistance mechanisms will be crucial. VAV interacting Kruppel-like factor (VIK) is a potential novel transcription factor known to be involved in cell cycle regulation, interacting with CDK4. Previously our lab reported a series of breast cancer patient tumour samples where VIK methylation was associated with an increased risk of recurrence and decreased survival in tamoxifen-treated patients, indicating a role for VIK in ER positive breast cancer.
Results: Methylation analysis of the VIK promoter region by pyrosequencing revealed dense methylation of the CpG island located in the 5’ regulatory sequences of the gene, in 8 out of 20 breast cancer cells lines. Methylation above 30% correlated with absent mRNA expression, confirming methylation transcriptionally silences VIK. Subsequently, VIK was knocked down in the ER positive, VIK unmethylated breast cancer cell lines T47D and SUM44. Knockdown resulted in significant cell death due to induction of apoptosis as measured by Annexin V staining and increased levels of cleaved PARP. The knockdown of VIK altered expression of regulatory cell cycle proteins decreasing levels of Rb, pRb, cyclin D and CDK4, whilst upregulating cyclin E. Treatment with the CDK4/6 inhibitor, palbociclib, in cells with reduced VIK expression resulted in decreased sensitivity to the drug. There was a shift in IC50 value towards resistance from 100nM to 500nM in T47D cells and 80nM to 1.2μM in SUM44 cells. In a model of acquired resistance, T47D cells were cultured long-term with palbociclib generating resistant clones with increased IC50 value 10 fold higher than sensitive cells. VIK was significantly down regulated in all resistant clones to barely detectable mRNA levels, indicating an important role for VIK in resistance to CDK4/6 inhibition.
Conclusions: VIK is a novel epigenetically regulated gene in breast cancer, playing a key role in the regulation of the G1/S phase transition in the cell cycle. VIK plays an important role in development of resistance to CDK4/6 inhibitors in estrogen receptor positive breast cancer.
Citation Format: Catherine Lenihan, Francesca Cavicchioli, Karen O’Leary, Alice Shia, Peter Schmid. The role of VAV-interacting Kruppel-like factor (VIK) in resistance to palbociclib in estrogen receptor-positive breast cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2110.