Modulation of hormone therapy resistance by CDK2-PELP1 axis.
Authors: Chandrasekharan Nair B , Nair SS , Chakravarty D , Yew RP , Tekmal RR , Vadlamudi RK 
Published: December 2010

Abstract
Abstract #3022
Background: The estrogen receptor (ER) plays a central role in the progression of breast cancer. Current endocrine therapy for ER+ve breast cancer involves modulating ER-pathway using Tamoxifen, and blocking peripheral estrogen (E2) synthesis by Aromatase inhibitors. Despite the positive effects, de novo and/or acquired resistance to endocrine therapies frequently occur. Although mechanisms for hormonal therapy resistance remains elusive, most downstream events in these pathways converge upon modulation of cell cycle regulatory proteins including upregulation of Cyclin E and A, along with activation of Cyclin Dependent Kinase 2 (CDK2). ER signaling complexes are known to recruit various co-regulatory proteins and recent evidences suggest that deregulated expression, localization and activity of ER coregulators also plays vital role in hormonal resistance. In this study, we found that CDK signaling regulates ER coregulator PELP1 function via phosphorylation leading to hormonal resistance.&#x2028; Methods: Significance of CDK2 axis in the therapy resistance was tested using breast cancer models cells that acquired resistance to endocrine therapy and by using chemical inhibitors that block CDK2 activity. Immunoprecipitation, and confocal analysis was used to confirm protein-protein interactions. In vitro kinase and ortho-phosphate labeling assays were used to test CDK2 phosphorylation of PELP1. Utilizing Breast cancer model cells that express PELP1 mutants that cannot be phosphorylated by CDK2, we examined the significance of PELP1 phosphorylation in cell cycle progression. Using PELP1siRNA nanoparticles, we tested the effect of PELP1 knockdown in hormone therapy resistance.&#x2028; Results: ER coregulator PELP1 interacts with CDK2 upon E2 stimulation. In vitro kinase assays using both purified CDK2/CyclinE and CDK2/CyclinA complexes showed that full length PELP1 is a potential substrate of CDK2. PELP1 exhibited phosphorylation at the time points that corresponds to CDK2 activation in MCF7 cells. PELP1 overexpression increases E2F luciferase activity while PELP1 mutants that lack CDK2 sites failed to enhance the E2F activity. CDK2 mediated phosphorylation of PELP1 is important for PELP1 regulation of E2F and ER target genes. Combination therapies using PELP1 siRNA nano particles or Roscovitine along with tamoxifen or letrozole, sensitized the therapy resistance cells for endocrine therapy.&#x2028; Conclusions: We have identified ER coregulator PELP1 as a novel substrate of CDK2. Because CDK2 activity is deregulated in breast tumors and implicated in therapy resistance, our findings suggests that CDK2-PELP1 axis deregulation may contribute therapy resistance. Combinatorial therapeutic strategy using Roscovitine along with PELP1 siRNA nanoparticles will provide new therapeutic opportunity to increase the sensitivity of hormone resistant cells to Tamoxifen and Letrozole therapy. These studies were supported by DOD breast cancer grant BC083207.
Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 3022.