Abstract 1829: Regulation of Mixed Lineage Kinase-3 activity by Her2 and its implication in death or survival

Human Epidermal Growth Factor Receptor 2 (Her2) is a receptor tyrosine kinase that regulates cell growth and differentiation signaling pathways. The protein Her2 is significantly overexpressed in 20-30% of breast cancer. The available clinical data proves that Her2 amplification in breast cancer is associated with worst overall survival. Therefore, Her2 has become a very important candidate for drug targeting and currently Her2 positive patients are treated with humanized monoclonal antibody, Trastuzumab (Herceptin) and a small molecule, Lapatinib that blocks Her2 tyrosine kinase activity. It is reported that Her2 amplification leads to activation of multiple survival pathways, including PI3K-Akt. However, it is not clearly known how Trastuzumab promotes cell death and what pro-apoptotic pathways are activated downstream of Her2 blockage? It is also reported that patients on Trastuzumab develop subsequent resistance to the therapy. Previously we published that in breast cancer tumors, a pro-apoptotic kinase, Mixed Lineage Kinase-3 (MLK-3) was inhibited, specifically in estrogen receptor (ER) positive tumors. The kinase activity of MLK-3 was down regulated by estrogen and this prevented MLK-3-mediated cell death. Since Her2 serves as a drug target in breast cancer, therefore, we asked whether Her2 plays any role in regulating MLK-3 kinase activation and whether MLK-3 plays any role in Trastuzumab resistance. Our results showed that the endogenous MLK-3 kinase activity was up-regulated in SkBr3 cells upon treatment with Trastuzumab and Lapatinib. Whether the activation of MLK-3 upon Her2 blockage leads to cell death or survival is still not known and experiments are underway to investigate these aspects. We also observed that Her3 was the dimerization partner of Her2 in Trastuzumab- or Lapatinib- mediated MLK-3 activation. Not knowing any role of MLK-3 in Trastuzumab resistance, we used Trastuzumab sensitive and resistant BT474 cell model to investigate the role of MLK-3 in Trastuzumab resistance. Our data clearly showed that knockdown of MLK-3 caused spontaneous cell death in resistant cells. The Trastuzumab treatment further amplified cell death in MLK-3 knockdown resistant cells. The Akt1 activation in these cells was also diminished upon MLK-3 knockdown. Taken together, our data demonstrate that like ER, the Her2 also down regulates MLK-3 kinase activity and perhaps promote cell survival. Furthermore, paradoxically, the MLK-3 knockdown promoted cell death in Trastuzumab resistance BT474 cells, suggesting that MLK-3 could also serve as survival factor in resistant cells. Whether MLK-3 serves as a pro-apoptotic kinase or survival kinase in Trastuzumab mediated pathway needs to be extensively examined in near future. Citation Format: Subhasis Das, Gautam Sondarva, Navin Viswakarma, Rakesh Sathish Nair, Clodia Osipo, Basabi Rana, Ajay Rana. Regulation of Mixed Lineage Kinase-3 activity by Her2 and its implication in death or survival. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1829. doi:10.1158/1538-7445.AM2014-1829