Abstract 4052: Targeting PDK1 in breast cancer: kinase-dependent regulation of tumor growth and kinase-independent regulation of cell migration and invasion

Abstract

Abstract 3-Phosphoinositide Dependent Kinase 1 (PDK1) is the kinase that phosphorylates the activation loop of several protein kinases such as Akt, PKC or SGK. In breast carcinomas PDK1 was found frequently overexpressed and this correlates with a more aggressive phenotype. This study is aimed to understand the role of PDK1 in breast tumor progression and the molecular mechanisms involved. Using stable gene silencing and kinase inhibitors, we studied PDK1 role in different in vitro and in vivo assays including anchorage independent growth, anoikis, xenografts, migration and invasion assays. In our study we found that PDK1 promotes resistance to anoikis, anchorage independent growth and tumor growth in breast cancer cell lines through the phosphorylation of its substrates, since the kinase-dead mutant abrogates all the PDK1-mediated effects. For this reasons either PDK1 gene silencing or its kinase inhibition with small compounds are sufficient to promote anoikis, to reduce anchorage independent growth and tumor growth. We also found that PDK1 silencing is able to reduce, while its overexpression to increase, cell migration and invasion in mammary cells. Surprisingly, the regulation of cell migration is operated in a kinase independent manner. In fact PDK1 kinase inhibitor failed to recapitulate the effects of gene silencing in cell migration. The molecular mechanism, underlying this unconventional kinase-independent regulation of cell migration, involves the binding of PDK1 to Myotonic Dystrophy kinase-related Cdc42-binding Kinase α (MRCKα). This binding increases MRCKα kinase activity on Myosin Light Chain-2 (MLC2) and on myosin phosphatase target subunit 1 (MyPT1). Both these phosphorylations increase of non-muscular myosin activity, which is required to pull forward the cell body during directional migration and invasion. In summary, our findings unveil that PDK1 regulates tumor growth, survival in absence of adhesion, resistance to anoikis, but also migration and invasion of breast cancer cells by two different mechanisms. These findings should be taken in account for a rational targeting of PDK1 in cancer. Citation Format: Paolo Armando Gagliardi, Laura di Blasio, Giorgio Seano, Roberto Sessa, Alberto Puliafito, Federico Bussolino, Luca Primo. Targeting PDK1 in breast cancer: kinase-dependent regulation of tumor growth and kinase-independent regulation of cell migration and invasion. [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 4052. doi:10.1158/1538-7445.AM2014-4052