Abstract 4008: Nuclear localization of Lymphocyte-specific protein tyrosine kinase (Lck) and its role in regulating LIM domain only 2 (LMO2) gene

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Lmo2 is a transcription factor that plays a critical role in both hematopoietic development and vascular remodeling. Lmo2 - associated chromosomal translocations are believed to play a major role in the development of T-cell acute lymphoblastic leukemia (T-ALL). Lmo2 is also shown to interact with the T-cell acute lymphocytic leukemia protein (TAL1) in leukemia. A previous report established a link between Lmo2 expression and the unexpected nuclear presence of oncogenic Janus kinase 2 (JAK2), a non-receptor protein tyrosine kinase. However, it is not known whether other oncogenic tyrosine kinases regulate Lmo2 expression through a similar mechanism. We showed previously that oncogenic Lck kinase promotes cellular transformation through the activation of signal transducer and activator of transcription 5b (Mol Cancer Res 4:39). In this report, we describe novel nuclear localization of Lck and its role in regulating Lmo2 expression. Lck, like most Src family kinases, is primarily known as a cytosolic or membrane-associated kinase. Using subcellular fractionation and immunofluorescence microscopy, we identified nuclear localization of Lck in murine leukemia cells (LSTRA) that overexpress Lck kinase. Nuclear Lck was also detected in 293 cells expressing constitutively active Y505FLck. Additionally, phosphorylation of the positive-regulatory tyrosine 394 confirmed the active status of nuclear Lck. These data suggest that oncogenic Lck may have an uncharacterized function within the nucleus that supports cellular transformation. Lmo2 expression was significantly elevated both in LSTRA leukemia and in 293 cells expressing oncogenic Y505FLck kinase. The oncogenic JAK2 kinase phosphorylates histone H3 on Y41 that leads to the relief of Lmo2 promoter repression and subsequent gene expression. However, expression of oncogenic Lck kinase does not result in significant increase in H3Y41 phosphorylation. Instead, chromatin immunoprecipitation experiment shows that oncogenic Y505FLck kinase binds to the Lmo2 promoter in vivo. This result raises the possibility that oncogenic Lck may activate Lmo2 promoter through direct interaction. The resulting increase in Lmo2 expression can potentially contribute to the transforming capacity of oncogenic Lck. All together, we report here a novel mechanism of Lmo2 gene regulation by the oncogenic Lck kinase. Previously, we have characterized the role of STAT5b in Lck-mediated transformation. The Lmo2 gene promoter lacks predicted STAT5 binding sites suggesting that Lck-mediated Lmo2 promoter activation is STAT5-independent. Our data suggest that oncogenic Lck can initiate both STAT5- dependent and independent signals that are likely to be controlled by the spatial localization of the oncoprotein. Subcellular compartmentalization of Lck therefore adds to the complexity that defines cellular transformation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4008. doi:1538-7445.AM2012-4008