Molecular mechanisms of Lck-mediated cellular transformation and SOCS-induced tumor suppression

Lymphocyte-specific protein tyrosine kinase (Lck) is a cytosolic Src family protein tyrosine kinase that has been implicated in lymphoid and non-lymphoid malignancies. We have previously shown that Lck-transformed cells exhibit persistent activation of the Janus kinase and signal transducer and activator of transcription (JAK-STAT) pathway. However, recent reports of nuclear Lck are suggestive of a novel, uncharacterized function by which Lck can induce cellular events. Here, we aim to understand the role of nuclear Lck during transformation. Similar to previous observations in breast cancer cells, we observed nuclear Lck in transformed 293 and murine T lymphoma (LSTRA) cells. Nuclear Lck was active, confirmed by the phosphorylation status of positive regulatory tyrosine 394. Using chromatin immunoprecipitation assay, we found that nuclear Lck directly binds the promoter region of lmo2, an oncogene involved in the development of T-cell acute lymphoblastic leukemia. The subsequent regulation of lmo2 promoter region by Lck results in increased Lmo2 expression. Furthermore, the absence of STAT5-binding site in the Lmo2 gene promoter supports the role of STAT5-independent mechanisms in Lck-mediated oncogenesis. These data are consistent with previous reports that link nuclear localization of tyrosine kinases such as JAK and Src with regulation of nuclear processes like chromatin restructuring and gene expression. It highlights the multiple distinct mechanisms by which oncoproteins induce atypical cellular activities.;In addition to aberrant Lck activity, the absence of Suppressor of cytokine signaling (SOCS) proteins also contributes to tumorigenesis. SOCS, a family of eight proteins, are endogenous negative regulators of tyrosine kinases. Our previous studies have shown that enforced expression of SOCS1 and SOCS3 inhibit Lck-mediated transformation leading to the hypothesis that SOCS function as tumor suppressors. However, the molecular mechanisms of tumor suppression are not understood. Here, we show that oncogenic Lck directly interacts with SOCS1, SOCS2, SOCS3 and CIS. Of the four SOCS proteins, SOCS1 has the highest affinity in interacting with oncogenic Y505FLck, mediated via an SH2-phosphotyrosine interaction. The interaction with kinase-active Lck, however, does not result in tyrosine phosphorylation of SOCS1. Instead, we detected phosphorylation of SOCS3 suggesting that different SOCS proteins respond to an oncoprotein in distinct ways. In addition, the identification of nuclear SOCS leads us to hypothesize that a nuclear interaction between SOCS and oncogenic Lck may contribute to the biological outcome of SOCS expression in Lck-transformed cells.;In summary, our results suggest that oncoproteins and tumor suppressors employ distinct mechanisms in the cytoplasmic and nuclear compartments to support their functions. Additional studies that characterize the contributions of nuclear oncoproteins and tumor suppressors in determining cell fate will lead to a better understanding of tumor biology and help develop better therapies.