ShcA regulates thymocyte proliferation through specific transcription factors and a c-Abl-dependent signaling axis

Signaling via the pre-T-cell receptor (pre-TCR), along with associated signals from Notch and chemokine receptors, regulates the ß-selection checkpoint that operates on CD4^- CD8^- doubly negative (DN) thymocytes. Since many hematopoietic malignancies arise at the immature developmental stages of lymphocytes, understanding the signal integration and how specific signaling molecules and distal transcription factors regulate cellular outcomes is of importance. Here, a series of molecular and genetic approaches revealed that the ShcA adapter protein critically influences proliferation and differentiation during ß-selection. We found that ShcA functions downstream of the pre-TCR and p56^Lck and show that ShcA is important for extracellular signal-regulated kinase (ERK)-dependent upregulation of transcription factors early growth factor 1 (Egr1) and Egr3 in immature thymocytes and, in turn, of the expression and function of the Id3 and E2A helix-loop-helix (HLH) proteins. ShcA also contributes to pre-TCR-mediated induction of c-Myc and additional cell cycle regulators. Moreover, using an unbiased Saccharomyces cerevisiae (yeast) screen, we identified c-Abl as a binding partner of phosphorylated ShcA and demonstrated the relevance of the ShcA-c-Abl interaction in immature thymocytes. Collectively, these data identify multiple modes by which ShcA can fine-tune the development of early thymocytes, including a previously unappreciated ShcA-c-Abl axis that regulates thymocyte proliferation.