Socs1 binds to multiple signalling proteins and suppresses Steel factor-dependent proliferation

Abstract

We have identified Socs1 as a downstream component of the Kit receptor tyrosine kinase signalling pathway. We show that the expression of Socs1 mRNA is rapidly increased in primary bone marrow-derived mast cells following exposure to Steel factor, and Socs1 inducibly binds to the Kit receptor tyrosine kinase via its Src homology 2 (SH2) domain. Previous studies have shown that Socs1 suppresses cytokine-mediated differentiation in M1 cells inhibiting Janus family kinases. In contrast, constitutive expression of Socs1 suppresses the mitogenic potential of Kit while maintaining Steel factordependent cell survival signals. Unlike Janus kinases, Socs1 does not inhibit the catalytic activity of the Kit tyrosine kinase. In order to define the mechanism by which Socs1-mediated suppression of Kit-dependent mitogenesis occurs, we demonstrate that Socs1 binds to the signalling proteins Grb-2 and the Rho-family guanine nucleotide exchange factors Vav. We show that Grb2 binds Socs1 via its SH3 domains to putative diproline determinants located in the N-terminus of Socs1, and Socs1 binds to the N-terminal regulatory region of Vav. These data suggest that Socs1 is an inducible switch which modulates proliferative signals in favour of cell survival signals and functions as an adaptor protein in receptor tyrosine kinase signalling pathways.

SOCS proteins in regulation of receptor tyrosine kinase signaling

“…One hour of SCF stimulation induces SOCS1 expression in KIT expressing Ba/F3 cells and in bone marrow derived mast cells [25], suggesting that SOCS1 is an early response gene and that KIT activation leads to transcriptional activation of the SOCS1 gene. SOCS1 mRNA and protein levels were down-regulated in an acute myeloid leukemia (AML) cell lines expressing oncogenic FLT3 upon treatment with sorafenib, a multi-kinase inhibitor that inhibits the oncogenic mutant FLT3-ITD [26].…”

“…Later it was found to associate with multiple RTKs including stem cell factor (SCF) receptor (KIT), FMS-like tyrosine kinase 3 (FLT3), platelet-derived growth factor (PDGF) receptor (PDGFR) and colony stimulating factor 1 (CSF1) receptor (CSF1R) in yeast two-hybrid assays [25]. The interaction in between SOCS1 and RTKs is mediated through SH2 domain of SOCS1 and is dependent on the presence of phosphorylated key tyrosine residues on the receptor.…”

“…Thus it is suggested that, like in the case of KIT, FLT3 activation also lead to transcriptional activation of SOCS1. The resulting expression of the SOCS1 protein then negatively regulates KIT and FLT3-induced mitogenic signaling in hematopoietic cells [25]. Therefore, it is apparent that activation of RTKs leads to transcriptional activation of SOCS1 which then counteracts RTK signaling.…”

“…A possible explanation is that SOCS proteins may not only act as ubiquitin ligases but also as adapters. Indeed, SOCS1 can recruits signaling proteins such as Grb2 and NCK to the KIT through its N-terminal proline-rich region [25].…”

“…Furthermore, expression of SOCS1 along with FLT3-ITD confers resistance to IFN-α and IFN-γ induced cell death in primary murine bone marrow cells. While SOCS1 plays a role in control of multiple RTKs including KIT, FLT3 and MET [25,38], it is unclear how oncogenic FLT3-ITD sustains SOCS1 regulation. Recently it was shown that tyrosine phosphorylation of the adaptor protein SLAP by an oncogenic mutant of KIT, D816V, impairs its ability to recruit the ubiquitin E3 ligase CBL and thereby reduces ubiquitination.…”