Submitted May 17, 2006
Accepted September 17, 2006
CD33 responses are blocked by SOCS3 through accelerated proteasomal-mediated turnover
Selinda J Orr, Nuala M Morgan, Joanne Elliott, James F Burrows, Christopher J Scott, Daniel W McVicar, and James A Johnston*
Queens University, Belfast, Northern Ireland
National Cancer Institute, Frederick, United States
* Corresponding author; email: jim.johnston{at}qub.ac.uk.
CD33 is a member of the Sialic acid binding immunoglobulin-like lectin (Siglec) family of inhibitory receptors and a therapeutic target for acute myeloid leukaemia (AML). It contains a cytoplasmic immunoreceptor tyrosine-based inhibitory motif (ITIM), which can recruit SHP-1 and SHP-2. How CD33 expression is regulated is unclear. Suppressor of cytokine signalling 3 (SOCS3) is expressed in response to cytokines, LPS and other PAMPs, and competes with SHP-1/2 binding to ITIMs of cytokine receptors thereby inhibiting signalling. In this study, using peptide pulldown experiments we find that SOCS3 can specifically bind to the phosphorylated ITIM of CD33. Additionally, following cross-linking SOCS3 can recruit the ECS E3 ligase resulting in accelerated proteasomal degradation of both CD33 and SOCS3. Our data suggests the tyrosine motifs in CD33 are not important for internalisation while they are required for degradation. Moreover, SOCS3 inhibited the CD33-induced block on cytokine-induced proliferation. This is the first receptor shown to be degraded by SOCS3 and where SOCS3 and its target protein are degraded concomitantly. Our findings clearly suggest that during an inflammatory response, the inhibitory receptor CD33 is lost by this mechanism. Moreover, this has important clinical implications as tumours expressing SOCS3 may be refractory to 
-CD33 therapy.
