Suppressor of cytokine signaling 3 controls lysosomal routing of G-CSF receptor

The hematopoietic system provides an attractive model for studying growth factor-controlled expansion and differentiation of cells in relation to receptor routing and its consequences for signal transduction. Suppressor of cytokine signaling (SOCS) proteins regulate receptor signaling partly via their ubiquitin ligase (E3)-recruiting SOCS box domain. Whether SOCS proteins affect signaling through modulating intracellular trafficking of receptors is unknown. Here, we show that a juxtamembrane lysine residue (K632) of the granulocyte colony-stimulating factor receptor (G-CSFR) plays a key role in receptor routing and demonstrate that the effects of SOCS3 on G-CSF signaling to a major extent depend on this lysine. Mutation of K632 causes accumulation of G-CSFR in early endosomes and leads to sustained activation of signal transducer and activator of transcription 5 and ERK, but not protein kinase B. Myeloid progenitors expressing G-CSFR mutants lacking K632 show a perturbed proliferation/differentiation balance in response to G-CSF. This is the first demonstration of SOCS-mediated ubiquitination and routing of a cytokine receptor and its impact on maintaining an appropriate signaling output

Engagement of SIRP alpha Inhibits Growth and Induces Programmed Cell Death in Acute Myeloid Leukemia Cells

<p>Background: Recent studies show the importance of interactions between CD47 expressed on acute myeloid leukemia (AML) cells and the inhibitory immunoreceptor, signal regulatory protein-alpha (SIRP alpha) on macrophages. Although AML cells express SIRP alpha, its function has not been investigated in these cells. In this study we aimed to determine the role of the SIRP alpha in acute myeloid leukemia.</p><p>Design and Methods: We analyzed the expression of SIRP alpha, both on mRNA and protein level in AML patients and we further investigated whether the expression of SIRP alpha on two low SIRP alpha expressing AML cell lines could be upregulated upon differentiation of the cells. We determined the effect of chimeric SIRP alpha expression on tumor cell growth and programmed cell death by its triggering with an agonistic antibody in these cells. Moreover, we examined the efficacy of agonistic antibody in combination with established antileukemic drugs.</p><p>Results: By microarray analysis of an extensive cohort of primary AML samples, we demonstrated that SIRP alpha is differentially expressed in AML subgroups and its expression level is dependent on differentiation stage, with high levels in FAB M4/M5 AML and low levels in FAB M0-M3. Interestingly, AML patients with high SIRP alpha expression had a poor prognosis. Our results also showed that SIRP alpha is upregulated upon differentiation of NB4 and Kasumi cells. In addition, triggering of SIRP alpha with an agonistic antibody in the cells stably expressing chimeric SIRP alpha, led to inhibition of growth and induction of programmed cell death. Finally, the SIRP alpha-derived signaling synergized with the activity of established antileukemic drugs.</p><p>Conclusions: Our data indicate that triggering of SIRP alpha has antileukemic effect and may function as a potential therapeutic target in AML.</p>