SOCS3 Plays an important negative regulatory role in myeloid colony formation

Abstract

The family of SOCS proteins are cytokine-responsive genes, which function as part of a negative feedback loop. In vitro, several growth factors including EPO, TPO, GM-CSF, IL3 and G-CSF have been shown to rapidly and transiently induce SOCS3 expression, and cycloheximide results in super-induction of SOCS3 message. We found that SOCS 3 basal expression was undetectable in human bone marrow and other tissues surveyed, but low levels of SOCS mRNA were detected in both human and murine peripheral blood leukocytes. Culture of BM with G-CSF for 3 to 7 days resulted in increased SOCS 3 mRNA, whereas SOCS 1 expression was not induced. In vivo expression of SOCS 3 was studied in mice treated twice daily with 50 ug/kg G-CSF for 4 days. SOCS 3 mRNA expression was rapidly induced in bone marrow and PBLs, but by 24 hours decayed to background levels in both tissues. Repeated daily injections of G-CSF yielded the same rapid and transient pattern, with some increase in maximal mRNA levels over the 4-day period. To determine if SOCS 3 regulates myeloid response to G-CSF, we tested anti-sense oiigonucleotides (ASO) on CFU-G formation in human bone marrow. SOCS 3 ASO increased the number of CFU-G colonies in the presence of sub-optimal or maximal G-CSF by 3-10-fold. The sensitivity to G-CSF was also increased up to ten-fold. The responses to SOCS3 ASO appear to be stage-specific, because the CFU-GEMM response was unchanged. Treatment with SOCS1, SOCS2 or CIS ASO had no significant effect on CFU-G assays. These results indicate that SOCS3 has an important regulatory role in regulation of normal myelopoiesis.