Suppressor of cytokine signaling 1 regulates an endogenous inhibitor of a mast cell protease

Suppressor of cytokine signaling 1 (SOCS1) is a negative regulator of c-Kit and interleukin-3 (IL-3) receptor signaling. We examined the role of SOCS1 in regulating IL-3-induced cell growth of primary bone marrow-derived mast cells (BMMCs) from SOCS1(-/-) mice. Instead of showing increased proliferation, SOCS1-deficient BMMCs responded poorly to IL-3 and stem cell factor. SOCS1(-/-) BMMCs showed increased apoptosis and defective cell cycle entry. We show that the growth retardation of SOCS1(-/-) BMMCs was due to a cell intrinsic defect. Protein tyrosine phosphorylation following IL-3 stimulation was markedly diminished in SOCS1(-/-) BMMCs. Intriguingly, JAK2 and STAT5 proteins were selectively diminished in SOCS1(-/-) BMMCs, which also showed lower molecular mass products of p85 and Vav suggesting proteolytic degradation. Incubation of the SOCS1(-/-) BMMC lysate with STAT5, p85, and Vav immunoprecipitated from SOCS1(+/+) cells directly demonstrated the dysregulated proteolytic activity in SOCS1(-/-) BMMCs. The proteolytic activity in SOCS1(-/-) BMMCs was selectively inhibited by phenylmethylsulfonyl fluoride and soybean trypsin inhibitor, suggesting that the protease regulated by SOCS1 is a tryptase. The dysregulated tryptase in SOCS1(-/-) BMMCs is unlikely to be mMCP6 or mMCP7, because the enzyme activity was not inhibited by Polybrene but was inhibited by normal mouse plasma. SOCS1(+/+) BMMC lysate inhibited the proteolytic activity present in SOCS1(-/-) BMMC lysate, indicating that SOCS1(-/-) BMMCs lack an endogenous protease inhibitor. These results show that SOCS1 is required for the expression and/or stability of an endogenous protease inhibitor, which protects mast cells from their own proteolytic enzymes.