SHPS-1 and a synthetic peptide representing its ITIM inhibit the MyD88, but not TRIF, pathway of TLR signaling through activation of SHP and PI3K in THP-1 cells

Background: Src homology 2 domain-containing protein tyrosine phosphatase substrate (SHPS)-1 is known to have regulatory effects on myeloid cells. However, its role in macrophage activation is not clearly understood. Methods and results: In order to investigate the role of SHPS-1 in Toll-like receptor (TLR)-mediated activation, human monocytic cell lines were treated with anti-SHPS-1 monoclonal antibody. The triggering of SHPS-1 blocked the expression of IL-8 and TNF-α in cells treated with a TLR4 ligand that induces a signaling pathway involving myeloid differentiation factor 88 (MyD88) and Toll-interleukin-1 receptor (TIR)-domain-containing adapter-inducing interferon-β (TRIF). Interestingly, SHPS-1 inhibited TLR9/MyD88-mediated, but not TLR3/TRIF-mediated, expression of IL-8. Accordingly, a synthetic peptide representing the immunoreceptor tyrosine-based inhibition motif (ITIM) of SHPS-1 suppressed only the MyD88 pathway. Utilization of specific inhibitors and Western blot analysis indicated that the inhibitory effects were mediated by Src homology 2 domain-containing phosphatases (SHPs) and phosphoinositide 3-kinase (PI3K). Conclusion: SHPS-1 negatively regulates the MyD88-dependent TLR signaling pathway through the inhibition of NF-κB activation.