The stimuli inducing expression of single immunoglobulin IL-1-related receptor 
(SIGIRR) and the relevant regulatory mechanisms are not well defined. 
Transforming growth factor β1 (TGFβ1) delays internalization of neurokinin-1 
receptor (NK1R) and subsequently enhances cellular signaling. This study 
investigated the effect of TGFβ1 on SIGIRR protein production by human M1 
macrophages in response to stimulation with substance P (SP). SP caused 
upregulation of SIGIRR expression in a concentration-dependent manner, whereas 
aprepitant (an NK1R inhibitor) blunted this response. Silencing p38γMAPK or 
TAK-1 partially attenuated the response to SP stimulation, while TGFβ1/2/3 siRNA 
dramatically diminished it. SP induced much greater SIGIRR protein production 
than either lipopolysaccharide (a TLR4 agonist) or resiquimod (a TLR7/8 
agonist). Unexpectedly, silencing of transcription factor specificity protein 1 
(Sp1) led to significant upregulation of SIGIRR expression after SP stimulation, 
while KLF2 siRNA only partially enhanced it and Fli-1 siRNA reduced it. SP also 
upregulated TGFβ1 expression, along with a corresponding increase of SIGIRR 
protein, whereas silencing TGFβ1/2/3 blunted these responses. Sp1 siRNA or 
mithramycin (a gene-selective Sp1 inhibitor) significantly enhanced the 
expression of TGFβ1 and SIGIRR by macrophages after SP stimulation. Importantly, 
this effect of Sp1 siRNA on TGFβ1 and SIGIRR was blunted by siRNA for Smad2, 
Smad3, or Smad4, but not by TAK-1 siRNA. Next, we investigated the influence of 
transcription factor cross-talk on SIGIRR expression in response to SP. 
Co-transfection of macrophages with Sp1 siRNA and C/EBPβ or TIF1β siRNA 
attenuated the upregulation of SIGIRR by SP, while a combination of Sp1 siRNA 
and Fli-1 siRNA dramatically diminished it. In conclusion, TGFβ1 may be an 
intermediary between SP/NK1R activation and SIGIRR expression in Sp1 
siRNA-transfected macrophages. In addition, Sp1 modulates TGFβ1/Smad signaling 
and negatively regulates SIGIRR protein production by macrophages after SP 
stimulation.