Sterile alpha motif and HD domain-containing
protein 1 (SAMHD1) is a deoxynucleoside triphosphohydrolase (dNTPase) with a
nuclear localization signal (NLS). SAMHD1 suppresses innate immune responses to
viral infection and inflammatory stimuli by inhibiting the NF-κB and
type I interferon (IFN-I) pathways. However, whether the dNTPase activity and nuclear localization of SAMHD1 are required for its suppression of innate immunity remains unknown. Here, we report that the dNTPase activity, but not nuclear localization of SAMHD1, is important for its suppression of innate immune responses in differentiated monocytic cells. We generated monocytic U937 cell lines stably expressing WT SAMHD1 or mutated variants defective in dNTPase activity (HD/RN) or nuclear localization (mNLS). WT SAMHD1 in differentiated U937 cells significantly inhibited
lipopolysaccharide-induced expression of
tumor necrosis factor α (TNF-α) and
interleukin-6 (IL-6) mRNAs, as well as IFN-α, IFN-β, and TNF-α
mRNA levels induced by Sendai virus
infection. In contrast, the HD/RN mutant did not exhibit this inhibition in either U937 or THP-1 cells, indicating that the dNTPase activity of SAMHD1 is important for suppressing NF-κB activation. Of note, in
lipopolysaccharide-treated or Sendai virus-infected U937 or THP-1 cells, the mNLS variant reduced TNF-α or IFN-β
mRNA expression to a similar extent as did WT SAMHD1, suggesting that SAMHD1-mediated inhibition of innate immune responses is independent of SAMHD1's nuclear localization. Moreover, WT and mutant SAMHD1 similarly interacted with key
proteins in NF-κB and IFN-I pathways in cells. This study further defines the role and mechanisms of SAMHD1 in suppressing innate immunity.