Selective Depletion Of Tumor-associated SAMHD1 By HSP90 Inhibitors Enhances Anti-AML Effect Of Cytarabine

Background:Acute myeloid leukemia(AML)is the most common acute type of leukemia in adults,with a disappointing prognosis and high death rate.Cytarabine(ara-C)is a primary first-line chemotherapy drug used in the treatment of AML,and resistance to it represents a major source of treatment failure.Sterile alpha motif and HD domain-containing protein 1(SAMHD1),a human dNTPase,is known to attenuate ara-C cytotoxicity in AML cells,resulting in treatment failure and a poor prognosis.Therefore,selective inhibition of SAMHD1 in AML should prove a promising strategy for increasing the efficacy of ara-C.However,no effective therapeutic strategy to target SAMHD1 in tumor is available yet.Methods:We used mass spectrometric(MS)system,co-immunoprecipitation(co-IP)assay and immunofluorescent(IF)assay to prove the interaction between SAMHD1 and HSP90 in AML cells and to determine the specific domain of SAMHD1 involved in the interaction.Moreover,Western-blot assay and real-time quantitative PCR assay are used to measure the expression changes of SAMHD1 or other proteins after treatment by HSP90 inhibitors.We also analyzed the combined effect of ara-C and HSP90 inhibitor in vitro by MTT assay and in vivo by orthotopic and heterotopic AML animal models Immune-tumor cell co-culture system is used to study the effects of HSP90 inhibitor on mice immune cells killing tumor cellsResults:Here,for the first time,we report that SAMHD1 is a novel HSP90-dependent protein in various tumor types.Pharmacological inhibition of HSP90 led to SAMHD1 depletion in a wide range of tumor cell types but had little effect on SAMHD1 in peripheral blood mononuclear cells(PBMCs)or macrophages from healthy donors.This is because SAMHD1 is usually phosphorylated in tumor cells whereas unphosphorylated in normal cells,and phosphorylated SAMHD1 binds more HSP90 therefore is more sensitive to HSP90 inhibitor treatment.Moreover,the combination of ara-C and HSP90 inhibitors showed a synergistic effect on killing AML cells.HSP90 inhibitor also significantly enhanced the therapeutic efficacy of ara-C in both orthotopic and heterotopic AML mouse models,alleviating tumor burden and prolonging survival.We also found that the depletion of SAMHD1 by HSP90 inhibitor STA9090 facilitated the production of interferon thereby enhanced the tumor cell killing ability of immune cells ex vivoConclusions:Overall,these data suggest that the addition of an HSP90 inhibitor to ara-C-based chemotherapy represents a potential new treatment strategy for SAMHD1 high-expressed AML patients and have implications for other important functions of SAMHD1.