title: Thiopurine Prodrugs Mediate Immunosuppressive Effects by Interfering with Rac1 Protein Function.
abstract: 6-Thiopurine (6-TP) prodrugs include 6-thioguanine and azathioprine. Both are widely used to treat autoimmune disorders and certain cancers. This study showed that a 6-thioguanosine triphosphate (6-TGTP), converted in T-cells from 6-TP, targets Rac1 to form a disulfide adduct between 6-TGTP and the redox-sensitive GXXXXGK(S/T)C motif of Rac1. This study also showed that, despite the conservation of the catalytic activity of RhoGAP (Rho-specific GAP) on the 6-TGTP-Rac1 adduct to produce the biologically inactive 6-thioguanosine diphosphate (6-TGDP)-Rac1 adduct, RhoGEF (Rho-specific GEF) cannot exchange the 6-TGDP adducted on Rac1 with free guanine nucleotide. The biologically inactive 6-TGDP-Rac1 adduct accumulates in cells because of the ongoing combined actions of RhoGEF and RhoGAP. Because other Rho GTPases, such as RhoA and Cdc42, also possess the GXXXXGK(S/T)C motif, the proposed mechanism for the inactivation of Rac1 also applies to RhoA and Cdc42. However, previous studies have shown that CD3/CD28-stimulated T-cells contain more activated Rac1 than other Rho GTPases such as RhoA and Cdc42. Accordingly, Rac1 is the main target of 6-TP in activated T-cells. This explains the T-cell-specific Rac1-targeting therapeutic action of 6-TP that suppresses the immune response. This proposed mechanism for the action of 6-TP on Rac1 performs a critical role in demonstrating the capability to design a Rac1-targeting chemotherapeutic agent(s) for autoimmune disorders. Nevertheless, the results also suggest that the targeting action of other Rho GTPases in other organ cells, such as RhoA in vascular cells, may be linked to cytotoxicities because RhoA plays a key role in vasculature functions.
authors.affiliation: From the Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019.
authors.forename: Jin-Young
authors.initials: JY
authors.lastname: Shin
authors.affiliation: From the Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019.
authors.forename: Michael
authors.initials: M
authors.lastname: Wey
authors.affiliation: From the Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019.
authors.forename: Hope G
authors.initials: HG
authors.lastname: Umutesi
authors.affiliation: the Department of Cell Biology and Immunology, University of North Texas Health Science Center, Fort Worth, Texas 76107, and.
authors.forename: Xiangle
authors.initials: X
authors.lastname: Sun
authors.affiliation: the Department of Cell Biology and Immunology, University of North Texas Health Science Center, Fort Worth, Texas 76107, and the Department of Pharmaceutical Sciences, UNT System College of Pharmacy, University of North Texas Health Science Center, Fort Worth, Texas 76107.
authors.forename: Jerry
authors.initials: J
authors.lastname: Simecka
authors.affiliation: From the Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019, jheo@uta.edu.
authors.forename: Jongyun
authors.initials: J
authors.lastname: Heo
mesh_terms: D020816:Amino Acid Motifs; D001327:Autoimmune Diseases / Q000188:drug therapy / Q000378:metabolism; D015496:CD4-Positive T-Lymphocytes / Q000378:metabolism*; D020690:GTPase-Activating Proteins / Q000378:metabolism; D006801:Humans; D007166:Immunosuppressive Agents / Q000493:pharmacokinetics*; D011355:Prodrugs / Q000494:pharmacology*; D064067:Rho Guanine Nucleotide Exchange Factors / Q000378:metabolism; D013872:Thionucleosides / Q000493:pharmacokinetics*; D020764:cdc42 GTP-Binding Protein / Q000378:metabolism; D020830:rac1 GTP-Binding Protein / Q000037:antagonists & inhibitors* / Q000378:metabolism*
chemical_list: D020690:GTPase-Activating Proteins; D007166:Immunosuppressive Agents; D011355:Prodrugs; C107953:RAC1 protein, human; D064067:Rho Guanine Nucleotide Exchange Factors; D013872:Thionucleosides; C085379:rho GTPase-activating protein; D020764:cdc42 GTP-Binding Protein; D020830:rac1 GTP-Binding Protein
keywords: 6-TGNP-Rac1 adduct; CD4+ cells; Rac1 inactivation; Ras-related C3 botulinum toxin substrate 1 (Rac1); T-cell; disulfide; immunosuppression; redox regulation; redox-sensitive GXXXXGK(S/T)C motif; thiopurine prodrugs