In vitro inhibition of the enzymatic activity of tumor suppressor FHIT gene product by carcinogenic transition metals.
FHIT (Fragile Histidine Triad) is a human tumor suppressor gene. The Fhit protein is believed to inhibit tumor growth by inducing apoptosis through interaction with diadenosine triphosphate (Ap(3)A). The latter is first sequestered and eventually hydrolyzed by Fhit to ADP and AMP. Thus, the balance between the cellular Ap(3)A level and Fhit enzymatic activity may affect cell death or survival. Increasing the Ap(3)A level, e.g., by inhibition of the enzyme, should prevent apoptosis and thus sustain tumorigenesis. To test if certain carcinogenic transition metals could inhibit the enzymatic activity of Fhit, purified human Fhit protein [30 nM in 1.25 mM poly(vinylpyrrolidone)], expressed in and isolated from E. coli, was incubated at pH 6.8 (50 mM HEPES buffer in 150 mM NaCl) with 120 microM Ap(3)A in the presence of 5 mM Mg(II) (activating cation) and 0-100 microM Ni(II), Cu(II), Zn(II), Cd(II), Co(II), Cr(III), As(III), or As(V). The reaction mixtures were analyzed by HPLC. The results revealed a strong inhibitory potential of Cu(II) [0.4], followed by Ni(II) [3.5] >or= Zn(II) [7.0] >> Cr(III) [73] > Cd(II) [98] >> Co(II) [432] [the numbers in brackets are IC(50) values, microM]. As(III) and As(V) had no effect. As revealed by spectrophotometry, mass spectrometry, and gel electrophoresis, the exceptionally strong inhibition by Cu(II) was associated with Fhit dimerization through formation of a disulfide bond. The other metals and also H(2)O(2) and NO did not cause the dimerization. Thus, the effect of Cu(II) must be due to its reaction with Cys-39 bearing the only thiol group in Fhit monomer. Since Cys-39 is not readily accessible in the Fhit molecule, the reaction is most likely facilitated by conformational changes which follow the coordination of Cu(II) by the surface histidines 35, 94, and/or 96. The observed inhibition of Fhit may be mechanistically involved in metal-mediated toxicity and carcinogenesis.
Kowara
Renata
R
Laboratory of Comparative Carcinogenesis, National Cancer Institute at Frederick, Frederick, Maryland 21702-1201, USA.
Karaczyn
Aldona A
AA
Fivash
Matthew J
MJ
Kasprzak
Kazimierz S
KS
D017766:Acid Anhydride Hydrolases / Q000037:antagonists & inhibitors*; D001665:Binding Sites; D002021:Buffers; D002273:Carcinogens / Q000494:pharmacology*; D002413:Cations, Divalent; D003545:Cysteine / Q000737:chemistry; D015226:Dinucleoside Phosphates / Q000737:chemistry; D004791:Enzyme Inhibitors / Q000494:pharmacology; D016147:Genes, Tumor Suppressor; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008274:Magnesium; D009363:Neoplasm Proteins / Q000037:antagonists & inhibitors* / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D028561:Transition Elements / Q000494:pharmacology*; D000099170:Bis(5'-adenosyl)triphosphatase
D017766:Acid Anhydride Hydrolases; D002021:Buffers; D002273:Carcinogens; D002413:Cations, Divalent; D003545:Cysteine; D015226:Dinucleoside Phosphates; D004791:Enzyme Inhibitors; D008274:Magnesium; D009363:Neoplasm Proteins; D028561:Transition Elements; D000099170:Bis(5'-adenosyl)triphosphatase; C011440:diadenosine triphosphate