Lead Inhibition of DNA-Binding Mechanism of Cys₂His₂Zinc Finger Proteins

Abstract

The association of lead with chromatin in cells suggests that deleterious metal effects may in part be mediated through alterations in gene function. To elucidate if and how lead may alter DNA binding of cysteine-rich zinc finger proteins, lead ions were analyzed for their ability to alter the DNA binding mechanism of the Cys(2)His(2) zinc finger protein transcription factor IIIA (TFIIIA). As assayed by DNase I protection, the interaction of TFIIIA with the 50-bp internal control region of the 5S ribosomal gene was partially inhibited by 5 microM lead ions and completely inhibited by 10 to 20 microM lead ions. Preincubation of free TFIIIA with lead resulted in DNA-binding inhibition, whereas preincubation of a TFIIIA/5S RNA complex with lead did not result in DNA-binding inhibition. Because 5S RNA binds TFIIIA zinc fingers, this result is consistent with an inhibition mechanism via lead binding to zinc fingers. The complete loss of DNase I protection on the 5S gene indicates the mechanism of inhibition minimally involves the N-terminal fingers of TFIIIA. Inhibition was not readily reversible and occurred in the presence of an excess of beta-mercaptoethanol. Inhibition kinetics were fast, progressing to completion in approximately 5 min. Millimolar concentrations of sulfhydryl-specific arsenic ions were not inhibitory for TFIIIA binding. Micromolar concentrations of lead inhibited DNA binding by Sp1, another Cys(2)His(2) finger protein, but not by the nonfinger protein AP2. Inhibition of Cys(2)His(2) zinc finger transcription factors by lead ions at concentrations near those known to have deleterious physiological effects points to new molecular mechanisms for lead toxicity in promoting disease.

Environmental lead exposure during early life alters granule cell neurogenesis and morphology in the hippocampus of young adult rats

“…As these author point out, one potential mechanism of Pb 2+ -induced decrease on progenitor cell proliferation may be due to its inhibitory effects on DNA binding of zinc finger transcription factors such as Sp1 and TFIIIA (Zawia et al, 1998;Hanas et al, 1999). An alternative mechanism by which chronic Pb 2+ exposure alters granule cell neurogenesis may be related to alterations in NMDAR function (Nacher and McEwen, 2006).…”

Zn-, Cd-, and Pb-transcription factor IIIA: properties, DNA binding, and comparison with TFIIIA-finger 3 metal complexes

“…Similarly, incubation of cell extracts with Cd 2+ inhibits MTF-1 binding to its cognate metal response elements according to an EMSA [19]. Hanas et al have also demonstrated that preincubation of Zn-TFIIIA with Pb 2+ prevents specific protein binding to the ICR [27,29]. Nevertheless, Cd 2+ -substituted Sp1 has been shown to be active in DNA binding [28].…”

“…As for Pb 2+ , in spite of its pervasiveness as an environmental contaminant, data about its interaction with zinc finger proteins has been lacking until recently. Hanas et al have reported that Pb 2+ inhibits the interaction of TFIIIA with the ICR [27,29]. Godwin and her coworkers have shown that Pb 2+ forms complexes with other zinc finger peptides which vary in the number of cysteine and histidine ligands [30].…”

Preventive action of zinc against heavy metals toxicity in honeybee

“…In another study, Azooz et al (2011) reported that a high dose combination of Zn and Pb produced a synergystic effect on the inhibition of plant growth parameters. Hanas et al (1999) also reported that Pb inhibited the binding of Zn to proteins. Protein binding was reported to be higher for Pb than for Ca and Zn (Hasiang and Diaz, 2011;Godwin, 2001).…”

“…Park et al (2001) reported that MT did not provide protection against Pb and Fe. The differences in results between the studies may be the result of the different doses used and variations between animal species utilised, which covered a wide scale, including rats and cows (Hanas et al,1999;Verster 2011).…”