Low level Hg2+ exposure modulates the B-cell cytoskeletal phosphoproteome

Abstract

Exposure of Wehi-231 B-cells to Hg²⁺ for 5min resulted in concentration dependent changes in protein phosphorylations. Phosphorylation was quantified using mass spectrometry to analyze TiO₂ and anti-pTyr antibody selected phosphopeptides from Wehi-231 digests. The most frequent and largest amplitude responses to Hg²⁺ exposure were increased phosphorylation although a decrease was observed for 1% of phosphoproteins detected in the untreated cells. A subset of proteins responded with an increase in phosphorylation to Hg²⁺ exposure at low micromolar concentrations. The majority of proteins required Hg²⁺ over 20μM in order to increase phosphorylation. Ser/Thr phosphorylations are prominent in the cytoskeletal organization and the GTPase signaling systems and these systems are notable as the primary ones responding to the lowest concentrations of Hg²⁺. Systems that required higher concentrations of Hg²⁺ to increase phosphorylation included immune receptor signaling. The proteins for which an increase in phosphorylation occurred at Hg²⁺ above 20μM have a higher proportion of pTyr sites. Anti Ig stimulation of Wehi-231 cells confirmed that cytoskeletal protein phosphorylation and GTPase signaling are modulated in physiologically relevant B-cell receptor activation. Candidate kinases that respond to Hg²⁺ exposure at the low μM concentrations include MAP Kinase 1, CaM Kinase II delta and PAK2.

Significance: Mercury (Hg) is a wide spread environmental toxicant. Epidemiological and laboratory studies suggest that exposure to environmental Hg at current levels, which have been perceived to be non-toxic, may contribute to immune system dysfunction and autoimmune disease in humans and animals respectively. While we have previously shown that exposure of B lymphocytes to low levels of mercury interferes with B-cell receptor signaling mediated by post transcriptional phosphorylation events, overall the mechanism that is responsible for increased autoimmunity in mercury exposed human or animal populations is not well understood. The current study evaluated the dose dependent actions of mercury to change phosphorylation in the Wehi-231 cell line, an immature B-cell model in which actions of mercury on development of cell function can be evaluated. The study identified the cytoskeletal proteins as the most sensitive to modulation by mercury with changes in Ser/Thr phosphorylation being observed at the lowest concentrations of mercury. These findings indicate that the actions of mercury on B-cell immune function and development are at least in part likely mediated through changes in cytoskeletal protein phosphorylation.

Keywords: B-cell; Cytoskeleton; Mercury; Phosphoproteomics; Wehi-231.