T-cell receptor signaling is mediated by transient Lck activity, which is inhibited by inorganic mercury.

Genetically susceptible rodents exposed to low nontoxic levels of inorganic mercury (Hg(2+)) develop idiosyncratic autoimmune disease associated with defective T-cell function. However, the molecular mechanisms underlying this phenomenon remain mostly unexplained. Brief exposure of T cells to micromolar concentrations of Hg(2+) leads to physiologically relevant nontoxic cellular mercury burdens, and as we have previously reported, attenuates T-cell receptor (TCR) signal strength by approximately 50%. We have found this to be the result of an inadequate activation of the tyrosine kinase ZAP-70, which is hypophosphorylated following TCR stimulation in Hg(2+) burdened cells when compared to untreated controls. In T cells, ZAP-70 phosphorylation is dependent on lymphocyte-specific protein tyrosine kinase (Lck) activity, which in turn is either positively or negatively regulated by the phosphorylation of specific Lck tyrosine residues. In particular, the general belief is that Lck is negatively regulated by phosphorylation of tyrosine 192 (Y192). We now demonstrate by Western blotting that, in Jurkat T cells, TCR signal transduction (and ZAP-70 phosphorylation) was positively associated with a rapid transient phosphorylation of Y192, which was inhibited in cells that were briefly (5 min) exposed to 5 microM Hg(2+). Thus, Hg(2+) inhibits a critical activating role played by Lck Y192 during the most proximal events of the TCR-induced cell signaling.

Low and nontoxic inorganic mercury burdens attenuate BCR-mediated signal transduction.

The ubiquitous environmental heavy metal contaminant mercury (Hg) is a potent immunomodulator that has been implicated as a factor contributing to autoimmune disease. However, the mechanism(s) whereby Hg initiates or perpetuates autoimmune responses, especially at the biochemical/molecular level, remain poorly understood. Recent work has established a relationship between impaired B-cell receptor (BCR) signal strength and autoimmune disease. In previous studies, we have shown that in mouse WEHI-231 B cells, noncytotoxic concentrations of inorganic mercury (Hg(+2)) interfered with BCR-mediated growth control, suggesting that BCR signal strength was impaired by Hg(+2). Extracellular signal-regulated kinase (ERK) 1,2 mitogen-activated protein kinase (MAPK) is responsible for the activation of several transcription factors in B cells. Phosphorylation of ERK serves as an essential node of signal integration for the BCR. Thus, the magnitude of ERK activation serves as an operational metric for BCR signal strength. Using Western blotting and phospho-specific flow cytometry, we now show that the kinetics and magnitude of BCR-mediated activation of ERK-MAPK are markedly attenuated in WEHI-231 cells and splenic B cells that have been exposed to low and nontoxic burdens of Hg(+2). However, Hg(+2) does not seem to act directly on ERK-MAPK but rather on an upstream element or elements of the BCR signal transduction pathway, above the level of the key protein tyrosine kinase Syk. Our data suggest that the site of action of Hg(+2) may very well be localized on the plasma membrane. These findings support a connection between Hg(+2) and attenuated BCR signal strength in the etiology of autoimmune disease.