Bruton's Tyrosine Kinase Activity and Inositol 1,4,5-Trisphosphate Production Are Not Altered in DT40 Lymphoma B Cells Exposed to Power Line Frequency Magnetic Fields

Abstract

Exposure of wild-type DT40 lymphoma B cells or Bruton's tyrosine kinase (BTK)-deficient DT40 cells reconstituted with the human btk gene to a 1-gauss 60-Hz electromagnetic field (EMF) has been reported to rapidly increase inositol 1,4,5-trisphosphate (Ins 1,4,5-P 3 ) production (1, 2). Here we have used BTK-deficient DT40 B cells reconstituted with the human btk gene to evaluate the reproducibility of these findings. An experimental design with blinded exposures and anti-IgM treatment to induce Ins 1,4,5-P 3 production as a positive control, showed no significant effect of a 1-gauss 60-Hz EMF on Ins 1,4,5-P 3 production. Because recent work has shown that the activation of BTK was required for EMF-responsiveness (2), we also evaluated the reproducibility of this finding in wild-type DT40 cells. BTK was activated in a dose-and time-dependent manner by treatment with the tyrosine phosphatase inhibitor pervanadate. However, the ability to detect BTK activation, as measured by increased autophosphorylation by immune complex kinase assay, was dependent on the kinase buffer. Using cells from the original investigators, no evidence was obtained to support the hypothesis that exposure to a 1-gauss 60-Hz EMF had a causal effect on protein-tyrosine kinase activities affecting Ins 1,4,5-P 3 production.Epidemiological studies suggesting a causal link between magnetic field exposure from 60-Hz power lines and childhood leukemia have focused attention on the possible molecular mechanisms by which 60-Hz electric and magnetic fields (EMFs) 1 may contribute to childhood leukemia. However, mechanistic research is complicated by the lack of established biomarkers of exposure or response. Although various biophysical mechanisms have been proposed, there is no generally accepted mechanism for the biological effects of EMFs (3). Because of the uncertainty of biologically meaningful exposure metrics (4), it is essential that in vitro exposure studies provide a precise, known, consistent condition of exposure to EMF with as much control over environmental factors as possible. However, our understanding of the interaction of weak magnetic fields with biological systems is severely limited by the lack of well understood model systems that have been shown to respond to EMF exposure.Exposure of human B-lineage leukemia cells to a 1-gauss 60-Hz EMF resulted in an enhanced tyrosine phosphorylation profile of substrate proteins and in LYN and SYK proteintyrosine kinase (PTK) activation, followed by protein kinase C stimulation (5). The activation of protein kinase C was dependent on the activation of LYN kinase, a PTK of the Src family that is known to be involved in signal cascades affecting proliferation and survival of B-lymphoid cells. More recently, the DT40 chicken lymphoma B-cell model was used to demonstrate that LYN is essential for increased inositol 1,4,5-trisphosphate (Ins 1,4,5-P 3 ) production that increased within seconds of EMF exposure through a mechanism involving phospholipase C␥2 activation (1). Further, in DT40 cells, BTK...

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Power-line frequency electromagnetic fields do not induce changes in phosphorylation, localization, or expression of the 27-kilodalton heat shock protein in human keratinocytes.

“…An alternative hypothesis is that EMF exposure alters the biochemical signaling pathways that regulate cell proliferation, differentiation, and apoptosis (12,13). A number of gene products involved in signal transduction, including tyrosine kinases, phospholipase C, protein kinase C, and protein kinase A, have been linked to effects of EMF exposure (13)(14)(15), but the results have not been consistent (12,16,17). A highly cited study by Lin and Goodman (18) reported that EMF exposure induced expression of c-myc, an oncogene implicated in a host of cancers including breast cancer.…”