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B
activation and tumor necrosis factor-
expression
1 Department of Biochemistry and Molecular Biology and 2 Department of Medicine, University of Maryland School of Medicine, Baltimore, Maryland 21201
Although tumor necrosis factor (TNF)-
is implicated in numerous cardiac pathologies, the intracellular events
leading to its production by heart cells are largely unknown. The goal
of the present study was to identify the role of the transcription factor nuclear factor (NF)-
B in this process. Among the many inducers of TNF-
expression in myeloid cells, only
lipopolysaccharide (LPS) led to its induction in cultured neonatal
myocytes. LPS also activated the NF-
B pathway, as evidenced by the
degradation of the inhibitory protein I
B and the appearance of
NF-
B-binding complexes in nuclear extracts. Furthermore, inhibitors
of NF-
B activation, such as lactacystin, MG132, and pyrrolidine
dithiocarbamate, were found to completely block the production of
TNF-
in response to LPS stimulation, indicating a requirement of
NF-
B for TNF-
expression. However, interleukin-1
and phorbol
12-myristate 13-acetate also activated NF-
B but did not evoke
TNF-
expression, revealing that this factor is not sufficient for
cytokine production. Detailed examination of the NF-
B cascade
revealed that cardiac cells displayed a unique pattern of I
B
degradation in response to LPS, with I
B
but not I
B
being
degraded upon stimulation. Additionally, two specific p65-containing
DNA-binding complexes were observed in the nuclear extracts of neonatal
cardiomyocytes: an inducible complex that is necessary for TNF-
expression and a constitutive species. Taken together, these results
reveal that NF-
B is not only involved in cytokine production but
also may be linked to other pathways that subserve a constitutive,
protective mechanism for the heart cell.
lipopolysaccharide; nuclear factor-
b; interleukin-1
; cytokine
production
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