Airway smooth muscle (ASM) is considered to be an end-target cell for the
effects of mediators released during airway wall inflammation. Several
reports suggest that activated ASM may be capable of generating various
proinflammatory cytokines. We investigated the effects of tumor necrosis
factor (TNF)-alpha, a potent proinflammatory cytokine, on cultured human
ASM cells by examining the expression and release of the cytokine
interleukin (IL)-6, cell proliferation, and the expression pattern of
c-fos and c-jun, two nuclear proto-oncogenes constituting the activator
protein-1 transcription factor. Growth-arrested cell monolayers were
stimulated with human recombinant TNF-alpha in a concentration- and
time-dependent manner. TNF-alpha stimulated the expression of IL-6
messenger RNA (mRNA), which was detected after 15 min, reaching a maximum
at 1 h. IL-6 protein was readily detected in ASM cell-conditioned medium
after 2 h of TNF-alpha stimulation. Protein levels increased in a time-
and concentration-dependent manner. Release of IL-6 elicited by TNF-alpha
was significantly inhibited by dexamethasone, cycloheximide, and
nordihydroguaiaretic acid (NDGA). TNF-alpha did not alter DNA biosynthesis
up to 48 h or cell numbers up to 120 h. Northern blot analysis of
proto-oncogene expression revealed that c-fos and c-jun mRNA levels were
elevated after 30 min of TNF-alpha incubation with maximum levels at 1 h
and 45 min, respectively. Expression of c-fos mRNA was downregulated by
NDGA. Four hours of TNF-alpha treatment resulted in translocation of c-jun
immunofluorescence from the cytoplasm to the nucleus in human ASM cells.
Our results suggest that despite the lack of a mitogenic response to
TNF-alpha, upregulation of primary response genes in human ASM cells may
account for the induction of proinflammatory cytokines, such as IL-6, in
human airways