Increased smooth muscle mass due to hyperplasia and hypertrophy of airway
smooth muscle (ASM) cells is a common feature in asthma. Angiotensin II
(Ang II), a potent vasoconstrictor and mitogen for a wide variety of
cells, has recently been implicated in bronchoconstriction in asthmatics.
However, a possible mitogenic role as well as underlying molecular
mechanisms of this octapeptide in human ASM cells are not yet known. We
studied the effects of Ang II on ASM cell proliferation and growth and on
the expression of three transcription factors, egr-1, c-fos, and c-jun, as
well as a cytokine, transforming growth factor-beta1 (TGF-beta1). Human
ASM cells were isolated by enzymatic digestion of bronchial smooth muscle
obtained from lung resection tissue. Confluent cells were growth-arrested
and subsequently incubated with Ang II (100 nM) for different time periods
and processed for the measurement of cell growth and gene expression. Ang
II significantly induced DNA and protein synthesis in human ASM cells at 8
h, resulting in a net increase in the accumulation of protein over DNA
(i.e., cellular hypertrophy) at 16 h of incubation. Cell counts and
MTT-reduction assay, however, showed no increase in cell number as a
result of Ang II stimulation. Ang II stimulated the expression of egr-1
and c-fos as early as 15 min, reaching maximum levels at 45 min, whereas
the expression of c-jun peaked at 2 h of Ang II exposure. Furthermore,
steady-state mRNA levels of TGF-beta1 were upregulated by Ang II after 4 h
and reached peak levels at 16 h of incubation. Secretion of biologically
active TGF-beta1 from human ASM cells was significantly (P <= 0.02)
enhanced by Ang II incubation after 8 h, which remained elevated until 24
h. Our results suggest that the Ang II-induced transient early expression
of transcription factors may regulate autocrine genes like TGF-beta1, of
which the subsequent late upregulation could contribute to cellular
hypertrophy during, for example, airway remodeling in asthma
