Activation of ERK and JNK1 MAP kinases in cultured lung tissue

The biochemical mechanisms responsible for lung cell growth and proliferation are not well defined during tissue injury. We previously showed stimulation of DNA synthesis in intact lung tissue cultured in vitro after exposure to elevated fractions of O2. By use of this in vitro model, the current study examined the enzymatic activities of the extracellular-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK) mitogen-activated protein (MAP) kinase pathways. ERK and JNK activities were transiently elevated in lung tissue cultured under serum-free conditions. Raf-1 kinase, the primary upstream activator of the ERK pathway, was also transiently activated, suggesting a receptor-mediated ERK activation. Phosphorylation of the guanine nucleotide exchange protein p170 son of sevenless further suggested a receptor-mediated activation of the ERK pathway. c-Fos and c-Jun expression, downstream targets of ERK and JNK, was dramatically increased in cultured tissue compared with uncultured tissue. After the initial transient activation, ERK and JNK could be reactivated with specific agonists, demonstrating that these signaling pathways were functional. These findings demonstrate activation of the ERK and JNK MAP kinase pathways in intact lung tissue and provide a model system to define signaling pathways involved in lung tissue remodeling injury.