Protein kinase C regulation of surfactant protein B expression: Role of mitogen activated protein kinase pathways

Surfactant protein B (SP-B) enhances the surface tension reducing function of surfactant by increasing the adsorption of surfactant to the alveolar surface and stabilizing surfactant monolayer through interactions with dipalmitoylphosphatidylcholine, the major phospholipid of surfactant. SP-B expression is reduced in a variety of infectious and inflammatory lung diseases such as acute respiratory distress syndrome (ARDS), pneumonia, respiratory syncytial virus (RSV) infection, asthma and others. Activation of protein kinase C (PKC) has been implicated to play a key role in the development of lung injury in infectious and inflammatory lung diseases. In this study we investigated cellular signaling pathways important for phorbol myristate acetate (PMA), an activator of PKC, inhibition of SP-B gene expression in H441 lung epithelial cells. We found that the PMA inhibited SP-B promoter activity, mRNA and protein levels indicating the involvement of transcriptional mechanisms in the inhibition. Inhibition of PKC with Go6983 but not Go6976 blocked PMA inhibition of SP-B expression indicating the involvement of novel and calcium insensitive PKC isoforms in the inhibition. PMA activated p44/42, p38 and JNK mitogen activated protein kinase (MAPK) signaling pathways and inhibited activation of Akt. Pharmacological inhibition of MAPK signaling pathways demonstrated that inhibitors of p44/42 but not p38, JNK, or phosphoinositide 3-kinase pathway blocked PMA inhibition of SP-B expression indicating a role for the p44/42 MAPK pathway in the inhibition. PMA treatment reduced the DNA binding activities of SP-B promoter TTF-1 and HNF-3 elements but increased AP-1 binding activity. In cells exposed to p44/42 MAPK inhibitors such as PD98059 and U0126 although PMA reduced TTF-1 and HNF-3 binding activities, they were still significantly higher than in cells treated with PMA alone. The p44/42 MAPK inhibitors also reduced PMA induction of AP-1 binding activity. We suggest that PD98059 and U0126 block PMA inhibition of SP-B expression by increasing TTF-1 and HNF-3 DNA binding activities and reducing AP-1 binding activity.