TGF- 1 stimulates human AT1 receptor expression in lung fibroblasts by cross talk between the Smad, p38 MAPK, JNK, and PI3K signaling pathways

Martin MM, Buckenberger JA, Jiang J, Malana GE, Knoell DL, Feldman DS, Elton TS. TGF1 stimulates human AT1 receptor expression in lung fibroblasts by cross talk between the Smad, p38 MAPK, JNK, and PI3K signaling pathways. Am J Physiol Lung Cell Mol Physiol 293: L790–L799, 2007. First published June 29, 2007; doi:10.1152/ajplung.00099.2007.—Both angiotensin II (ANG II) and transforming growth factor1 (TGF1) are thought to be involved in mediating pulmonary fibrosis. Interactions between the reninangiotensin system (RAS) and TGF1 have been well documented, with most studies describing the effect of ANG II on TGF1 expression. However, recent gene expression profiling experiments demonstrated that the angiotensin II type 1 receptor (AT1R) gene was a novel TGF1 target in human adult lung fibroblasts. In this report, we show that TGF1 augments human AT1R (hAT1R) steady-state mRNA and protein levels in a doseand time-dependent manner in primary human fetal pulmonary fibroblasts (hPFBs). Nuclear run-on experiments demonstrate that TGF1 transcriptionally activates the hAT1R gene and does not influence hAT1R mRNA stability. Pharmacological inhibitors and specific siRNA knockdown experiments demonstrate that the TGF1 type 1 receptor (T RI/ALK5), Smad2/3, and Smad4 are essential for TGF1-stimulated hAT1R expression. Additional pharmacological inhibitor and small interference RNA experiments also demonstrated that p38 MAPK, JNK, and phosphatidylinositol 3-kinase (PI3K) signaling pathways are also involved in the TGF1-stimulated increase in hAT1R density. Together, our results suggest an important role for cross talk among Smad, p38 MAPK, JNK, and PI3K pathways in mediating the augmented expression of hAT1R following TGF1 treatment in hPFB. This study supports the hypothesis that a self-potentiating loop exists between the RAS and the TGF1 signaling pathways and suggests that ANG II and TGF1 may cooperate in the pathogenesis of pulmonary fibrosis. The synergy between these systems may require that both pathways be simultaneously inhibited to treat fibrotic lung disease.