The functions of inhibitor of DNA-binding proteins in endothelial cells during lung development and disease
The inhibitor of DNA-binding (Id) genes encode a family of helix-loop-helix proteins lacking the basic DNA-binding domain. The Id proteins function as dominant negative factors by dimerizing with other transcription factors, inhibiting DNA binding and transcriptional activation. Four members of the Id family (Id1-4) have been identified in mammals, expressed in spatially and temporally restricted patterns in many developing organs and have been implicated in the regulation of cell proliferation, apoptosis, differentiation and migration. Both Id1 and Id3 are highly expressed in the lung mesenchyme during vascular development, suggesting that they play indispensable roles in endothelial morphogenesis. In agreement, we found that Id1-/-Id3-/- lungs exhibit defects in distal angiogenesis after prolonged culture and implantation in renal capsules. MMP-2 is significantly under-expressed in Id1-/-Id3-/- lung endothelial cells, suggesting its contribution to the phenotype. These findings indicate that Id proteins are key components of embryonic lung vascular development. In this study we also found that, upon bleomycin treatment, Id1 expression was upregulated in a number of lung cell types but predominantly in endothelial cells, as revealed by double immunolabeling and quantitative FACS sorting analysis. As the result of ablated Id1 function, bleomycin-injured Id1-/- lungs showed increased vascular permeability and endothelial apoptosis. Accordingly, bleomycin-treated Id1-/- lung microvascular endothelial cells also showed decreased survival in culture. We detected a decrease in the level of Bcl-2, a primary anti-apoptotic protein, in Id1-/- endothelial cells, suggesting that downregulated Bcl-2 may promote endothelial apoptosis in the lung. Therefore, we propose that Id1 plays an important role in promoting endothelial survival in the adult lung upon injury. In addition, mice lacking Id1 function displayed increased collagen accumulation and fibrogenesis after long-term bleomycin exposure, suggesting that Id1 upregulation in the endothelium and other lung cell types may play a critical role in lung homeostasis.