Soluble signals from mechanically disrupted lung tissue induce lung-related gene expression in bone marrow-derived cells in vitro

Stem cells and cell therapy approaches in lung biology and diseases

Cell-based therapies with embryonic or adult stem cells, including induced pluripotent stem cells, have emerged as potential novel approaches for several devastating and otherwise incurable lung diseases, including emphysema, pulmonary fibrosis, pulmonary hypertension, and the acute respiratory distress syndrome. Although initial studies suggested engraftment of exogenously administered stem cells in lung, this is now generally felt to be a rare occurrence of uncertain physiologic significance. However, more recent studies have demonstrated paracrine effects of administered cells, including stimulation of angiogenesis and modulation of local inflammatory and immune responses in mouse lung disease models. Based on these studies and on safety and initial efficacy data from trials of adult stem cells in other diseases, groundbreaking clinical trials of cell-based therapy have been initiated for pulmonary hypertension and for chronic obstructive pulmonary disease. In parallel, the identity and role of endogenous lung progenitor cells in development and in repair from injury and potential contribution as lung cancer stem cells continue to be elucidated. Most recently, novel bioengineering approaches have been applied to develop functional lung tissue ex vivo. Advances in each of these areas will be described in this review with particular reference to animal models.

Regulation of surfactant protein B gene expression in bone marrow-derived cells

While investigating the differentiation potential of bone marrow-derived cells, we previously demonstrated upregulated expression of the lung-related surfactant protein B (SP-B) gene in hematopoietic progenitor cells (HPCs) when they were cocultured with macerated lung tissue. During coculture, HPCs differentiated toward a dendritic-like myeloid cell phenotype (hematopoietic progenitor cell-derived dendritic-like cells [HPC-DCs]). However, immature dendritic cells (iDCs) cocultured under identical conditions did not express SP-B mRNA before or after coculture. We have now further examined the regulation of SP-B expression in HPC-DCs and iDCs. Of the transcription factors involved in SP-B gene expression, neither cell type expressed TTF-1, HNF3alpha, or HNF3beta, but both cell types expressed Sp1 and Sp3. Sp1 binding to the SP-B promoter was investigated in these cells. Three novel Sp1 binding motifs were identified in the mouse SP-B promoter. Using chromatin immunoprecipitation, it was demonstrated that Sp1 was bound to all three sites in HPC-DCs after coculture with lung tissue, but not in iDCs. We hypothesized that although genes from multiple lineages may be active in HPCs, gene silencing events, such as methylation, may subsequently occur to suppress expression of these genes in more mature myeloid cells, such as iDCs. Treatment with the demethylating agent 5-azacytidine resulted in expression of the SP-B gene in iDCs. These data indicate that tissue-specific transcription factors are not required to express the lung-related gene SP-B in hematopoietic progenitor cells. Furthermore, silencing events, such as methylation, may occur to suppress lung-related gene expression as progenitor cells become committed toward more mature hematopoietic cell phenotypes.