Quantitative Proteome Analysis of Alveolar Type-II Cells Reveals a Connection of Integrin Receptor Subunits Beta 2/6 and WNT Signaling

Alveolar type-II cells (ATII cells) are lung progenitor cells responsible
for regeneration of alveolar epithelium during homeostatic turnover
and in response to injury. Characterization of ATII cells will have
a profound impact on our understanding and treatment of lung disease.
The identification of novel ATII cell-surface proteins can be used
for sorting and enrichment of these cells for further characterization.
Here we combined a high-resolution mass spectrometry-based membrane
proteomic approach using lungs of the SILAC mice with an Affymetrix
microarray-based transcriptome analysis of ATII cells. We identified
16 proteins that are enriched in the membrane fraction of ATII cells
and whose genes are highly expressed in these cells. Interestingly,
we confirmed our data for two of these genes, integrin beta 2 and
6 (Itgb2 and Itgb6), by qRT-PCR
expression analysis and Western blot analysis of protein extracts.
Moreover, flow cytometry and immunohistochemistry in adult lung revealed
that ITGB2 and ITGB6 are present in subpopulations of surfactant-associated-protein-C-positive
cells, suggesting the existence of different types of ATII cells.
Furthermore, analysis of the Itgb2–/– mice showed that Itgb2 is required for proper WNT
signaling regulation in the lung.