Airway epithelium is composed of 4 major cell types, including ciliated cells, secretory cells, undifferentiated columnar cells, and basal cells (BC). The earliest changes associated with the development of smoking-induced lung diseases, such as chronic obstructive pulmonary disease (COPD) and lung cancer, occur in the airway epithelium, including BC hyperplasia, squamous metaplasia, mucous cell hyperplasia and metaplasia, impaired ciliated cell structure and function, loss of Clara cells, and increased epithelial permeability due to impaired junctional barrier. We hypothesize that fundamental to these changes are smoking-induced derangements of BC, the stem/progenitor cell population that can self-renew and differentiate into ciliated and secretory cells. Using technologies established in our laboratory to culture pure population of BC from the human airway epithelium, to induce differentiation of these BC in air-liquid interface, and to assess the transcriptome of purified BC compared to that of the complete differentiated airway epithelium, our preliminary data indicates that: (1) airway BC exhibit a distinct gene expression signature relevant to stem/progenitor cell function, including high expression of the epidermal growth factor receptor (EGFR); (2) airway BC from healthy smokers have a different gene expression pattern compared to nonsmokers, with enrichment of functional categories related to cell cycle and proliferation and down-regulation of differentiation-associated genes; and (3) constitutive EGF expression in BC and differentiated cells is barely detectable, but smoking selectively up-regulates EGF expression in differentiated cells of the airway epithelium in vivo. Based on these data and on the knowledge that EGFR signaling plays a central role in the regulation of cell proliferation and differentiation in the airway epithelium, the central concept of this proposal is that smoking-induced expression by differentiated cells activates BC via EGFR altering the molecular phenotype of airway BC and impairing their ability to generate normal differentiated airway epithelium. To assess this concept, the following aims will be addressed: