TGFβ1 induces growth arrest and apoptosis but not ciliated cell differentiation in rat tracheal epithelial cell cultures

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Abstract
We are studying the regulation of ciliated cell differentiation using an in vitro model of tracheal regeneration. Previously, we reported that removal of growth stimulating compounds such as epidermal growth factor (EGF) and cholera toxin reduced DNA synthesis and cell number while increasing ciliated cell differentiation (Clark et al., 1995). This result suggested that the induction of growth arrest may stimulate terminal differentiation of airway epithelial cells into ciliated cells. Transforming growth factor βs (TGFβs) inhibit epithelial cell proliferation and have also been shown to stimulate epithelial cell differentiation. In this study, the effect of TGFβ1 on growth and ciliated cell differentiation of rat tracheal epithelial (RTE) cells was examined. TGFβ1 inhibited [3H]thymidine incorporation by RTE cells in a dose-dependent manner. A 40% inhibition was observed after a 24-h incubation with 10 pM TGFβ1. Continuous treatment with TGFβ1 (1–50 pM) also reduced cell number during the time when ciliogenesis occurs. This reduction resulted in part from a loss of cells through exfoliation, in addition to the inhibition of proliferation. The exfoliated cells exhibited several morphological features characteristic of apoptosis, including shrunken cells, condensed and fragmented nuclei, and intact organelles. In addition, electrophoretic analysis of genomic DNA analysis isolated from exfoliated cells demonstrated the presence of a nucleosomal ladder. However, in contrast to the removal of EGF, treatment with TGFβ1 for 7 d did not increase ciliated cell differentiation. TGFβ1 is, therefore, capable of inhibiting proliferation and increasing apoptosis in RTE cells without stimulating ciliated cell differentiation.