Abstract
Retinoic acid (RA) plays an essential role in maintaining normal differentiation of tracheal epithelial cells. During vitamin A deficiency, tracheocytes undergo squamous metaplasia, an abnormal differentiation that can be reversed by RA. We used rat tracheal 2C5 cells to study the mechanism of inhibition of squamous differentiation by RA. 2C5 cells grown to confluence in the presence of serum underwent squamous differentiation as marked by an increase in their level of crosslinked envelope formation. The serum-induced crosslinked envelope formation was blocked by RA in 2C5 cells with an ED50 < 0.1 nM. However, the activity of the crosslinking enzyme, keratinocyte transglutaminase, did not correlate with the formation of crosslinked envelopes in 2C5 cells. Changes in biochemical markers of squamous differentiation such as an altered expression of specific cytokeratins also accompanied serum-induced squamous differentiation of 2C5 cells. The expression of the keratin squamous differentiation markers (i.e. K13) were inhibited by RA, although the ED50 for K13 expression was > 1 nM. The different dose responses for RA inhibiting differentiation markers suggests multiple mechanisms of regulation by RA. These results indicate that 2C5 cells remain responsive to differentiation factors such as RA and serum despite being an immortalized cell line.
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