THE MECHANISM OF THE INHIBITION OF SQUAMOUS DIFFERENTIATION OF RAT TRACHEAL 2C5 CELLS BY RETINOIC ACID

Retinoic acid (RA) plays an essential role in maintaining normal diffe rentiation 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 th e mechanism of inhibition of squamous differentiation by RA. 2C5 cells grown to confluence in the presence of serum underwent squamous diffe rentiation as marked by an increase in their level of crosslinked enve lope formation. The serum-induced crosslinked envelope formation was b locked by RA in 2C5 cells with an ED(50) <0.1 nM. However, the activit y of the crosslinking enzyme, keratinocyte transglutaminase, did not c orrelate with the formation of crosslinked envelopes in 2C5 cells. Cha nges in biochemical markers of squamous differentiation such as an alt ered expression of specific cytokeratins also accompanied serum-induce d squamous differentiation of 2C5 cells. The expression of the keratin squamous differentiation markers (i.e. K13) were inhibited by RA, alt hough the ED(50) for K13 expression was >1 nM. The different dose resp onses for RA inhibiting differentiation markers suggests multiple mech anisms of regulation by RA. These results indicate that 2C5 cells rema in responsive to differentiation factors such as RA and serum despite being an immortalized cell line.