Retinoid-induced differentiation regulates prostaglandin H synthase and cPLA2 expression in tracheal epithelium

Rat tracheal epithelial cells cultured in vitro at an air-liquid interface can differentiate into a mucociliary or squamous phenotype depending on the presence or absence of retinoic acid (RA). The airway epithelium is known to produce a number of eicosanoids. We propose that eicosanoid biosynthesis is dependent on the differentiation status of the epithelium. Therefore, prostaglandin production and the expression of cytosolic phospholipase A2 (cPLA2) and the prostaglandin H synthase (PGHS) isoforms were investigated during differentiation to these two phenotypes. The major eicosanoid produced by both phenotypes was prostaglandin E2 (PGE2). Proliferating undifferentiated cultures produced low levels of PGE2 regardless of retinoid status. Differentiated mucociliary cultures produced high levels of PGE2 (50 ng/10(6) cells), whereas differentiated squamous cultures produced low levels of PGE2 (< 5 ng/10(6) cells). Mucociliary cultures expressed high levels of cPLA2 and PGHS-2 isoform mRNA and protein. In contrast squamous cultures expressed very low levels of cPLA2 and PGHS-2 transcript and protein. The PGHS-1 isoform was expressed in squamous but not in mucociliary cultures. We investigated changes in expression of these enzymes during retinoid treatment of established squamous cultures. Treatment with RA resulted in a rapid (24 h) downregulation of PGHS-1 mRNA expression. However, the cPLA2 and PGHS-2 genes were expressed in squamous cultures only after 3 days of RA treatment coincident with redifferentiation of the culture to a mucociliary phenotype. These studies reveal that retinoid-induced differentiation of airway epithelium into either a mucociliary or squamous phenotype results in profound changes in the expression of cPLA2 and PGHS isozymes that regulate prostaglandin formation.