ITO CELL EXPRESSION OF A NUCLEAR RETINOIC ACID RECEPTOR

Although it has been suggested that retinoids regulate Ito cell proliferation and collagen synthesis, little is known about the ability of Ito cells to respond to retinoids in vivo. Because retinoids may mediate their molecular effects through nuclear receptors, Ito cells were examined for the presence of one of these receptors, nuclear retinoic acid receptor-beta. The modulation of nuclear retinoic acid receptor-beta expression was also studied during cell culture and hepatic fibrogenesis. Northern hybridization analysis revealed that Ito cells freshly isolated from normal rat liver contained nuclear retinoic acid receptor-beta messenger RNA at levels significantly higher than those found in other hepatic cell types. Ito cells also contained messenger RNA for two other nuclear retinoic acid receptors, nuclear retinoic acid receptor-alpha and nuclear retinoic acid receptor-gamma. Using an antibody to human nuclear retinoic acid receptor-beta, the nuclear presence of this receptor was demonstrated in normal Ito cells. In contrast, Ito cells cultured for at least 7 days had no detectable messenger RNA or nuclear staining for nuclear retinoic acid receptor-beta despite a 20 +/- 5-fold increase in the messenger RNA level of another retinoid binding protein, cellular retinol binding protein. Analysis of Ito cells isolated from rats with carbon tetrachloride-induced hepatic fibrosis revealed an 81% +/- 3% decrease in nuclear retinoic acid receptor-beta messenger RNA levels in these cells when compared with normal Ito cells. No difference in the messenger RNA levels of cellular retinol binding protein was found in Ito cells isolated from either normal or fibrotic liver. The effect of retinoid treatment on Ito cell nuclear retinoic acid receptor-beta messenger RNA level was also studied because retinoic acid treatment of other cells has been shown to induce nuclear retinoic acid receptor-beta gene expression. Treatment for 48 hr with either 10(-6) mol/L retinoic acid or 10(-5) mol/L retinyl acetate induced the expression of nuclear retinoic acid receptor-beta messenger RNA in primary cultured Ito cells. These data demonstrate that Ito cells possess a nuclear retinoic acid receptor allowing them to respond to retinoic acid in vivo. Increased Ito cell proliferation and collagen synthesis during cell culture or hepatic fibrosis may result in part from decreased Ito cell retinoid responsiveness as reflected by the loss or decrease in nuclear retinoic acid receptor-beta gene expression observed under these two conditions. Furthermore, the ability of retinoids to induce Ito cell nuclear retinoic acid receptor-beta messenger RNA expression suggests that Ito cell retinoid unresponsiveness during cell culture and hepatic fibrogenesis is reversible.