Retinoid-induced differentiation of acute promyelocytic leukemia involves PML-RARα-mediated increase of type II transglutaminase

All-trans retinoic acid (t-RA) administration leads to complete remission in acute promyelocytic leukemia (APL) patients by inducing growth arrest and differentiation of the leukemic clone. In the present study, we show that t- RA treatment dramatically induced type II transglutaminase (type II TGase) expression in cells carrying the t(15; 17) translocation and expressing the PML-RARα product such as the APL-derived NB4 cell line and fresh leukemic cells from APL patients. This induction correlated with the t-RA-induced growth arrest, granulocytic differentiation, and upregulation of the leukocyte adherence receptor β subunit (CD18) gene expression. The increase in type II TGase was not abolished by cycloheximide treatment, suggesting that synthesis of a protein intermediate was not required for the induction. t-RA did not significantly alter the rate of growth arrest and did not stimulate differentiation and type II TGase activity in NB4.306 cells, a t- RA-resistant subclone of the NB4 cell line, or in leukemic cells derived from two patients morphologically defined as APL but lacking the t(15;17). However, in NB4.306 cells, t-RA treatment was able to increase CD18 mRNA expression in a manner similar to NB4 cells. The molecular mechanisms involved in the induction of these genes were investigated. In NB4 cells, using novel receptor-selective ligands such as 9-cis-RA, TTNPB, AM580, and SR11217, we found that RAR- and RARα-selective retinoids were able to induce growth arrest, granulocytic differentiation, and type II TGase, whereas the RXR-selective retinoid SR11217 was inactive. Moreover, an RARα-antagonist completely inhibited the expression of type II TGase and CD18 induced by these selective retinoids in NB4 cells. In NB4.306 cells, an RARα-dependent signaling pathway was found involved in the modulation of CD18 expression. In addition, expression of the PML-RARα gene in myeloid U937 precursor cells resulted in the ability of these cells to induce type II TGase in response to t-RA. On the basis of these results we hypothesize a specific involvement of a signaling pathway involving PML-RARα for the induction of growth arrest, granulocytic differentiation, and type II TGase by retinoids in APL cells.