The AML1/ETO Target LAT2 Membrane Adaptor Molecule Is Regulated during Normal Monocytic Differentiation.

LAT2 (NTAL/LAB/WBSCR5) is a 28 KDa membrane protein which acts as adaptor molecule in the signalling pathways of FcεR I, c-Kit, B cell and T cell receptor. Bone marrow-derived mast cells from knock-out (KO) mice are hyperresponsive to stimulation via FcεR I. Although LAT2 is highly expressed in B cells, no major changes were found in function or development of B cells from LAT2 KO mice. An autoimmunity syndrome in LAT2 KO mice is caused, at least in part, by hyperreactivity and higher proliferation of T cells. Previously, we showed that LAT2 mRNA is repressed in vivo by AML1/ETO which was confirmed by others in several large series of primary AML blasts. We wished to elucidate the possible role of LAT2 during the myelopoiesis. AML1/ETO was induced by Ponasterone A in an ecdysone-inducible system in U937 cells (9/14/18 cell line). AML bone marrow samples from 43 patients (pts) were analyzed for LAT2 expression. Several myeloid cell lines were treated either with ATRA, DMSO or PMA for 3 days. Normal CD34+ cells were differentiated ex vivo by G-CSF towards granulocytes and by GM-CSF plus IL-4 towards monocytes and dendritic cells. LAT2 expression was determined by Northern and Western blot. LAT2 protein was repressed not only in AML1/ETO positive primary AML blasts (6/6), but also in blasts from patients with deletions of chromosome 7 (3/4) and the t(15;17) (4/4); expression was moderate to high in AML blasts with normal karyotype (14/15). LAT2 was expressed in normal monocytes and even higher in alveolar macrophages but not in granulocytes of healthy donors. It was downregulated after ATRA-induced granulocytic differentiation of NB4, HL60 and U937 cells but upregulated after DMSO-induced granulocytic differentiation of HL60 cells and PMA-induced monocytic-macrophage differentiation of HL60, U937 and Kasumi-1 cells. In normal CD34+ cells, LAT2 was strongly induced 7 days after the addition of G-CSF and GM-CSF+IL4 respectively, but after 14 days it was downregulated (0.7 +/− 0.4-fold) by G-CSF-induced granulocytic differentiation and upregulated (5.8 +/− 2.8-fold) by GM-CSF+IL4-induced monocytic-DC differentiation. Conditional expression of AML1/ETO in 9/14/18-U937 cells partially inhibited the PMA- and vitamin D3-induced monocytic differentiation of these cells, as determined by FACS for CD11b and CD11c. In conclusion, LAT2 protein is strongly repressed by AML1/ETO in primary leukemias and is upregulated during the monocytic differentiation in several cell lines and normal CD34+ cells. Further studies in a LAT2 knock-down by shRNAs in U937 cells are warranted to functionally address its possible role in monocytic differentiation.