RESTRICTED EXPRESSION OF E2A PROTEIN IN PRIMARY HUMAN TISSUES CORRELATES WITH PROLIFERATION AND DIFFERENTIATION

E2A is a basic helix-loop-helix (bHLH) transcription factor required f or B cell lymphopoiesis and implicated in myogenesis and the regulatio n of insulin expression. As E2A is expressed widely in tissues, tissue -specific downstream effects are thought to result primarily from dime rization with other bHLH proteins. To investigate the degree to which regulation of E2A protein abundance may serve to regulate E2A function , expression of E2A was evaluated using immunohistochemistry on histol ogical sections of primary human tissues. Somewhat surprisingly, nucle ar staining for E2A was restricted in all tissues examined, often to a small subpopulation of cells. In some tissues, such as adult liver, e xpression was absent or Limited to rare infiltrating lymphocytes. E2A- expressing cells were most abundant in lymphoid tissues. In tonsil, ly mph node, and spleen, expression appeared most abundant and prevalent among rapidly proliferating centroblasts of the germinal center dark z one. Scattered E2A-expressing thymocytes were more numerous in the thy mic cortex than medulla. In developing skeletal muscle, E2A was detect able in striated myotubes but not in more primitive mononucleated prog enitors or mature muscle. Differential E2A expression was also noted i n proliferating periventricular neuroepithelial cells in the developin g brain. These results suggest that regulation of EA abundance complem ents protein-protein interactions in modulating E2A function.