PAN/E2A EXPRESSION PRECEDES IMMUNOGLOBULIN HEAVY-CHAIN EXPRESSION DURING B-LYMPHOPOIESIS IN NONTRANSFORMED CELLS, AND PAN/E2A PROTEINS ARE NOT DETECTED IN MYELOID CELLS

A newly developed rat long term bone marrow culture system was used to study the role of Pan/E2A basic helix-loop-helix transcription factors during B-cell development. In this system, B-lymphocyte progenitors actively differentiate into mature B cells. Monoclonal (Yae) and polyclonal (anti-Pan) antibodies were employed to characterize the expression of Pan proteins by Western blot assay during hematopoiesis and to examine the components of immunoglobulin heavy-chain gene enhancer element-binding species by electrophoretic mobility shift assay. During B-cell development, the appearance of Pan/E2A proteins preceded the expression of immunoglobulin heavy-chain protein. A Pan-containing immunoglobulin heavy-chain enhancer element (mu E5)-binding species (BCF1), composed of immunoreactive Pan-1/E47 but not Pan-2/E12, was observed concomitantly with the detection of Pan/E2A proteins. In addition to BCF1, other mu E5-binding species were detected which were not recognized by the Yae antibody. Two of these species were present in primary B-lymphocyte and myeloid cultures and were recognized by an anti-upstream stimulatory factor antiserum. Although Pan/E2A proteins have been proposed to be ubiquitous, Pan/E2A proteins were not detected in primary myeloid cultures composed mainly of granulocytes and macrophages or in the macrophage cell line J774. The absence of Pan/E2A proteins in differentiated myeloid cells correlated with low steady-state levels of Pan/E2A RNA. However, Pan/E2A proteins were present in a promyeloid cell line, 32DCL3, suggesting that extinction of Pan/E2A expression may play a role in myelopoiesis.