The LIM-domain binding protein Ldb1 and its partner LMO2 act as negative regulators of erythroid differentiation

Abstract

The nuclear LIM domain protein LMO2, a T cell oncoprotein, is essential for embryonic erythropoiesis. LIM-only proteins are presumed to act primarily through protein-protein interactions. We, and others, have identified a widely expressed protein, Ldb1, whose C-terminal 76-residues are sufficient to mediate interaction with LMO2. In murine erythroleukemia cells, the endogenous Lbd1 and LMO2 proteins exist in a stable complex, whose binding affinity appears greater than that between LMO2 and the bHLH transcription factor SCL. However, Ldb1, LMO2, and SCL͞E12 can assemble as a multiprotein complex on a consensus SCL binding site. Like LMO2, the Ldb1 gene is expressed in fetal liver and erythroid cell lines. Forced expression of Ldb1 in G1ER proerythroblast cells inhibited cellular maturation, a finding compatible with the decrease in Ldb1 gene expression that normally occurs during erythroid differentiation. Overexpression of the LMO2 gene also inhibited erythroid differentiation. Our studies demonstrate a function for Ldb1 in hemopoietic cells and suggest that one role of the Ldb1͞LMO2 complex is to maintain erythroid precursors in an immature state.

The LIM-only protein, LMO4, and the LIM domain-binding protein, LDB1, expression in squamous cell carcinomas of the oral cavity

“…Investigation of the molecular mechanism of carcinoma cell differentiation/ dedifferentiation is a momentous subject in cancer cell biology and may contribute to the development of a novel strategy to relieve patient suffering from the disease. Although investigators reported that various factors are involved in carcinoma cell differentiation and dedifferentiation, accumulating evidence implies that transcriptional misregulation takes a critical part in the events (Angel et al, 1996;Hunter, 1997;Visvader et al, 1997;Battle et al, 2000;Cano et al, 2000). In the present study, our data demonstrate for the first time that LMO4 and LDB1 form protein complexes in the nucleus and are expressed in carcinoma cells at the invasive front, and their immunoreactivity is increased in lessdifferentiated carcinomas and in metastasised lymph nodes.…”

“…LMO4 and LDB1 are ubiquitously expressed in the mouse embryos, including epithelial and mesenchymal areas, when compared to the exclusive expression of other LMO family members and LDB2 in neuronal and haematopoietic cells (Visvader et al, 1997;Kenny et al, 1998;Sugihara et al, 1998;Toyama et al, 1998;Hermanson et al, 1999;Thaler et al, 2002). The LIM domain of LDBs contributes to the binding of transcription factors, including LIM-homeodomain, zinc-finger and basic helixloop -helix (bHLH) proteins (Agulnick et al, 1996;Jurata et al, 1996;Bach et al, 1997;Morcillo et al, 1997).…”

“…In the presence of LMO protein, it mediates the transcription factor binding to the LDB protein or competes with the direct binding between LDB and the transcription factor (Rabbitts, 1998;Thaler et al, 2002). These combinational actions are involved in the diversity of transcriptional regulation and specification of cell types (Visvader et al, 1997;Thaler et al, 2002). Misexpression of LMO1 and LMO2 by the chromosomal translocation is observed in T-cell leukaemia and …”

Identification and characterization of Grainyhead‐like epithelial transactivator (GET‐1), a novel mammalian Grainyhead‐like factor

“…Although LMOs are transcriptional regulators that do not associate directly with DNA (Bach, 2000), work with LMO-2 in erythroid cells suggests that LMOs may act by interacting with DNA-binding proteins (Osada et al, 1995;Visvader et al, 1997;Wadman et al, 1997). Therefore, we decided to search for LMO-4 -interacting DNA-binding proteins in epidermis.…”

“…Like LIM-HD proteins, LMO factors exhibit nuclear localization, but because these proteins do not bind DNA directly, they must regulate gene transcription by associating with other DNA-binding proteins or coregulators. Several laboratories have isolated highly related and functionally redundant cofactors, referred to as Clim-1 and Clim-2/ Lbd1/NLI, which interact strongly with the LIM domains of LIM-HD proteins (Agulnick et al, 1996;Jurata et al, 1996;Bach et al, 1997;Visvader et al, 1997) and are required for the transactivation function of LIM-HD proteins on enhancers (Bach et al, 1997;Hobert and Westphal, 2000).…”