Levels of Smad7 regulate Smad and mitogen activated kinases (MAPKs) signaling and controls erythroid and megakaryocytic differentiation of erythroleukemia cells

• Published in: Platelets (Edinburgh) 2007-01, Vol.18 (8), p.566-578
• Main Authors: Akel, Salem, Bertolette, Daniel, Petrow-Sadowski, Cari, Ruscetti, Francis W.
• Format: Article
• Language: English
• Subjects: activin; Activins - metabolism; Autocrine Communication; Cell Differentiation; Erythroid Cells - cytology; erythropoiesis; Gene Expression Regulation; Humans; K562; K562 Cells; Leukemia, Erythroblastic, Acute - pathology; MAP Kinase Signaling System; Megakaryocytes - cytology; megakaryopoiesis; Signal Transduction; Smad Proteins - metabolism; Smad7; Smad7 Protein - analysis; Smad7 Protein - genetics; TGF-α; Transforming Growth Factor beta - metabolism
• ISSN: 0953-7104; 1369-1635
• DOI: 10.1080/09537100701549546

Smad and MAPK signaling cascades are involved in erythroid and megakaryocytic differentiation. The inhibitory Smad for TGF-β/activin signaling, Smad7, may directly or indirectly affect these signaling pathways. By modulating Smad7 expression, we attempted to delineate the relevance of Smad7 during erythro-megakaryocytic (E/M) differentiation of human erythroleukemia cells. Smad7 transcripts were detected at low levels in different erythroleukemia cell lines (TF-1, HEL and K562). Reduction of expression of endogenous Smad7 by RNA interference enhanced erythroid differentiation of K562 cells in response to physiological doses of activin-A/TGF-β1. Stable over-expression of Smad7 in K562 cells (K562/7) prevented activation of Smad2/3 and MAPK (ERK1/2, p38 and JNK1/2) proteins by activin-A/TGF-β1and subsequent induction of erythroid differentiation. High levels of Smad7 also interfered with hydroxyurea- and butyrate-, but not hemin-induced erythroid differentiation. Interestingly, K562/7 cells were found to harbor a significant proportion (about 35%) of large ploy nucleated cells compared to fewer than 12% in control cells. K562/7 cells treated with phorbol 12-myristate 13-acetate (PMA), showed a great shift in ploidy towards high ploidy classes (≥8N) accompanied with an increase in the expression of the maturation marker CD42b. We showed here that: (a) low levels of endogenous Smad7 in erythroleukemia cells are physiologically relevant, and (b) high levels of Smad7 interferes with TGF-β/activin-induced Smad/MAPK signaling and erythro-differentiation and promotes megakaryocytic differentiation, possibly by blocking autocrine TGF-β.