EXPERIMENTAL STUDY mRNA expression of type I and type II receptors for activin, transforming growth factor-b, and bone morphogenetic protein in the murine erythroleukemic cell line, F5-5.fl

Objective: Intracellular signaling of activin and transforming growth factor-b (TGF-b) is thought to be mediated by the same molecules (Smad2/3 and Smad4). Although differentiation of murine erythroleukemia F5-5.fl cells is induced by activin, it is not induced by TGF-b, suggesting that at some point TGF-b signaling is defective. The aim of this study was to investigate the unresponsiveness of F5-5.fl cells to TGF-b. Design: mRNA expression of ligands, receptors, and signal mediators for the TGF-b family was examined in F5-5.fl cells using RT-PCR. Results: Activin induced erythrodifferentiation of F5-5.fl cells in a dose-dependent manner. Neither TGFb1 nor bone morphogenetic protein (BMP)-4 affected the differentiation of F5-5.fl cells in the presence or absence of activin. Although mRNAs of TGF-bs (TGF-b1, TGF-b2 and TGF-b3) were detected, those of inhibin/activin (a -, bAand bB-subunits) and BMPs (BMP-2, BMP-4 and BMP-7) could not be detected in the cells, suggesting that neither activins nor BMPs are produced in F5-5.fl cells. The expression of both type I (ALK-4/ActRIB) and type II (ActRII) receptors for activin was detected in F5-5.fl cells. In contrast, while the expression of type I receptor for TGF-b (ALK-5/TbRI) was detected, that of type II receptor (TbRII) was not. The mRNA of all Smads examined was detected in F5-5.fl cells. Conclusions: A defect in the type II receptor might cause unresponsiveness to TGF-b in F5-5.fl cells. An erythrodifferentiation assay using F5-5.fl cells would be useful for measuring net activin activity because it would not be necessary to consider endogenous activins and BMPs. European Journal of Endocrinology 143 705±710