Predominant intracellular localization of the type I transforming growth factor-β receptor and increased nuclear accumulation after growth arrest

Transforming growth factor-beta (TGF-beta) signaling requires the functional interaction of two distinct receptors, type I (RI) and type II (RII), at the cell surface. Exposure of cells to TGF-beta results in receptor internalization and down-regulation (Zwaagstra et al, 1999, Exp, Cell Res, 252, 352-362); however, little is known about the subsequent fate of RI or RII, In this study the cellular distribution of RI was examined in cells before and after treatment with ligand, RI was localized by immunocytochemistry and confocal microscopy using two polyclonal antisera directed against two different epitopes, one in the C-terminal region and one in the N-terminal region of the cytoplasmic domain. The majority of RI molecules in untreated Mv1Lu and A549 cells were found to be intracellular, Treatment of Mv1Lu and A549 cells with 100 pM TGF-beta 1 for 24 h at 37 degrees C caused a redistribution of surface RI on Mv1Lu cells, as evidenced by surface RI aggregation. Unexpectedly, this TGF-beta 1 treatment also caused redistribution and accumulation of intracellular RI in and around the nucleus for both Mv1Lu and A549 cells. Nuclear accumulation of RI was also promoted independently of ligand receptor activation by treatment of Mv1Lu cells with olomoucine, an agent that results in growth arrest. The capacity of RI to localize in the nucleus was confirmed by microscopic examination of 293 cells transiently expressing RI fused to green fluorescent protein (RI-GFP). Olomoucine treatment of these cells resulted in the movement of RI-GFP into the nucleus. Our results indicate that growth arrest alters intracellular transport/routing of RI and may indicate that RI functions not only at the cell surface but inside the cell as well.