A Novel Transforming Growth Factor-β Receptor-interacting Protein That Is Also a Light Chain of the Motor Protein Dynein

The phosphorylated, activated cytoplasmic domains of the transforming growth factor-β (TGFβ) receptors were used as probes to screen an expression library that was prepared from a highly TGFβ-responsive intestinal epithelial cell line. One of the TGFβ receptor-interacting proteins isolated was identified to be the mammalian homologue of the LC7 family (mLC7) of dynein light chains (DLCs). This 11-kDa cytoplasmic protein interacts with the TGFβ receptor complex intracellularly and is phosphorylated on serine residues after ligand-receptor engagement. Forced expression of mLC7-1 induces specific TGFβ responses, including an activation of Jun N-terminal kinase (JNK), a phosphorylation of c-Jun, and an inhibition of cell growth. Furthermore, TGFβ induces the recruitment of mLC7-1 to the intermediate chain of dynein. A kinase-deficient form of TGFβ RII prevents both mLC7-1 phosphorylation and interaction with the dynein intermediate chain (DIC). This is the first demonstration of a link between cytoplasmic dynein and a natural growth inhibitory cytokine. Furthermore, our results suggest that TGFβ pathway components may use a motor protein light chain as a receptor for the recruitment and transport of specific cargo along microtublules

Functional Heterogeneity of Bone Morphogenetic Protein Receptor-II Mutants Found in Patients with Primary Pulmonary Hypertension

Germline mutations in the BMPR2 gene encoding bone morphogenetic protein (BMP) type II receptor (BMPR-II) have been reported in patients with primary pulmonary hypertension (PPH), but the contribution of various types of mutations found in PPH to the pathogenesis of clinical phenotypes has not been elucidated. To determine the biological activities of these mutants, we performed functional assays testing their abilities to transduce BMP signals. We found that the reported missense mutations within the extracellular and kinase domains of BMPR-II abrogated their signal-transducing abilities. BMPR-II proteins containing mutations at the conserved cysteine residues in the extracellular and kinase domains were detected in the cytoplasm, suggesting that the loss of signaling ability of certain BMPR-II mutants is due at least in part to their altered subcellular localization. In contrast, BMPR-II mutants with truncation of the cytoplasmic tail retained the ability to transduce BMP signals. The differences in biological activities among the BMPR-II mutants observed thus suggest that additional genetic and/or environmental factors may play critical roles in the pathogenesis of PPH

Rapid Up-Regulation of α4 Integrin-mediated Leukocyte Adhesion by Transforming Growth Factor-β1

The α4 integrins (α4β1 and α4β7) are cell surface heterodimers expressed mostly on leukocytes that mediate cell-cell and cell-extracellular matrix adhesion. A characteristic feature of α4 integrins is that their adhesive activity can be subjected to rapid modulation during the process of cell migration. Herein, we show that transforming growth factor-β1 (TGF-β1) rapidly (0.5–5 min) and transiently up-regulated α4 integrin-dependent adhesion of different human leukocyte cell lines and human peripheral blood lymphocytes (PBLs) to their ligands vascular cell adhesion molecule-1 (VCAM-1) and connecting segment-1/fibronectin. In addition, TGF-β1 enhanced the α4 integrin-mediated adhesion of PBLs to tumor necrosis factor-α–treated human umbilical vein endothelial cells, indicating the stimulation of α4β1/VCAM-1 interaction. Although TGF-β1 rapidly activated the small GTPase RhoA and the p38 mitogen-activated protein kinase, enhanced adhesion did not require activation of both signaling molecules. Instead, polymerization of actin cytoskeleton triggered by TGF-β1 was necessary for α4 integrin-dependent up-regulated adhesion, and elevation of intracellular cAMP opposed this up-regulation. Moreover, TGF-β1 further increased cell adhesion mediated by α4 integrins in response to the chemokine stromal cell-derived factor-1α. These data suggest that TGF-β1 can potentially contribute to cell migration by dynamically regulating cell adhesion mediated by α4 integrins