Members of the transforming growth factor ␤ (TGF-␤) family of proteins signal through cell surface transmembrane serine/threonine protein kinases known as type I and type II receptors. The TGF-␤ signal is extended through phosphorylation of receptor-associated Smad proteins by the type I receptor. Although numerous investigations have established the sequence of events in TGF-␤ receptor (TGF-␤R) activation, none have examined the role of the endocytic pathway in initiation and/or maintenance of the signaling response. In this study we investigated whether TGF-␤R internalization modulates type I receptor activation, the formation of a functional receptor/Smad/SARA complex, Smad2/3 phosphorylation or nuclear translocation, and TGF-␤-dependent reporter gene activity. Our data provide evidence that, whereas type I receptor phosphorylation and association of SARA and Smad2 with the TGF-␤R complex take place independently of clathrin lattice formation, Smad2 or Smad3 activation and downstream signaling only occur after endocytic vesicle formation. Thus, TGF-␤R endocytosis is not simply a way to dampen the signaling response but instead is required to propagate signaling via the Smad pathway.

Transforming growth factor ␤ (TGF-␤) causes growth arrest in epithelial cells and proliferation and morphological transformation in fibroblasts. Despite the ability of TGF-␤ to induce various cellular phenotypes, few discernible differences in TGF-␤ signaling between cell types have been reported, with the only well-characterized pathway (the Smad cascade) seemingly under identical control. We determined that TGF-␤ receptor signaling activates the STE20 homolog PAK2 in mammalian cells. PAK2 activation occurs in fibroblast but not epithelial cell cultures and is independent of Smad2 and/or Smad3. Furthermore, we show that TGF-␤-stimulated PAK2 activity is regulated by Rac1 and Cdc42 and dominant negative PAK2 or morpholino antisense oligonucleotides to PAK2 prevent the morphological alteration observed following TGF-␤ addition. Thus, PAK2 represents a novel Smad-independent pathway that differentiates TGF-␤ signaling in fibroblast (growth-stimulated) and epithelial cell (growth-inhibited) cultures.

Transforming growth factor ␤ (TGF␤) superfamily polypeptides regulate cell growth and differentiation by binding to single pass serine/threonine kinases referred to as TGF␤ type I and type II receptors. Signal propagation is dependent upon heteromeric (type I-type II) complex formation and transphosphorylation of the type I receptor by the type II receptor. While many of the phosphorylation events necessary for receptor signaling have recently been characterized, the role of TGF␤ receptor kinase activity in modulating receptor endocytosis has not been addressed. To that end, we have used chimeric receptors consisting of the extracellular domain of the granulocyte/macrophage colony-stimulating factor ␣ and ␤ receptors spliced to the TGF␤ type I and type II transmembrane and cytoplasmic domains to address the specific role of type I and/or type II receptor kinase activity in TGF␤ receptor internalization, downregulation, and signaling. To inactivate chimeric receptor kinase activity, point mutations in the ATP binding site were made at amino acids 232 and 277 in the type I and type II receptor, respectively. Either of these mutations abolished plasminogen activator inhibitor 1 protein expression stimulated by granulocyte/macrophage colony-stimulating factor activation of chimeric heteromeric type I-type II TGF␤ receptors. They did not, however, modulate TGF␤ signaling stimulated through the endogenous TGF␤ receptor. Although TGF␤ receptor signaling was dependent upon the kinase activity of both chimeric receptors, the initial endocytic response was distinctly regulated by type I and/or type II receptor kinase activity. For instance, while heteromeric receptor complexes containing a kinase-inactive type I receptor were endocytosed similarly to wild type complexes, the kinase activity of the type II TGF␤ receptor was necessary for optimal internalization and receptor down-regulation. Furthermore, these responses were shown to occur independently of type II receptor autophosphorylation but require a type II receptor capable of transphosphorylation.The transforming growth factor ␤ (TGF␤) 1 superfamily of proteins regulate a number of diverse biologic processes (1-3). While the cellular response can be as distinct as growth stimulation or growth inhibition, it appears as though a similar receptor system is utilized for both pathways. Understanding how the receptors are regulated for one family of proteins will ultimately extend the knowledge for the entire superfamily. The model most commonly accepted for receptor activation requires oligomerization of a type I and type II TGF␤ receptor (4 -7). This occurs through ligand binding to a type II receptor and recruitment of a type I receptor into a dimeric and/or tetrameric complex (7-11). The serine/threonine kinase activity of the type I receptor is then activated by specific type II receptor phosphorylations in the juxtamembrane region of the type I receptor (12-16). This cascade of receptor interactions and phosphorylations ultimately results in the propagation of the T...

Matrix remodeling, degradation, inflammation and invasion liberate peptide fragments that can subsequently interact with cells in an attachment-independent manner. Such 'soluble' matrix components, including collagens, fibronectin and laminin, induced Smad activation (termed crosstalk signaling), which follows a similar chronological sequence and R-Smad specificity as induced by transforming growth factor (TGF)-b1. Smad4 nuclear translocation occurred in response to collagen binding, indicating downstream signal propagation. TGF-b scavenging antibody affected only TGF-b1, but not crosstalk-induced responses. TGF-b type II receptor mutation (DR26D25), which is deficient in TGF-b type I receptor recruitment to the ligand, induced a heterotetramer signaling complex, and propagated Smad2 activation only through collagen induction and not TGF-b signaling. Consequentially, TGF-b ligand participation is not required for crosstalk signaling. This signaling requires a functional integrin b1 receptor as showed by RNA interference. Co-immunoprecipitation (co-IP) and fluorescent microscopy indicate the involvement of focal adhesion kinase (FAK) and Src activity in collagen-induced signal propagation, and suggest a membrane signaling complex formation that includes both TGF-b receptors and integrins. The related gene expressional responses are distinct from that evoked by TGF-b1, supporting its separate function. This signaling mechanism expands and partially explains TGFb receptor dynamics and consequential signaling diversityrelated gene expressional plasticity.

Transforming growth factor-␤ (TGF-␤) induces distinct responses dependent upon the cellular context. It is unclear whether the initial receptor interactions identified in one cell type will be operative in another. Utilizing a chimeric receptor strategy we have examined the signaling and endocytic activity of both heteromeric (type I/type II) and homomeric (type I/type I or type II/type II) TGF-␤R interactions in Mv1Lu epithelial cells. In agreement with that observed in mesenchymal cells, all TGF-␤R signaling in Mv1Lu cells required the formation of a heteromeric type I-type II receptor complex. However, the initial endocytic response to TGF-␤R oligomerization was distinctly regulated in the two cell types. While heteromeric TGF-␤ receptors were internalized and down-regulated, homomeric TGF-␤R interactions showed diminished endocytic activity in Mv1Lu cells. This contrasts to that observed in mesenchymal cultures where ligand bound to TGF-␤R homomers was internalized, yet the receptors were not down-regulated. Moreover, while previous reports have suggested that mutations at serine 172 or threonine 176 in the type I TGF-␤R separated transcriptional from proliferative responses, we found no separation of pathways or effect on initial endocytic activity when the analogous mutations were made in the chimeric receptors.

Transforming growth factor-␤s (TGF-␤) are multifunctional proteins capable of either stimulating or inhibiting mitosis, depending on the cell type. These diverse cellular responses are caused by stimulating a single receptor complex composed of type I and type II receptors. Using a chimeric receptor model where the granulocyte/monocyte colony-stimulating factor receptor ligand binding domains are fused to the transmembrane and cytoplasmic signaling domains of the TGF-␤ type I and II receptors, we wished to describe the role(s) of specific amino acid residues in regulating ligand-mediated endocytosis and signaling in fibroblasts and epithelial cells. Specific point mutations were introduced at Y182, T200, and Y249 of the type I receptor and K277 and P525 of the type II receptor. Mutation of either Y182 or Y249, residues within two putative consensus tyrosine-based internalization motifs, had no effect on endocytosis or signaling. This is in contrast to mutation of T200 to valine, which resulted in ablation of signaling in both cell types, while only abolishing receptor down-regulation in fibroblasts. Moreover, in the absence of ligand, both fibroblasts and epithelial cells constitutively internalize and recycle the TGF-␤ receptor complex back to the plasma membrane. The data indicate fundamental differences between mesenchymal and epithelial cells in endocytic sorting and suggest that ligand binding diverts heteromeric receptors from the default recycling pool to a pathway mediating receptor down-regulation and signaling. INTRODUCTIONTransforming growth factor-␤s (TGF-␤) control a variety of cellular processes as diverse as mitotic inhibition or stimulation (Massagué, 1996;Moses and Serra, 1996). It is unclear how the same receptor complex can mediate such different cellular phenotypes. The most commonly accepted receptor model for TGF-␤ action consists of a heteromeric complex composed of type I and type II receptors (Wrana et al., 1992(Wrana et al., , 1994. Once associated, the type I receptor becomes phosphorylated primarily within the juxtamembrane GS domain (amino acids 185-192) by the constitutive serine/threonine kinase activity of the type II receptor (Franzén et al., 1995;Wieser et al., 1995). Phosphorylation of the GS domain is proposed to activate the type I receptor, resulting in signal propagation to downstream effector molecules (Massagué, 1998). In addition, specific residues in nearby regions have also been suggested to have both positive and negative regulatory functions (Wieser et al., 1995;Charng et al., 1996;Souchelnytskyi et al., 1996;Doré et al., 1998). For instance, threonine 200 has been shown to have a fundamental role in mediating all aspects of TGF-␤ signaling (Wieser et al., 1995), whereas replacement of threonine 204 with an acidic residue, such as aspartate, can generate a type I receptor capable of signaling (albeit to a lesser extent) independent of ligand or an associated type II receptor (Wieser et al., 1995;Charng et al., 1996;Luo and Lodish, 1996). What makes these data most i...

Noribogaine is the long-lived human metabolite of the anti-addictive substance ibogaine. Noribogaine efficaciously reaches the brain with concentrations up to 20 μM after acute therapeutic dose of 40 mg/kg ibogaine in animals. Noribogaine displays atypical opioid-like components in vivo, anti-addictive effects and potent modulatory properties of the tolerance to opiates for which the mode of action remained uncharacterized thus far. Our binding experiments and computational simulations indicate that noribogaine may bind to the orthosteric morphinan binding site of the opioid receptors. Functional activities of noribogaine at G-protein and non G-protein pathways of the mu and kappa opioid receptors were characterized. Noribogaine was a weak mu antagonist with a functional inhibition constants (Ke) of 20 μM at the G-protein and β-arrestin signaling pathways. Conversely, noribogaine was a G-protein biased kappa agonist 75% as efficacious as dynorphin A at stimulating GDP-GTP exchange (EC50=9 μM) but only 12% as efficacious at recruiting β-arrestin, which could contribute to the lack of dysphoric effects of noribogaine. In turn, noribogaine functionally inhibited dynorphin-induced kappa β-arrestin recruitment and was more potent than its G-protein agonistic activity with an IC50 of 1 μM. This biased agonist/antagonist pharmacology is unique to noribogaine in comparison to various other ligands including ibogaine, 18-MC, nalmefene, and 6'-GNTI. We predict noribogaine to promote certain analgesic effects as well as anti-addictive effects at effective concentrations>1 μM in the brain. Because elevated levels of dynorphins are commonly observed and correlated with anxiety, dysphoric effects, and decreased dopaminergic tone, a therapeutically relevant functional inhibition bias to endogenously released dynorphins by noribogaine might be worthy of consideration for treating anxiety and substance related disorders.

Transforming growth factor beta (TGF-beta) coordinates a number of biological events important in normal and pathophysiological growth. In this study, deletion and substitution mutations were used to identify receptor motifs modulating TGF-beta receptor activity. Initial experiments indicated that a COOH-terminal sequence between amino acids 482-491 in the kinase domain of the type I receptor was required for ligand-induced receptor signaling and down-regulation. These 10 amino acids are highly conserved in mammalian, Xenopus, and Drosophila type I receptors. Although mutation or deletion of the region (referred to as the NANDOR BOX, for nonactivating non-down-regulating) abolishes TGF-beta-dependent mitogenesis, transcriptional activity, type I receptor phosphorylation, and down-regulation in mesenchymal cultures, adjacent mutations also within the kinase domain are without effect. Moreover, a kinase-defective type I receptor can functionally complement a mutant BOX expressing type I receptor, documenting that when the BOX mutant is activated, it has kinase activity. These results indicate that the sequence between 482 and 491 in the type I receptor provides a critical function regulating activation of the TGF-beta receptor complex.