Sonic hedgehog signaling by the patched-smoothened receptor complex.
BACKGROUND: The Hedgehog (Hh) family of secreted proteins is involved in a number of developmental processes as well as in cancer. Genetic and biochemical data suggest that the Sonic hedgehog (Shh) receptor is composed of at least two proteins: the tumor suppressor protein Patched (Ptc) and the seven-transmembrane protein Smoothened (Smo). RESULTS: Using a biochemical assay for activation of the transcription factor Gli, a downstream component of the Hh pathway, we show here that Smo functions as the signaling component of the Shh receptor, and that this activity can be blocked by Ptc. The inhibition of Smo by Ptc can be relieved by the addition of Shh. Furthermore, oncogenic forms of Smo are insensitive to Ptc repression in this assay. Mapping of the Smo domains required for binding to Ptc and for signaling revealed that the Smo-Ptc interaction involves mainly the amino terminus of Smo, and that the third intracellular loop and the seventh transmembrane domain are required for signaling. CONCLUSIONS: These data demonstrate that Smo is the signaling component of a multicomponent Hh receptor complex and that Ptc is a ligand-regulated inhibitor of Smo. Different domains of Smo are involved in Ptc binding and activation of a Gli reporter construct. The latter requires the third intracellular loop and the seventh transmembrane domain of Smo, regions often involved in coupling to G proteins. No changes in the levels of cyclic AMP or calcium associated with such pathways could be detected following receptor activation, however. (+info)
Concomitant activation of pathways downstream of Grb2 and PI 3-kinase is required for MET-mediated metastasis.
The Met tyrosine kinase - the HGF receptor - induces cell transformation and metastasis when constitutively activated. Met signaling is mediated by phosphorylation of two carboxy-terminal tyrosines which act as docking sites for a number of SH2-containing molecules. These include Grb2 and p85 which couple the receptor, respectively, with Ras and PI 3-kinase. We previously showed that a Met mutant designed to obtain preferential coupling with Grb2 (Met2xGrb2) is permissive for motility, increases transformation, but - surprisingly - is impaired in causing invasion and metastasis. In this work we used Met mutants optimized for binding either p85 alone (Met2xPI3K) or p85 and Grb2 (MetPI3K/Grb2) to evaluate the relative importance of Ras and PI 3-kinase as downstream effectors of Met. Met2xPI3K was competent in eliciting motility, but not transformation, invasion, or metastasis. Conversely, MetP13K/Grb2 induced motility, transformation, invasion and metastasis as efficiently as wild type Met. Furthermore, the expression of constitutively active PI 3-kinase in cells transformed by the Met2xGrb2 mutant, fully rescued their ability to invade and metastasize. These data point to a central role for PI 3-kinase in Met-mediated invasiveness, and indicate that simultaneous activation of Ras and PI 3-kinase is required to unleash the Met metastatic potential. (+info)
Plasma membrane recruitment of RalGDS is critical for Ras-dependent Ral activation.
In COS cells, Ral GDP dissociation stimulator (RalGDS)-induced Ral activation was stimulated by RasG12V or a Rap1/Ras chimera in which the N-terminal region of Rap1 was ligated to the C-terminal region of Ras but not by Rap1G12V or a Ras/Rap1 chimera in which the N-terminal region of Ras was ligated to the C-terminal region of Rap1, although RalGDS interacted with these small GTP-binding proteins. When RasG12V, Ral and the Rap1/Ras chimera were individually expressed in NIH3T3 cells, they localized to the plasma membrane. Rap1Q63E and the Ras/Rap1 chimera were detected in the perinuclear region. When RalGDS was expressed alone, it was abundant in the cytoplasm. When coexpressed with RasG12V or the Rap1/Ras chimera, RalGDS was detected at the plasma membrane, whereas when coexpressed with Rap1Q63E or the Ras/Rap1 chimera, RalGDS was observed in the perinuclear region. RalGDS which was targeted to the plasma membrane by the addition of Ras farnesylation site (RalGDS-CAAX) activated Ral in the absence of RasG12V. Although RalGDS did not stimulate the dissociation of GDP from Ral in the absence of the GTP-bound form of Ras in a reconstitution assay using the liposomes, RalGDS-CAAX could stimulate it without Ras. RasG12V activated Raf-1 when they were coexpressed in Sf9 cells, whereas RasG12V did not affect the RalGDS activity. These results indicate that Ras recruits RalGDS to the plasma membrane and that the translocated RalGDS induces the activation of Ral, but that Rap1 does not activate Ral due to distinct subcellular localization. (+info)
Arrestin function in G protein-coupled receptor endocytosis requires phosphoinositide binding.
Internalization of agonist-activated G protein-coupled receptors is mediated by non-visual arrestins, which also bind to clathrin and are therefore thought to act as adaptors in the endocytosis process. Phosphoinositides have been implicated in the regulation of intracellular receptor trafficking, and are known to bind to other coat components including AP-2, AP180 and COPI coatomer. Given these observations, we explored the possibility that phosphoinositides play a role in arrestin's function as an adaptor. High-affinity binding sites for phosphoinositides in beta-arrestin (arrestin2) and arrestin3 (beta-arrestin2) were identified, and dissimilar effects of phosphoinositide and inositol phosphate on arrestin interactions with clathrin and receptor were characterized. Alteration of three basic residues in arrestin3 abolished phosphoinositide binding with complete retention of clathrin and receptor binding. Unlike native protein, upon agonist activation, this mutant arrestin3 expressed in COS1 cells neither supported beta2-adrenergic receptor internalization nor did it concentrate in coated pits, although it was recruited to the plasma membrane. These findings indicate that phosphoinositide binding plays a critical regulatory role in delivery of the receptor-arrestin complex to coated pits, perhaps by providing, with activated receptor, a multi-point attachment of arrestin to the plasma membrane. (+info)
The MAP kinase ERK2 inhibits the cyclic AMP-specific phosphodiesterase HSPDE4D3 by phosphorylating it at Ser579.
The extracellular receptor stimulated kinase ERK2 (p42(MAPK))-phosphorylated human cAMP-specific phosphodiesterase PDE4D3 at Ser579 and profoundly reduced ( approximately 75%) its activity. These effects could be reversed by the action of protein phosphatase PP1. The inhibitory state of PDE4D3, engendered by ERK2 phosphorylation, was mimicked by the Ser579-->Asp mutant form of PDE4D3. In COS1 cells transfected to express PDE4D3, challenge with epidermal growth factor (EGF) caused the phosphorylation and inhibition of PDE4D3. This effect was blocked by the MEK inhibitor PD98059 and was not apparent using the Ser579-->Ala mutant form of PDE4D3. Challenge of HEK293 and F442A cells with EGF led to the PD98059-ablatable inhibition of endogenous PDE4D3 and PDE4D5 activities. EGF challenge of COS1 cells transfected to express PDE4D3 increased cAMP levels through a process ablated by PD98059. The activity of the Ser579-->Asp mutant form of PDE4D3 was increased by PKA phosphorylation. The transient form of the EGF-induced inhibition of PDE4D3 is thus suggested to be due to feedback regulation by PKA causing the ablation of the ERK2-induced inhibition of PDE4D3. We identify a novel means of cross-talk between the cAMP and ERK signalling pathways whereby cell stimuli that lead to ERK2 activation may modulate cAMP signalling. (+info)
p50(cdc37) acting in concert with Hsp90 is required for Raf-1 function.
Genetic screens in Drosophila have identified p50(cdc37) to be an essential component of the sevenless receptor/mitogen-activated kinase protein (MAPK) signaling pathway, but neither the function nor the target of p50(cdc37) in this pathway has been defined. In this study, we examined the role of p50(cdc37) and its Hsp90 chaperone partner in Raf/Mek/MAPK signaling biochemically. We found that coexpression of wild-type p50(cdc37) with Raf-1 resulted in robust and dose-dependent activation of Raf-1 in Sf9 cells. In addition, p50(cdc37) greatly potentiated v-Src-mediated Raf-1 activation. Moreover, we found that p50(cdc37) is the primary determinant of Hsp90 recruitment to Raf-1. Overexpression of a p50(cdc37) mutant which is unable to recruit Hsp90 into the Raf-1 complex inhibited Raf-1 and MAPK activation by growth factors. Similarly, pretreatment with geldanamycin (GA), an Hsp90-specific inhibitor, prevented both the association of Raf-1 with the p50(cdc37)-Hsp90 heterodimer and Raf-1 kinase activation by serum. Activation of Raf-1 via baculovirus coexpression with oncogenic Src or Ras in Sf9 cells was also strongly inhibited by dominant negative p50(cdc37) or by GA. Thus, formation of a ternary Raf-1-p50(cdc37)-Hsp90 complex is crucial for Raf-1 activity and MAPK pathway signaling. These results provide the first biochemical evidence for the requirement of the p50(cdc37)-Hsp90 complex in protein kinase regulation and for Raf-1 function in particular. (+info)
Dominant activity of activation function 1 (AF-1) and differential stoichiometric requirements for AF-1 and -2 in the estrogen receptor alpha-beta heterodimeric complex.
Estrogenic responses are now known to be mediated by two forms of estrogen receptors (ER), ERalpha and ERbeta, that can function as homodimers or heterodimers. As homodimers the two have been recently shown to exhibit distinct transcriptional responses to estradiol (E2), antiestrogens, and coactivators, suggesting that the ER complexes are not functionally equivalent. However, because the three possible configurations of ER complexes all recognize the same estrogen response element, it has not been possible to evaluate the transcriptional properties of the ER heterodimer complex by transfection assays. Using ER subunits with modified DNA recognition specificity, we were able to measure the transcriptional properties of ERalpha-ERbeta heterodimers in transfected cells without interference from the two ER homodimer complexes. We first demonstrated that the individual activation function 1 (AF-1) domains act in a dominant manner within the ERalpha-ERbeta heterodimer: the mixed agonist-antagonist 4-hydroxytamoxifen acts as an agonist in a promoter- and cell context-dependent manner via the ERalpha AF-1, while activation of the complex by the mitogen-activated protein kinase (MAPK) pathway requires only the ERalpha- or ERbeta-responsive MAPK site. Using ligand-binding and AF-2-defective mutants, we further demonstrated that while the ERalpha-ERbeta heterodimer can be activated when only one E2-binding competent partner is present per dimer, two functional AF-2 domains are required for transcriptional activity. Taken together, the results of this study of a retinoid X receptor-independent heterodimer complex, the first such study, provide evidence of different stoichiometric requirements for AF-1 and -2 activity and demonstrate that AF-1 receptor-specific properties are maintained within the ERalpha-ERbeta heterodimer. (+info)
A human sequence homologue of Staufen is an RNA-binding protein that is associated with polysomes and localizes to the rough endoplasmic reticulum.
In the course of a two-hybrid screen with the NS1 protein of influenza virus, a human clone capable of coding for a protein with high homology to the Staufen protein from Drosophila melanogaster (dmStaufen) was identified. With these sequences used as a probe, cDNAs were isolated from a lambda cDNA library. The encoded protein (hStaufen-like) contained four double-stranded RNA (dsRNA)-binding domains with 55% similarity and 38% identity to those of dmStaufen, including identity at all residues involved in RNA binding. A recombinant protein containing all dsRNA-binding domains was expressed in Escherichia coli as a His-tagged polypeptide. It showed dsRNA binding activity in vitro, with an apparent Kd of 10(-9) M. Using a specific antibody, we detected in human cells a major form of the hStaufen-like protein with an apparent molecular mass of 60 to 65 kDa. The intracellular localization of hStaufen-like protein was investigated by immunofluorescence using a series of markers for the cell compartments. Colocalization was observed with the rough endoplasmic reticulum but not with endosomes, cytoskeleton, or Golgi apparatus. Furthermore, sedimentation analyses indicated that hStaufen-like protein associates with polysomes. These results are discussed in relation to the possible functions of the protein. (+info)