Gs alpha stimulates transcytosis and apical secretion in MDCK cells through cAMP and protein kinase A

Evidence that the 50-kDa substrate of brefeldin A-dependent ADP-ribosylation binds GTP and is modulated by the G-protein beta gamma subunit complex

Brefeldin A, a fungal metabolite that inhibits membrane transport, induces the mono(ADP-ribosyl)ation of two cytosolic proteins of 38 and 50 kDa as judged by SDS/PAGE. The 38-kDa substrate has been previously identified as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). We report that the 50-kDa BFA-induced ADP-ribosylated substrate (BARS-50) has native forms of 170 and 130 kDa, as determined by gel filtration of rat brain cytosol, indicating that BARS-50 might exist as a multimeric complex. BARS-50 can bind GTP, as indicated by blot-overlay studies with [alpha-32P]GTP and by photoaffinity labeling with guanosine 5'-[gamma-32P] [beta,gamma-(4-azidoanilido)]triphosphate. Moreover, ADP-ribosylation of BARS-50 was completely inhibited by the beta gamma subunit complex of G proteins, while the ADP-ribosylation of GAPDH was unmodified, indicating that this effect was due to an interaction of the beta gamma complex with BARS-50, rather than with the ADP-ribosylating enzyme. Two-dimensional gel electrophoresis and immunoblot analysis shows that BARS-50 is a group of closely related proteins that appear to be different from all the known GTP-binding proteins.

Evidence for a regulated interaction between heterotrimeric G proteins and caveolin

Caveolae are flask-shaped plasma membrane specializations. A 22-kDa protein, caveolin, is a principal component of caveolar membranes in vivo. As recent evidence suggests that caveolae may participate in G protein-coupled signaling events, we have investigated the potential interaction of caveolin with heterotrimeric G proteins. Using cell fractionation techniques, we found that mutational or pharmacologic activation of Gs alpha prevents its cofractionation with caveolin. In a second independent approach, we directly examined the interaction of G proteins with caveolin. For this purpose, we recombinantly expressed caveolin as a glutathione S-transferase fusion protein. Using an in vitro binding assay, we found that caveolin interacts with G protein alpha subunits (Gs, Go, and Gi). Mutational or pharmacologic activation (with guanosine 5'-O-(thiotriphosphate)) of G alpha subunits prevents this interaction, indicating that the inactive GDP-bound form of G alpha subunits preferentially interacts with caveolin. This G protein binding activity is located within a 41-amino acid region of caveolin's cytoplasmic N-terminal domain (residues 61-101). Further functional analysis shows that a polypeptide derived from this region of caveolin (residues 82-101) effectively suppresses the basal activity of purified G proteins, apparently by inhibiting GDP/GTP exchange. This caveolin sequence is homologous to a region of the Rab GDP dissociation inhibitor, a known inhibitor of GDP/GTP exchange for Rab proteins. These data suggest that caveolin could function to negatively regulate the activation state of heterotrimeric G proteins.

Rapid sequestration of DPP IV/CD26 and other cell surface proteins in an autophagic-like compartment in Caco-2 cells treated with forskolin

The enterocytic differentiation of Caco-2 cells, a human colon adenocarcinoma cell line, is accompanied by the transcriptionally regulated expression of a subset of proteins and their correct sorting towards the cell surface. In the present work we have explored the possibility that post-translational events may interfere with this process by investigating the short term effects of a potent adenylyl cyclase activator, forskolin, on cell surface expression of dipeptidyl peptidase IV. Previous works have shown that this protein is targeted towards the apical domain through either a direct or an indirect route. Domain specific biochemical experiments demonstrate that cell surface expression of neosynthesized dipeptidyl peptidase IV rapidly decreases after a 1 hour forskolin treatment. Both initial basolateral and apical dipeptidyl peptidase IV membrane delivery were altered by forskolin treatment. Decrease of dipeptidyl peptidase IV cell surface expression was not restricted to this protein, since membrane expression of '525' antigen, a basolateral protein and of sucrase-isomaltase, an apically targeted hydrolase, which unlike dipeptidyl peptidase IV mainly follows a direct route to the brush border membrane, also decreases. In addition endocytosis of proteins from the apical and from the basolateral domain was essentially unchanged, suggesting that forskolin's target may be located on the exocytic pathway. Confocal laser scanning microscopy and immuno-electron microscopy studies demonstrate that, within 5 minutes of forskolin treatment, the cell surface proteins studied accumulate in intracellular vesicles which were co-labeled with a polyclonal antibody raised against Lamp-1, a lysosomal membrane marker. Electron microscopy studies show that these vesicles display an autophagic-like morphology. Finally, biochemical experiments indicate that dibutyryl cAMP does not mimick the forskolin effect, thus suggesting that it is a cAMP-independent phenomenon.

Cyclic AMP modulates the rate of ‘constitutive’ exocytosis of apical membrane proteins in Madin-Darby canine kidney cells

Madin-Darby canine kidney and other epithelial cell lines (e.g. Caco-2, MCF-10A and MCF-7) develop intracellular vacuoles composed of apical membrane displaying microvilli (VACs) when impaired from forming normal cell-to-cell contacts. In a previous publication, we showed that VACs are rapidly exocytosed upon treatment with 8-Br-3′,5′-cyclic adenosine monophosphate (8-Br-cAMP), a membrane-permeable analog of cAMP, and that this exocytosis correlates with variations in the cellular cAMP concentration in response to the cell-cell contacts. In the present work, we tested the hypothesis that cAMP may be a positive modulator of the ‘constitutive’ exocytic pathway. To mimic conditions in cells with incomplete intercellular contacts, the intracellular levels of cAMP were decreased by means of two independent approaches: (i) pores were induced in the plasma membrane with the polypeptidic antibiotic subtilin, thus allowing small molecules (including cAMP) to permeate and move out of the cytoplasm; and (ii) adenylate cyclase and protein kinase A were blocked with specific inhibitors. In all cases, the intracellular levels of cAMP were measured and, in porated cells, equilibrated to simulate the corresponding physiological intracellular concentrations. The decrease in cAMP within the physiological range resulted in a decreased rate of transport of an apical marker of the constitutive pathway (influenza virus hemagglutinin) from the trans-Golgi network to the apical plasma membrane. Likewise, the delivery of a number of cellular apical proteins to the plasma membrane was retarded at low cAMP concentrations. The inhibitors of adenylate cyclase failed to block basolateral delivery of vesicular stomatitis virus G protein. This differential modulatory effect may represent a differentiation-dependent control of the insertion of apical membrane in epithelial cells.

Regulation of protein traffic in polarized epithelial cells

The plasma membrane of polarized epithelial cells is divided into apical and basolateral surfaces, with different compositions. Proteins can be sent directly from the trans-Golgi network (TGN) to either surface, or can be sent first to one surface and then transcytosed to the other. The glycosyl phosphatidylinositol anchor is a signal for apical targeting. Signals in the cytoplasmic domain containing a beta-turn determine basolateral targeting and retrieval, and are related to other sorting signals. Transcytosed proteins, such as the polymeric immunoglobulin receptor (pIgR), are endocytosed from the basolateral surface and then accumulate in a tubular compartment concentrated underneath the apical surface. This compartment, tentatively termed the apical recycling compartment, may be a central sorting station, as it apparently receives material from both surfaces and sorts them for delivery to the correct surface. Delivery to the apical surface from both the TGN and the apical recycling compartment appears to be regulated by protein kinases A and C, and endocytosis from the apical surface is also regulated by kinases. Transcytosis of the pIgR is additionally regulated by phosphorylation of the pIgR and by ligand binding to the pIgR. Regulation of traffic in polarized epithelial cells plays a central role in cellular homeostasis, response to external signals and differentiation.