The V2 vasopressin receptor stimulates ERK1/2 activity independently of heterotrimeric G protein signalling

The V2 vasopressin receptor (V2R) activates the mitogen activated protein kinases (MAPK) ERK1/2 through a mechanism involving the scaffolding protein beta arrestin. Here we report that this activating pathway is independent of G alpha s, G alpha i, G alpha q or G betagamma and that the V2R-mediated activation of G alpha s inhibits ERK1/2 activity in a cAMP/PKA-dependent manner. In the HEK293 cells studied, the beta arrestin-promoted activation was found to dominate over the PKA-mediated inhibition of the pathway, leading to a strong vasopressin-stimulated ERK1/2 activation. Despite the strong MAPK activation and in contrast with other GPCR, V2R did not induce any significant increase in DNA synthesis, consistent with the notion that the stable interaction between V2R and beta arrestin prevents signal propagation to the nucleus. Beta arrestin was found to be essential for the ERK1/2 activation, indicating that the recruitment of the scaffolding protein is necessary and sufficient to initiate the signal in the absence of any other stimulatory cues. Based on the use of selective pharmacological inhibitors, dominant negative mutants and siRNA, we conclude that the beta arrestin-dependent activation of ERK1/2 by the V2R involves c-Src and a metalloproteinase-dependent trans-activation event. These findings demonstrate that beta arrestin is a genuine signalling initiator that can, on its own, engage a MAPK activation machinery upon stimulation of a GPCR by its natural ligand.     

Differential inhibition by α- and β-tocopherol of human erythroleukemia cell adhesion: role of integrins

The effect of α- and β-tocopherol on human erythroleukemia cell (HEL) adhesion induced by phorbol 12-myristate 13-acetate (PMA) has been studied. Adhesion induced by PMA stimulation was prevented by 44.5% by physiological concentrations of α-tocopherol. Under the same experimental conditions, β-tocopherol, an analogue of α-tocopherol, produced 11% inhibition of adhesion. Cell response gradually increased from 0 to 24 h of α-tocopherol treatment. Only a slight time dependency of β-tocopherol inhibition was observed. Another human erythroleukemia cell line (K562) and the human monocyte tumor cell line U937 showed 5.0 and 11.2% inhibition, respectively. Similar to α-tocopherol, the protein kinase C inhibitor, Calphostin C, and the MAPK inhibitor, PD98059, prevented PMA-induced cell adhesion. An inhibition of ERK-1 phosphorylation was observed for α-tocopherol only in HEL, implying that MAP kinase pathway is involved in this cell line. Fluorescence-activated cell sorting (FACS), by using various integrin-specific monoclonal antibodies, has shown that α (1–6), β1, and αv integrins are less expressed at the cell surface after α-tocopherol treatment. Beta-tocopherol treatment was less effective.     

Swelling and shrinkage phenomena in liver mitochondria VI. Metabolism-independent swelling coupled to ion movement

1. Water uptake coupled to ion movement has been studied in respiratory-inhibited liver mitochondria, of which the permeability to cations was increased by valinomycin, gramicidin or EDTA, and to anions by raising the pH of the medium. The movement of water was accounted for by the osmotic pressure of the penetrating solutes.

2. The rate of movement of water was inversely proportional to the concentration of solutes in the medium, and was dependent on the presence of permeating cations and anions. The above findings are interpreted within the concept of an osmotic movement of water.

3. The flow of anions through the membrane was inhibited by Ca²⁺ and Mn²⁺ Mitochondrial swelling was inhibited by sucrose.

4. Ion movement was independent of energy supply from metabolism. The nature of the force driving the ion movement is discussed.     

Interaction of cytochrome c with cytochrome bc1 complex of the mitochondrial respiratory chain

The binding of cytochrome c to the cytochrome bc1 complex of bovine heart mitochondria was studied. Cytochrome c derivatives, arylazido-labeled at lysine 13 or lysine 22, were prepared and their properties as electron acceptors from the bc1 complex were measured. Mixtures of bc1 complex with cytochrome c derivatives were illuminated with ultraviolet light and afterwards subjected to polyacrylamide gel electrophoresis. The gels were analysed using dual-wavelength scanning at 280 minus 300 and 400 minus 430 nm. It was found that illumination with ultraviolet light in the presence of the lysine 12 derivative produced a diminution of the polypeptide of the bc1 coplex having molecular weight 30 000 (band IV) and formation of a new polypeptide composed of band IV and cytochrome c. Band IV was identified as cytochrome c1, and it was concluded that this hemoprotein interacts with cytochrome c and contains its binding site in complex III of the mitochondrial respiratory chain. Illumination of the bc1 complex in presence of the lysine 22 derivative did not produce changes of the polypeptide pattern.     

Selective labeling of beef heart cytochrome oxidase subunit III with eosin-5-maleimide

Cytochrome c oxidase has been isolated from beef heart mitochondria and labeled with the fluorochrome eosin-5-maleimide (EMA) after pretreatment with mersalyl. On SDS-polyacrylamide gels, EMA fluorescence and absorption occurred at a single band corresponding to subunit III. Since only Cys 115 of the two cysteinyl residues of subunit III had been shown to be reactive towards water-soluble SH-reagents, it was concluded that this residue was the one labeled by EMA. The EMA/enzyme ratio was about 1. Gel filtration experiments have shown that upon treatment with dicyclohexylcarbodiimide, subunit III was loosened from the complex; this result suggests that the inhibitory effect of dicyclohexylcarbodiimide on the H+-translocation activity may be related to such a phenomenon.