Heparin stimulates epidermal growth factor receptor-mediated phosphorylation of tyrosine and threonine residues

Protein kinase C recognizes the protein kinase A-binding motif of nonstructural protein 3 of hepatitis C virus

The nonstructural protein 3 (NS3) of hepatitis C virus (HCV) inhibits the nuclear transport and the enzymatic activity of the catalytic subunit of protein kinase A. This inhibition is mediated by an arginine-rich domain localized between amino acids 1487-1500 of the HCV polyprotein. The data presented here indicate that the arginine-rich domain, when embedded in recombinant fragments of NS3, interacts with the catalytic site of protein kinase C (PKC) and inhibits the phosphorylation mediated by this enzyme in vitro and in vivo. Furthermore, a direct binding of PKC to the NS3 fragments leads to an inhibition of the free shuttling of the kinase between the cytoplasm and the particulate fraction. In contrast, a peptide corresponding to the arginine-rich domain (HCV (1487-1500)), despite also being a PKC inhibitor, did not influence the PKC shuttling process and was transported to the particulate fraction by the translocating kinase upon activation with tetradecanoylphorbol-13-acetate. Using the tetradecanoylphorbol-13-acetate -stimulated respiratory burst of NS3-introduced neutrophils as a model system, we could demonstrate that NS3 is able to block PKC-mediated functions within intact cells. Our data support the possibility that NS3 disrupts the PKC-mediated signal transduction.

Purification of catalytic domain of rat spleen p72syk kinase and its phosphorylation and activation by protein kinase C

The catalytic domain of p72(syk) kinase (CDp72(syk)) was purified from a 30000 g particulate fraction of rat spleen. The purification procedure employed sequential chromatography on columns of DEAE-Sephacel and Superdex-200, and elution from HA-Ultrogel by chloride. The analysis of the final CDp72(syk) preparation by SDS/PAGE revealed a major silver-stained 40 kDa protein. The kinase was identified by covalent modification of its ATP-binding site with [14C]5'-fluorosulphonylbenzoyladenosine and by immunoblotting with a polyclonal antibody against the 'linker' region of p72(syk). By using poly(Glu4, Tyr1) as a substrate, the specific activity of the enzyme was determined as 18.5 nmol Pi/min per mg. Casein, histones H1 and H2B and myelin basic protein were efficiently phosphorylated by CDp72(syk). The kinase exhibited a limited ability to phosphorylate random polymers containing tyrosine residues. CDp72(syk) autophosphorylation activity was associated with an activation of the kinase towards exogenous substrates. The extent of activation was dependent on the substrates added. CDp72(syk) was phosphorylated by protein kinase C (PKC) on serine and threonine residues. With a newly developed assay method, we demonstrated that the PKC-mediated phosphorylation had a strong activating effect on the tyrosine kinase activity of CDp72(syk). Studies extended to conventional PKC isoforms revealed an isoform-dependent manner (alpha > betaI = betaII > gamma) of CDp72(syk) phosphorylation. The different phosphorylation efficiencies of the PKC isoforms closely correlated with the ability to enhance the tyrosine kinase activity.

Non-structural protein 3 of hepatitis C virus inhibits phosphorylation mediated by cAMP-dependent protein kinase

Inspection of the amino acid sequence of the non-structural region of the hepatitis C virus (HCV) gene product reveals a sequence of 14 amino acids, Arg1487-Arg-Gly-Arg-Thr-Gly-Arg-Gly-Arg-Arg-Gly-Ile-Tyr-Arg1500 , located in the non-structural protein, NS3. This sequence is highly similar to the inhibitory site of the heat-stable inhibitor of cAMP-dependent protein kinase (PKA) and to the autophosphorylation site in the hinge region of the PKA type II regulatory domain. A synthetic peptide that corresponds to the HCV sequence above and a set of shorter analogues act as competitive inhibitors of PKA. A 43.5-kDa fragment of NS3 that consists of residues 1189-1525 of the HCV polyprotein inhibits PKA in a similar range to the investigated synthetic peptides. In contrast to the short peptides, which show competitive inhibition, HCV-polyprotein-(1189-1525) influences PKA in a mixed-inhibition-type manner. A possible mechanism explaining these differences is the formation of complexes that consist of the protein substrate, the enzyme and the HCV-polyprotein-(1189-1525). Binding studies with PKA and the non-hydrolysable ATP analogue [14C]fluorosulfonylbenzoyladenosine and [3H]cAMP do not reveal any influence of the short HCV-derived peptides or HCV-polyprotein-(1189-1525) upon the affinity of PKA for these nucleotides. The complex interactions of the NS3 fragments could influence one of the most important signal pathways of the cell and, therefore, could possibly provide new pathological mechanisms for HCV infections of liver.

Autophosphorylation of purified c-Src at its primary negative regulation site

The phosphorylating and transforming activities of c-Src are negatively regulated by phosphorylation at Tyr-527 near its carboxyl terminus. Previous studies have indicated that c-Src preferentially autophosphorylates Tyr-416, a residue in the middle of the catalytic domain, in vitro, and that Tyr-527 is phosphorylated by the carboxyl-terminal Src kinase, Csk. However, indirect evidence suggests that c-Src may also autophosphorylate Tyr-527 as part of a negative feedback loop. While some in vivo evidence suggests that Tyr-527 can be autophosphorylated in an intermolecular interaction, it has not previously been possible to directly demonstrate significant autophosphorylation in vitro. Here we show that c-Src purified from recombinant bacteria can autophosphorylate Tyr-527 to high levels in vitro when incubated with sufficiently high concentrations of ATP (KM(Mg2+/ATP) approximately equal to 20 microM) that are well above those that have been used previously. In vitro Tyr-527 autophosphorylation can occur both as an intra- and intermolecular interaction; higher enzyme concentrations are required for intermolecular Tyr-527 phosphorylation than for Tyr-416 autophosphorylation. These results support the possibility that, like G-proteins, c-Src can switch itself off in vivo by its own enzymatic activity.

Dopamine-induced lymphoma cell death by inhibition of hormone release

Dopamine inhibits prolactin release from pituitary cells and seems to affect the release of several other hormones as well. We report here that dopamine may have similar effects on human B lymphoma cells leading to inhibition of production or release of endogenous factors required for cell viability and proliferation. Thus, addition of dopamine to serum-free cultures of Burkitt lymphoma cells (Raji, Namalwa, Daudi and Jijoye) resulted in rapid and extensive cell death while a myeloma cell line, SKO, appeared to be refractory to this treatment. The addition of FCS or supernatant from serum-free cultures of Raji or T24 bladder carcinoma cells could, to a variable degree, counteract the effect of dopamine, suggesting that dopamine acts by inhibiting the production of essential autocrine factors. When two of the hormones known to be under dopamine control, i.e. prolactin (PRL) and thyrotropin (TSH), were tested, they were able to prevent dopamine-induced cell death if combined with heparin. We further observed that the reducing agent 2-mercaptoethanol (2-ME), which is known to inhibit the binding of TSH to its receptor, displayed similar effects to those of dopamine and was strongly inhibitory for Burkitt lymphoma but not for myeloma cells. As expected from its blocking activity at the receptor level, the effect of 2-ME could not be reversed by adding exogenous factors. Contrary to its effect on B lymphoma cells, 2-ME is essential for growth of the murine T-cell lymphoma line CTLL. However, we show here that dopamine can fully compensate for 2-ME, suggesting that TSH or another factor under dopamine control is intimately involved in the regulation of T-cell growth. This study lends further support to the notion of an active interplay between the neuroendocrine and immune systems and emphasizes PRL and TSH as important regulators of lymphoid cell function. It also shows that these hormones may contribute to the autonomous growth pattern of B lymphoma cells and suggests a potential role for dopamine in the treatment of B-cell tumours.

Proteoglycans in cell regulation

Proteoglycans are a diverse group of proteins carrying one or more glycosaminoglycan side chains linked to the protein as O-glycosides. Our appreciation of these structures has matured from a curiosity about unusual structural glycoproteins, to confer upon them a central role in cell biology. The major classes of glycosaminoglycans are heparan sulfate and heparin, chondroitin and dermatan sulfates, keratan sulfate and hyaluronic acid. The latter is unique in that it does not contain sulfate residues, and appears to be synthesized, at least sometimes, free of a carrier protein. There is now a wealth of information on the ability of these structures to influence the growth and development of cells and tissues. Many direct and specific effects of proteoglycans will undoubtedly be found, and there are likely to be indirect effects of the glycosaminoglycans relating to their polyelectrolyte nature. Convincing arguments that biological activity resides in certain proteoglycan core proteins are also appearing. The following discussion concerns the role of proteoglycans in the regulation and action of autocrine and polypeptide growth factors, direct mitogenic and antimitogenic actions of glycosaminoglycans, the role of these structures in regulating gene expression, and the biological activities of proteoglycan core proteins. The probable role of proteoglycans in normal glomerular cell function, and in progressive renal disease, will be presented as a harbinger of the significant role we can expect them to play in diagnosis and therapy in the near future.