No abstract

The effects of the Ca2+-mobilizing hormones noradrenaline, vasopressin and angiotensin on the unidirectional influx of Ca2+ were investigated in isolated rat liver cells by measuring the initial rate of 45Ca2+ uptake. The three hormones increased Ca2+ influx, with EC50 values (concentrations giving half-maximal effect) of 0.15 microM, 0.44 nM and 0.8 nM for noradrenaline, vasopressin and angiotensin respectively. The actions of noradrenaline and angiotensin were evident within seconds after their addition to the cells, whereas the increase in Ca2+ influx initiated by vasopressin was slightly delayed (by 5-15s). The activation of Ca2+ influx was maintained as long as the receptor was occupied by the hormone. The measurement of the resting and hormone-stimulated Ca2+ influxes at different external Ca2+ concentrations revealed Michaelis-Menten-type kinetics compatible with a saturable channel model. Noradrenaline, vasopressin and angiotensin increased both Km and Vmax. of Ca2+ influx. It is proposed that the hormones increase the rate of translocation of Ca2+ through a common pool of Ca2+ channels without changing the number of available channels or their affinity for Ca2+.

Interleukin 1 (IL1) increased phosphorylation of the small heat-shock protein (hsp 27) in MRC5 fibroblasts. The increase was maintained for at least 30 min, but levels had returned to pre-stimulation values by 2 h. When hsp 27 was metabolically labelled with [3H]leucine, about 15% was phosphorylated in resting confluent cells; this rose to 90% upon stimulation by IL1. Peptide maps of the three differently charged phosphorylated forms were consistent with their arising by phosphorylation of increasing numbers of serine residues. IL1 had the same effect on hsp 27 in pig articular chondrocytes, endothelial cells from human umbilical vein and an epidermoid carcinoma cell line (KB). Certain other agents were found selectively to increase phosphorylation of hsp 27 in MRC5 cells besides IL1 [and tumour necrosis factor (TNF)]. Platelet-derived growth factor had a similar effect to that of IL1; bradykinin, acid fibroblast growth factor and ATP caused an intermediate effect; phorbol myristate acetate (PMA) and 1-oleoyl-2-acetylglycerol had smaller effects. Dibutyryl cyclic AMP and forskolin had no effects on hsp 27 phosphorylation. When cells had been depleted of protein kinase C (PKC) by prolonged treatment with PMA, stimulation by IL1, TNF or bradykinin still increased hsp 27 phosphorylation. The stimulation by all three agents was also unaffected by the PKC inhibitor staurosporine. IL1, TNF and bradykinin each caused hsp 27 phosphorylation by a pathway independent of PKC. The results are consistent with IL1 activating a serine kinase which remains to be identified.

The inflammatory cytokines interleukin 1 (IL1) and tumour necrosis factor (TNF) have a broad range of physiological effects. Whereas their immediate post-receptor events are not well understood, both have the potential to activate a range of protein kinases. These include the three types of mitogen activated protein (MAP) kinase (ERK, JNK/p54 and p38) and a beta-casein kinase. The mechanisms by which these kinases are activated is discussed and the significance of their activation for particular biological responses is assessed.

Tumor necrosis factor (TNF) and interleukin 1 (IL1) activate a protein kinase, TIP kinase, which phosphorylates ␤ casein in vitro. We have now identified its main phosphorylation site on ␤ casein, Ser 124 (K m Ϸ 28 M), and a minor phosphorylation site, Ser 142 (K m Ϸ 0.7 mM). The sequence motif that determined the phosphorylation of Ser 124 by the kinase was studied with synthetic peptides bearing deletions or substitutions of the neighboring residues. This allowed synthesis of improved substrates (K m Ϸ 6 M) and showed that efficient phosphorylation of Ser 124 was favored by the presence of large hydrophobic residues at positions ؉1, ؉9, ؉11, and ؉13 (counted relative to the position of the phosphoacceptor amino acid) and of a cysteine at position ؊2. Peptides in which Ser124 was replaced by tyrosine were also phosphorylated by TIP kinase, showing it to have dual specificity. It is unable to phosphorylate the MAP kinases in vitro and is therefore not directly involved in their activation. Its biochemical characteristics indicate that TIP kinase is a novel dual specificity kinase, perhaps related to the mixed lineage kinases. It copurified with a phosphoprotein of about 95 kDa, which could correspond either to the autophosphorylated kinase or to an associated substrate.