Purification and characterization of a cytosolic insulin-stimulated serine kinase from rat liver

Characterization of a eukaryotic type serine/threonine kinase in Spodoptera litura nucleopolyhedrovirus-I (SpltNPV-I)

An open reading frame (ORF) of 819nt coding for a predicted protein of 272 amino acids was identified in the genome of Spodoptera litura nucleopolyhedrovirus (SpltNPV-I). Sequence derived amino acid sequence analysis of this ORF suggested it to be a eukaryotic type protein kinase having conserved I-XI subdomains of Hanks kinase. In addition to kinase catalytic domains, this hypothetical protein had two bromodomains which could play regulatory roles in transcription. The ORF was expressed as approximately 31 kDa apoprotein in E. coli and approximately 33 kDa glycoprotein in Sf9 cells, and was called SpltNPV-I pk1 or pk1. The protein was localized in the nuclei of infected cells of the SpltNPV-I permissive cell line, NIV-HA-197. The recombinant protein had autophosphorylation and substrate phosphorylation activities in presence of Mn(2+) or Mg(2+), and these activities were inhibited by staurosporine. Mutation of Lys-50 to Met but not Lys-44 to Gln abolished pk1 kinase activity. Kinetics of pk1 showed that the rate of phosphorylation of SpltNPV-I pk1>MBP>histone H1, and both MBP and histone H1 had the K(m)s of 3muM. Analysis of phosphorylated protein showed the phosphorylation of serine and threonine residues, but not tyrosine. All these results suggested that identified SpltNPV-I ORF codes for a serine/threonine kinase.

A non-radioactive assay for the cAMP-dependent protein kinase activity in rat brain homogenates and age-related changes in hippocampus and cortex

Cyclic AMP-dependent protein kinase (PKA) activity was involved in a number of brain functions such as cognitive process or aging. The measurement of PKA activity is traditionally based on the use of [(32)P]ATP in phosphorylation of specific protein. Recently non-isotopic PKA assays have been developed, but none has been tested on brain homogenates. This work aimed to adapt a fluorimetric method of PKA activity into a novel assay never applied before in brain homogenate, and to characterize the enzyme activity and ratio in hippocampus and cortex from rats of different ages. Optimal conditions of homogenization and enzyme protection were determined. The method was sensitive and reproducible (intra-assay and interassay variation was 5.0% and 9.0%, respectively). In hippocampal cytosol, PKA activity was 27+/-8 and 80+/-9 nmol/min per mg protein in basal and cAMP-stimulated activity, respectively, and accounted for 80% of total cell PKA activity. The non-PKA activity, assessed by the use of the PKA specific inhibitor (PKI) accounted for 49.0% and 65.0% of endogenous levels in cytosol and membrane, respectively. cAMP-augmenting drugs effects were measured and increase of 53%, 273% and 118% over basal by 10 microM isoproterenol, 100 microM forskolin, 1 microM Sp-AMP, respectively, was observed. With respect to the changes in animal age, PKA activity increased from newborn to the mature rats but decreased in older rats. The PKA ratio was higher in cytosol than in particulate fraction, and was decreased in hippocampal sample from old rats (P<0.05). This last result was interpreted as related to the loss of cognitive capacities in old animals.

Novel piperidinedione analogs as inhibitors of breast cancer cell growth

We previously reported the utility of antineoplaston-A10 (3-phenylacetylamino-2,6-piperidinedione) as an endogenous cancer protector and immune modulator in breast cancer patients (Cancer Lett., 2000, 157, 57). In this study, four new piperidinedione A10 analogs were synthesized and tested for their antimitotic activity on a human breast cancer cell line against the prototype A10 and the antibreast cancer drug tamoxifen. Moreover, the DNA binding capacity of such compounds was evaluated against A10, (E)-3-(4-Nitrocinnamoylamino)-2,6-piperidinedione "3B" and (E)-3-(4-hydroxycinnamoylamino)-2,6-piperidinedione "3D" were several-fold more potent antiproliferative agents than A10 and tamoxifen. They also had significantly higher capacity to bind DNA than A10. Conversely, (E)-3-(cinnamoylamino)-2,6-piperidinedione "3A" and (E)-3-(4-methoxycinnamoylamino)-2,6-piperidinedione) "3C" had weaker biological profiles than the lead compound A10. Detailed synthetic, spectroscopic, and biological data are reported.

TECHNICAL NOTE: measurement of cAMP-dependent protein kinase activity using a fluorescent-labeled kemptide

BACKGROUND

Traditional protein kinase assays include the use of [32P] labeled ATP as phosphate donor and a substrate protein or peptide as phosphoreceptor. Since this approach has a number of drawbacks in addition to generating ionizing radiation, several non-isotopic methods have been developed. Although shown to reflect the activity of purified enzymes, none have been demonstrated to detect physiological changes in endogenous enzyme activity in cell homogenates.

METHODS

Studies were performed to examine the kinetics, reproducibility, and optimal assay conditions of a novel non-radioisotopic kinase assay that detects PKA activity by phosphorylation of the peptide substrate Kemptide covalently bound to a fluorescent molecule (f-Kemptide). Basal and agonist-induced PKA activity in epithelial cell homogenates was measured.

RESULTS

The kinetics of f-Kemptide were similar to the standard radioisotopic method with intraassay and interassay variations of 5.6 +/- 0.8% and 14.3 +/- 2.6%, respectively. Neither fluorescence quenching nor enhancing effects were found with consistent amounts of homogenate protein. Specific PKA activity was determined as the IP20-inhibitable fraction to account for nonspecific phosphorylation, perhaps due to S6 kinase or a similar enzyme. The basal activity of 38% of total PKA in A6 cells increased by 84% after exposure to vasopressin and by 58% after short exposure to forskolin. In T84 cells exposed to VIP there was a 360% increase over basal activity.

CONCLUSIONS

These results show that f-Kemptide exhibits acceptable kinetics, and that the assay system can quantitatively and reproducibly measure basal and stimulated PKA activity in cell homogenates.

Identification of a novel Ca2+-stimulated S6-kinase in rat liver

Extracellular calcium addition transiently stimulated two S6 peptide kinase activities in isolated rat hepatocytes. Mono Q chromatography revealed that the activities eluting at 0.15 M NaCl and 0.18 M NaCl were stimulated 4-fold and 2-fold, respectively. The kinase stimulated by calcium was a 40000-Mr S6 peptide kinase, as demonstrated by partial purification from whole liver. The protein kinase did not crossreact with antibodies directed against the N- or C-terminal part of p70 ribosomal S6 kinase (p70(S6K)) and the C-terminal part of p90 ribosomal S6 kinase (p90(rsk)). Following digestion of 40000-Mr S6 peptide kinase with trypsin, six peptides were sequenced. There was no similarity with the sequences of p70(S6K) and p90(rsk). Moreover, the obtained sequences could not be identified in the SwissProt or EMBL-genebank databases, suggesting that 40000-Mr S6 peptide kinase probably represents a novel protein kinase.

The role of protein phosphorylation in the regulation of cyclic nucleotide phosphodiesterases

The cyclic nucleotide phosphodiesterases constitute a complex superfamily of enzymes responsible for catalyzing the hydrolysis of cyclic nucleotides. Regulation of cyclic nucleotide phosphodiesterases is one of the two major mechanisms by which intracellular cyclic nucleotide levels are controlled. In many cases the fluctuations in cyclic nucleotide levels in response to hormones is due to the hormone responsiveness of the phosphodiesterase. Isozymes of the cGMP-inhibited, cAMP-specific, calmodulin-stimulated and cGMP-binding phosphodiesterases have been demonstrated to be substrates for protein kinases. Here we review the evidence that hormonally responsive phosphorylation acts to regulate cyclic nucleotide phosphodiesterases. In particular, the cGMP-inhibited phosphodiesterases, which can be phosphorylated by at least two different protein kinases, are activated as a result of phosphorylation. In contrast, phosphorylation of the calmodulin-stimulated phosphodiesterases, which coincides with a decreased sensitivity to activation by calmodulin, results in decreased phosphodiesterase activity.