G2 cell cycle arrest induced by glycopeptides isolated from the bovine cerebral cortex.

The ability of glycopeptides, isolated from bovine cerebral cortex, to alter cell division was studied by cell-cycle analyses. The results showed that glycopeptides arrested baby hamster kidney (BHK)-21 cells and Chinese hamster ovary (CHO) cells in the G2 phase of the cell cycle. Upon removal of the growth inhibition from arrested BHK-21 cells, the mitotic index in colchicine-treated cultures increased from 5 to 40% within 6 h and the increase in mitotic activity was accompanied by a complete doubling of all arrested cells within this 6-h time period. Determination of DNA content in growth-arrested BHK-21 cells showed that growth-arrested cells contained about twice the DNA of control cell cultures. Although CHO cells treated in a like manner with growth inhibitor could not be arrested for the same length of time as BHK-21 cells (18 h vs. 72 h before initiation of escape) and to the same degree (60% of the cell population vs. 99% of BHK-21 cells), the escape kinetics of CHO cells did indicate a G2 arrest. Approximately 3.5 h after escape began, CHO cell numbers in treated cultures attained the cell numbers found in control cultures. This rapid growth phase occurring in less than 4 h indicated that the growth inhibitor induced a G2 arrest-point in CHO cells that was not lethal since the entire arrested cell population divided.

Effects of phorbol ester on translocation and down-regulation of protein kinase C and phosphorylation of endogenous proteins in human acute myeloid leukemia cell line KG-1 and its phorbol ester-resistant subline KG-1a.

12-O-Tetradecanoylphorbol-13-acetate (TPA) induced decreases in the catalytic activity and immunoreactivity of protein kinase C (PK-C) in the soluble fraction, accompanied by increases in their activities in the particulate fraction, of a human myeloid leukemia cell line KG-1. TPA also caused a similar down-regulation and translocation of PK-C in KG-1a, a cloned subline shown to be resistant to the differentiating effect of TPA. The activity levels of enzyme in the soluble and particulate fractions from KG-1 cells, however, were about three times higher than those from KG-1a cells. Immunocytochemical studies showed that, when KG-1 cells were treated with 10 nM TPA for 30 min, PK-C was translocated to the plasma membrane in the adherent subpopulation of cells, whereas the enzyme remained largely in the cytoplasm and perinuclear area of the nonadherent cells. TPA, in contrast, caused a PK-C translocation primarily to the perinuclear region in KG-1a cells. Phosphorylation patterns of PK-C substrate proteins in the two cell lines were similar, except that phosphorylation of the Mr 33,000 and 97,000 proteins were predominant in KG-1 and KG-1a cells, respectively. The present findings showed existence of certain differential effects of TPA on the PK-C system in the two leukemia cell lines, suggesting a molecular basis for the selective resistance of KG-1a cells to the differentiating action of TPA.

Inhibition of DNA repair by trifluoperazine.

We examined the possible role of calmodulin in the excision repair of ultraviolet light-induced pyrimidine dimers in damaged DNA by means of specialized assay systems. These assays included bromodeoxyuridine photolysis, dimer chromatography and cytosine arabinoside incorporation in conjunction with hydroxyurea. The calmodulin antagonist, trifluoperazine, and the calcium-chelating agent, EGTA, were employed to ascertain what affect calmodulin played in the repair process. Normal human fibroblast cells were used in all studies described in this report. After exposure to 10 J/m2 of 254 nm light, we observed a decrease of about 30% in the number of single-strand breaks produced in the presence of 25 microM trifluoperazine (1.9 vs. 3.3) in controls although the numbers of bases re-inserted in the repaired regions were similar (64 vs. 72). Measurement of thymine-containing dimers remaining throughout a 24 h time period indicated a 30% difference in the excision of dimers when tested with either EGTA or trifluoperazine. We also observed a significant decrease in the number of cytosine arabinoside arrested repair sites in the presence of either EGTA or trifluoperazine. The results are discussed with relation to the possibility of calmodulin altering the initial incision by repair endonuclease.

DNA repair in human cells: methods for the determination of calmodulin involvement.

These results of these assays, therefore, suggested that calmodulin functions by modulating the initial step of excision repair of UV-damaged DNA. The initial step in this process is the endonuclease function. The reasoning follows these lines of thought: Dimer chromatography indicates that incision and excision occur albeit at a decreased rate. If no excision occurs, the dimer region will remain bound to the DNA and there would be no difference between the control and treated samples. Therefore any difference detected should be due to a decrease in the incision step of repair. The photolysis assay also indicates a decrease in the incision step. Since the patch size between the control and treated cultures were identical within experimental error (64 vs 72 bases), there is no inhibition of polymerase activity. If excision were affected during the photolysis assay, it is possible that the free region of the incised strand could be used as a primer strand and the repaired DNA could have a higher molecular weight than the control strand. This was not observed. Finally, the cytosine arabinoside procedures indicated that less cytosine arabinoside molecules were incorporated into the damaged regions. Since the photolysis assay indicated that the polymerase reaction was not affected, this would indicate that less initial sites were available for repair, that is, less nicks were available indicative of decreased endonuclease activity.

Inhibition of protein kinase C by defensins, antibiotic peptides from human neutrophils.

Defensins, human neutrophil peptide (HNP) antibiotics, potently inhibited phospholipid/Ca2+ protein kinase (protein kinase C, PKC) and phosphorylation of endogenous proteins from rat brains catalyzed by the enzyme. Of the three defensin peptides, HNP-2 appeared to be more potent than HNP-1 and HNP-3. Kinetic studies indicated that defensins inhibited PKC noncompetitively with respect to phosphatidylserine (a phospholipid cofactor), Ca2+ (an activator), ATP (a phosphoryl donor) and histone H1 (a substrate protein) with Ki values ranging from 1.2 to 1.7 microM. Defensins, unlike polymyxin B (another peptide inhibitor of PKC), did not inhibit the binding of [3H]phorbol 12,13-dibutyrate to PKC; however, defensins, like polymyxin B, inhibited the PKC activity stimulated by 12-O-tetradecanoylphorbol-13-acetate. Defensins had little or no effect on myosin light chain kinase (a calmodulin/Ca2+-dependent protein kinase) and the holoenzyme or catalytic subunit of cyclic AMP-dependent protein kinase, indicating a specificity of action of defensins. It is suggested that defensins, among the most potent peptide inhibitors of PKC so far identified, may have profound effects on functions of neutrophils and other mammalian cells, in addition to their well-recognized antimicrobial activities.

Regulation of protein kinase C by lysophospholipids. Potential role in signal transduction.

Certain lysophospholipids, lysophosphatidylcholine (lyso-PC) in particular, stimulated protein kinase C at low concentrations (less than 20 microM) but, conversely, inhibited it at high concentrations (greater than 30 microM). Protein kinase C stimulation by lyso-PC required the presence of phosphatidylserine (PS) and Ca2+ and was associated with a decreased Ka for PS and increased Ka for Ca2+ of the enzyme. Cardiolipin and phosphatidic acid could partially substitute for PS in supporting the stimulatory effect of lyso-PC. Lyso-PC also biphasically regulated protein kinase C activated by diolein. Of several synthetic lyso-PC preparations tested, the oleoyl, myristoyl and palmitoyl derivatives were most active. Data from the Triton X-100 mixed micellar assay indicated that 1.4 and 14.0 mol of lyso-PC/micelle produced a maximal stimulation and a complete abolishment of the stimulation of protein kinase C, respectively. Protein kinase C stimulation by lyso-PC, with a pH optimum of about 7.5, was observed for phosphorylation of histone H1, myelin basic protein, and the 35- and 47-kDa proteins from the rat brain, but not for that of other histone subfractions and protamine. Lyso-PC acted synergistically with diacylglycerol in stimulating protein kinase C, whereas the stimulation by lyso-PC was additive to that by oleic acid. Protein kinase C inhibitors (alkyllysophospholipid, sphingosine, tamoxifen, and polymyxin B) inhibited more potently the protein kinase C activity stimulated by PS/Ca2+/lyso-PC than that stimulated by PS/Ca2+. The stimulatory and inhibitory effects of lyso-PC were not observed for myosin light chain kinase and cAMP-dependent protein kinase, indicating a specificity of its actions. The present findings suggested that lyso-PC, likely derived from membrane PC by the action of phospholipase A2, might play a role in signal transduction via a dual regulation of protein kinase C, and that it could further modulate the enzyme and hence the cellular activity by interplaying with diacylglycerol and unsaturated fatty acid, the two other classes of cellular mediators also shown to be activators of protein kinase C.

Synthetic branched-chain analogues of distearoylphosphatidylcholine: structure-activity relationship in inhibiting and activating protein kinase C.

A series of distearoylphosphatidylcholine (DSPC) analogues having various branched alkyl chains were synthesized and tested for their abilities to regulate protein kinase C (PKC). The greatest improvement (about 3-fold) in the PKC inhibitory activity over that seen for the parental lipid (i.e., DSPC) was accomplished by substitution of 8-methylstearate at sn-2 and 16-methylstearate at both sn-1 and sn-2 positions of glycerol; substitutions at both sn-1 and sn-2 with 8-methylstearate, on the other hand, caused a decrease (about 4-fold) in its inhibitory activity. Introduction of butyl, phenyl, or keto functions to various positions in the fatty alkyl chain substituted at both sn-1- and sn-2 positions imparted upon the DSPC analogues an ability to potently stimulate PKC to an extent comparable to those attainable by diacylglycerol or phorbol ester; the analogues having substitution only at the sn-2 position, in comparison, had no or reduced stimulatory activity. The butyl, phenyl, and keto analogues of DSPC, as with DSPC itself and its methyl analogues, inhibited PKC at high concentrations. Kinetic analysis indicated that the methyl DSPC analogues inhibited the enzyme competitively with respect to phosphatidylserine (PS; a phospholipid cofactor) and Ca2+. The butyl analogues activated the enzyme without affecting its affinity for PS or Ca2+, indicating a mechanism different from that seen for diacylglycerol or phorbol ester. The inhibitory activity of the methyl DSPC analogues and the stimulatory activity of the butyl DSPC analogues were reduced when PKC was activated by phorbol ester. Both classes of the analogues were unable to compete for the binding of [3H]phorbol dibutyrate to PKC.(ABSTRACT TRUNCATED AT 250 WORDS)