Regulation of cyclic AMP level and lactic acid production in Ehrlich ascites tumor cells

Therapeutic effect of natural polyphenols against glioblastoma

Glioblastoma (GBM) is the most common and aggressive tumor of the central nervous system, which has a highly invasive growth pattern, which creates poor prospects for patient survival. Chemotherapy and tumor surgery are limited by anticancer drug resistance and tumor invasion. Evidence suggests that combinations of treatments may be more effective than single drugs alone. Natural polyphenolic compounds have potential as drugs for the treatment of glioblastoma and are considered as potential anticancer drugs. Although these beneficial effects are promising, the efficacy of natural polyphenolic compounds in GBM is limited by their bioavailability and blood-brain barrier permeability. Many of them have a significant effect on reducing the progression of glioblastoma through mechanisms such as reduced migration and cell invasion or chemosensitization. Various chemical formulations have been proposed to improve their pharmacological properties. This review summarizes natural polyphenolic compounds and their physiological effects in glioblastoma models by modulating signaling pathways involved in angiogenesis, apoptosis, chemoresistance, and cell invasion. Polyphenolic compounds are emerging as promising agents for combating the progression of glioblastoma. However, clinical trials are still needed to confirm the properties of these compounds in vitro and in vivo.

The biochemistry and medical significance of the flavonoids

Flavonoids are plant pigments that are synthesised from phenylalanine, generally display marvelous colors known from flower petals, mostly emit brilliant fluorescence when they are excited by UV light, and are ubiquitous to green plant cells. The flavonoids are used by botanists for taxonomical classification. They regulate plant growth by inhibition of the exocytosis of the auxin indolyl acetic acid, as well as by induction of gene expression, and they influence other biological cells in numerous ways. Flavonoids inhibit or kill many bacterial strains, inhibit important viral enzymes, such as reverse transcriptase and protease, and destroy some pathogenic protozoans. Yet, their toxicity to animal cells is low. Flavonoids are major functional components of many herbal and insect preparations for medical use, e.g., propolis (bee's glue) and honey, which have been used since ancient times. The daily intake of flavonoids with normal food, especially fruit and vegetables, is 1-2 g. Modern authorised physicians are increasing their use of pure flavonoids to treat many important common diseases, due to their proven ability to inhibit specific enzymes, to simulate some hormones and neurotransmitters, and to scavenge free radicals.

The constitutive heat shock protein-70 is required for optimal expression of myelin basic protein during differentiation of oligodendrocytes

To further elucidate the role of the constitutive heat shock protein-70 (HSC70) as a chaperone for the synthesis of myelin basic protein (MBP), HSC70 content was decreased in oligodendrocyte precursor cells prior to MBP expression either by transfection with an antisense oligonucleotide specific for HSC70, or by exposure to low levels of quercetin, a bioflavonoid known to decrease synthesis of HSC70. As these cells underwent differentiation in vitro, antisense treatment decreased HSC70 levels to 66% of controls. At the same time, a sharp induction resulted in the stress-inducible heat shock protein-70 (HSP70). Levels of two other stress proteins increased as well, namely, the 25-kDa heat shock protein (HSP25) and the 78-kDa glucose regulated protein (GRP78). MBP synthesis proceeded over a normal time course, but at only 50% of control values. As HSC70 content returned to normal, MBP synthesis was also restored to normal levels. Quercetin reduced the expression of HSC70 to an even greater extent than transfection, and prevented the induction of HSP70. In contrast to antisense-treated cells, MBP synthesis was essentially blocked in quercetin-treated cells even though levels of HSP25 and GRP78 increased. Taken together, these observations (a) indicate that HSP70 partially compensates for decreased chaperoning of nascent MBP by HSC70 (HSC70 and HSP70 are closely related and perform similar functions); (b) preclude the involvement of HSP25 and GRP78 in MBP synthesis; and (c) emphasize the requirement of HSC70 for optimal synthesis of MBP.

Inhibition of glutathione reductase by plant polyphenols

The effects of forty-one plant polyphenols on the activity of glutathione reductase (GSH-RD) were studied. These polyphenols showed varying degrees of concentration-dependent inhibition on the enzyme, with IC50 values that varied from approximately 40 microM to 1 mM. 4'-Hydroxychalcone and tannic acid were among the more potent inhibitors, with IC50 values of 47.3 and 50.4 microM, respectively. Different classes of polyphenols varied in potency in the following order: chalcones > tannic acid > flavonoids > coumarins > catechins. Analysis of structure-activity relationships showed certain chemical structures to be important for the inhibition of GSH-RD: (a) C-5 and C-7 hydroxylations in the A-ring, a carbonyl group at C-4, and the B-ring attached to C-2 in flavonoids; (b) C-2' and C-4' hydroxylations in chalcones; and (c) C-6 and C-7 hydroxylations in coumarins. The inhibition of GSH-RD by tannic acid and quercetin was time dependent and irreversible, whereas that by 4'-hydroxychalcone and esculin was reversible but not time dependent. Enhanced inhibition of GSH-RD by the four polyphenols 4'-hydroxychalcone, quercetin, butein, and acacetin was observed in the presence of NADPH. Kinetic studies showed that both tannic acid and 4'-hydroxychalcone exhibited non-competitive inhibition on GSH-RD towards glutathione disulfide.

Ambazone as a membrane active antitumor drug

Ambazone (1,4-benzoquinone guanylhydrazone thiosemicarbazone) was found to be active against various transplantable tumors in mice as well as rats. When administered orally for 4-9 days, the effective therapeutic dose ranged between 60 and 125 mg/kg. The antineoplastic effect of ambazone appeared to be mediated, at least in part, by the immune system. In order to characterize the drug, biophysical and biophysicochemical studies were carried out using thin-layer chromatography, absorption spectroscopy and polarographic measurements. The distribution of ambazone in an n-octanol/water system indicated low hydrophobicity, thereby excluding the possibility of a preferential contribution from hydrophobic forces to the mode of action of ambazone. Ambazone undergoes three protonation reactions with pK values at 10.69 (equilibrium between the negatively charged and neutral forms), 7.39 (equilibrium between the neutral and singly positively charged form) and 6.22 (equilibrium between the singly and doubly positively charge form). Interaction of the drug with model membrane system was monitored by spectrophotometric and fluorescence measurements. Using the fluorescence label 1-anilino-8-naphthalenesulfonic acid (ANS) as a probe pointed to the interaction of ambazone with the inner area of the phospholipid bilayer matrix of liposomes as being nonspecific. Ambazone induces an overall increase in the cellular cAMP content of leukemia cells and macrophages. So far, membrane interaction has provided a molecular basis for both immunological and antineoplastic activities of the drug. By performing DNA melting experiments, it was shown that neutral or singly positively charged ambazone species stabilize the secondary structure of DNA, while the doubly positively charged form binds more strongly and destabilizes the DNA. After oral administration to rats and mice, ambazone was found to be incompletely absorbed from the gastrointestinal tract, to an extent of about 35-50%. Absorbed ambazone binds only weakly to plasma proteins, whereas its binding to red blood cells is relatively strong. The mutagenic potential of ambazone shown in bacterial systems and human lymphocytes corresponds to its relatively weak interaction with DNA. The toxic action of ambazone on the intestine is believed to be due to inhibition by the drug of bacterial DNA, RNA and protein syntheses. It is assumed that the reported affinity of ambazone for different cellular targets, i.e., membranes, nucleic acids and proteins, contributes to the overall antibacterial effect. The weak antiviral activity of ambazone in the Sendai virus/chicken embryo fibroblast system is probably the result of the interaction with Sendai virus NH glycoprotein.

Catalytic properties of a purified phosphatidylinositol-4-phosphate kinase from rat brain

A phosphatidylinositol-4-phosphate (PIP) kinase activity was purified from rat brain extract through several chromatographic steps to yield an active preparation (specific activity 1 mumol of 32P incorporated into phosphatidylinositol 4,5-bisphosphate/min per mg of protein) with an apparent molecular size of 100-110 kDa in the native form. The isolated PIP kinase required Mg2+ (optimally 20-30 mM) for its activity and was not influenced by Ca2+. The enzyme used ATP (Km 25 microM) and GTP (Km 133 microM) as phosphate sources and appeared specific for PIP (Km 3.3 micrograms/ml) as the lipid substrate. The PIP-phosphorylation reaction was inhibited by micromolar concentrations of heparin [ID50 (concn. giving 50% inhibition) 2 micrograms/ml] and the flavonoid quercetin (ID50 0.2 microM). Whereas heparin behaves as a competitive inhibitor to PIP, quercetin was competitive towards ATP (or GTP). Phosphorylation of the preparation by a highly active purified protein kinase C did not detectably alter PIP kinase activity. Whereas 12-O-tetradecanoylphorbol acetate and various phospholipids had no effect, phosphatidylserine elicited a dose-dependent activation of PIP activity. This suggests that a phosphatidylserine-PIP kinase interaction may be considered as a possible regulatory process at the cell-membrane level.

Forskolin mimics the hydrosmotic action of vasopressin in the urinary bladder of toads Bufo marinus

Net water flow JW was measured across the urinary bladder of toads Bufo marinus and averaged over periods of 1 min by means of a volumetric, automatic technique. The diterpene forskolin, an activator of adenylate cyclase bypassing the hormonal receptor subunit, induced a rapid, reversible, dose-dependent increase in osmotic water permeability, Pf, very similar to that induced by vasopressin. At 1.1 microM, forskolin induced a half-maximal response. At 5 microM forskolin caused a near maximal response and Pf increased from 1.66 +/- 0.15 to 66.6 +/- 2.99 microns s-1. In bladders pre-exposed to 5 microM-forskolin, further significant increases in Pf were obtained by their subsequent exposure to vasopressin, cyclic AMP, theophylline or serosal hypertonicity. The similarity of the forskolin and vasopressin actions was further demonstrated by the finding that substances causing enhancement (quercetin) or inhibition (trifluoperazine, vanadate, silver, cobalt, manganese and Ca2+-free Ringer solution) of the vasopressin response, induced parallel changes in the forskolin response. Three agents, however, induced dissimilar effects on vasopressin and forskolin: high K+ potentiated vasopressin but inhibited forskolin; methohexital and diamide inhibited vasopressin but had no effect on forskolin. The forskolin-induced hydrosmotic response can be viewed as a new criterion for ascertaining the messenger role of cycle AMP in the the hydrosmotic effect of vasopressin.

The effect of quercetin on the phosphorylation activity of the Rous sarcoma virus transforming gene product in vitro and in vivo

The phosphotransferase activity of the Rous sarcoma virus src gene product, pp60src, was inhibited both in vitro and in vivo by the bioflavonoid quercetin. The Ki for the inhibitory effect was in the range of 6-11 microM under conditions in vitro. The inhibitory effect of quercetin was competitive towards the nucleotides ATP and GTP as substrates for pp60src and was non-competitive towards alpha-casein as the protein substrate of this kinase activity. In contrast, studies in vitro of the phosphotransferase activity of the catalytic subunit of the cAMP-dependent protein kinase showed that this flavonoid did not inhibit the phosphorylation of physiological substrates of this enzyme. In cultured cells the half-maximal inhibition of tyrosine phosphorylation of pp60src as well as the phosphorylation of the Mr = 34000 protein, a physiological substrate of pp60src, was in the range 0.06-0.08 mM.

Inhibition of aldose reductases from rat and bovine lenses by flavonoids

Thirty flavones, four isoflavones and thirteen coumarins were tested as inhibitors of lens aldose reductase, which is believed to participate in the initiation of cataract formation in diabetes. Many were found to be potent inhibitors, and the two most potent ones were axillarin (5,7,3',4'-tetrahydroxy-3,6-dimethoxyflavone) and 6,3',4'-trihydroxy-5,7,8-trimethoxyflavone (LARI 1). These two flavones inhibited aldose reductase purified from rat lens with IC50 values of 2.6 X 10(8) and 3.6 X 10(8) M respectively. They also inhibited aldose reductase purified from bovine lens with IC50 values of 1.8 X 10(7) M. The potencies of the two compounds were superior to those of all the previously reported inhibitors of aldose reductase. Inhibition of rat and bovine lens aldose reductases by the two compounds was of a non-competitive type with DL-glyceraldehyde as the variable substrate. Some flavones including axillarin and LARI 1 were found to be poorly or scarcely inhibitory against several adeninenucleotide-requiring enzymes, which are involved in glycolysis and other metabolic reactions. These results obtained show that the two flavones have some features which may be required in clinically useful drugs for diabetic patients. All the potent inhibitors of the compounds tested had a flavone skeleton, one (or two free) hydroxyl(s) in ring C, and more than three hydroxyls (free or methylated) in ring A. The possible relationships of structures to inhibitory potencies of the compounds tested are discussed.

Selective inhibition of a cyclic nucleotide independent protein kinase (G type casein kinase) by quercetin and related polyphenols

The effect of quercetin and a number of structurally related phenolic compounds upon the activity of three different purified protein kinases was examined. Whereas the catalytic subunit of a cyclic AMP-dependent protein kinase and an A type (using only ATP) cyclic nucleotide-independent casein kinase (CKA) were not affected, a G type (using GTP as well as ATP) casein kinase (CKG) was selectively inhibited by several bioflavonoid structures. Kinetic studies showed that quercetin behaved as a competitive inhibitor toward the nucleotidic substrate and exhibited a high affinity for the ATP (Ki = 0.75 microM) and GTP (Ki = 0.22 microM) site of the enzyme. Considering the CKG inhibitory potency of a series of flavonoid, cinnamic acid and coumarin derivatives, it is suggested that the biological activity lays upon a common structural feature involving a phenolic ring bearing a side chain with conjugated double bonds and an oxygenated function, as found in the coumaroyl residue. These observations suggest that quercetin and related compounds may lead to a shift in intracellular protein phosphorylations by selectively inhibiting a particular type of protein kinase activity (CKG). It remains to be established whether this process may contribute to the mechanism of action of flavonoids upon cellular metabolism, particularly in the case of malignant cells.

Quercetin enhances water transport in toad bladder

A highly significant enhancement of the hydrosmotic actions both of vasopressin and of exogenous cAMP was seen in the presence of quercetin. The hypothesis is advanced that quercetin affects the intracellular coupling between Ca++ in cAMP.