Relationship of the methylenetetrahydrofolate reductase C677T polymorphism with microsatellite instability and promoter hypermethylation in sporadic colorectal cancer

The methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism is associated with the expression of a thermolabile enzyme with decreased activity that influences the pool of methyl-donor molecules. Several studies have reported an association between C677T polymorphism and susceptibility to colorectal cancer (CRC). Considering that methylation abnormalities appear to be important for the pathogenesis of CRC, we examined the correlation between the genotype of the MTHFR C677T polymorphism, hypermethylation of the promoter region of five relevant genes (DAPK, MGMT, hMLH1, p16(INK4a), and p14(ARF)), and microsatellite instability, in 106 patients with primary CRCs in Brazil. We did not find significant differences in the genotypic frequencies of the MTHFR C677T polymorphism when one or more loci were hypermethylated. However, we did find a significant excess of 677TT individuals among patients with CRC who had microsatellite instability. This strong association was independent of the methylation status of hMLH1 and of the biogeographical genomic ancestry of the patients. Although the mechanism responsible for the link between the C677T polymorphism and microsatellite instability was not apparent, this finding may provide a clue towards a better understanding of the pathogenesis of microsatellite instability in human colorectal cancer.

Control of the binding of a vesamicol analog to the vesicular acetylcholine transporter.

4-Aminobenzovesamicol was used to test whether activation of protein kinase C protects the vesicular acetylcholine transporter from interaction with vesamicol-like drugs. The essentially irreversible vesamicol analog inhibits the release of newly synthesized [3H]acetylcholine from stimulated hippocampal slices. Prior activation of protein kinase C with a phorbol ester prevented the inhibition of [3H]acetylcholine release, but activation of protein kinase C after the exposure to the irreversible analog did not prevent the effect of the drug. Binding of 4-aminobenzovesamicol in hippocampal synaptosomes, assayed using [3H]vesamicol and back-titration, was decreased by activation of protein kinase C prior to analog exposure but not by activation subsequent to exposure. We propose that phosphorylation of the vesicular acetylcholine transporter prevents the binding of vesamicol-like drugs.

Role of protein kinase C in the release of [3H]acetylcholine from myenteric plexus treated with vesamicol.

The present experiments investigated the release of [3H]acetylcholine ([3H]ACh) from the guinea pig myenteric plexus treated with 2-(4-phenylpiperidino)cyclohexanol (vesamicol), a drug that impairs ACh accumulation by synaptic vesicles. Ouabain, an Na+-K+ ATPase inhibitor, released [3H]ACh synthesised in the presence of (-)-vesamicol, while electrical field stimulation or KCl depolarisation were not effective to release the transmitter in this condition. The effect of ouabain was Ca2+-dependent and in the presence of (-)-vesamicol it was blocked by calphostin C, an inhibitor of protein kinase C (PKC). In addition, stimulation of kinase C activity by a phorbol ester, but not by its inactive isomer, prevented (-)-vesamicol from interfering with the release of [3H]ACh in electrically-stimulated myenteric plexus, similar to the effect of ouabain. We conclude that release of [3H]ACh induced by ouabain in the presence of (-)-vesamicol depends on PKC activation.

Effect of protein kinase C activation on the release of [3H]acetylcholine in the presence of vesamicol.

The present work tested whether pharmacological activation of protein kinase C (PKC) influences the release of [3H]acetylcholine ([3H]ACh) synthesized in the presence of vesamicol, an inhibitor of the vesicular acetylcholine transporter (VAChT). Newly synthesized [3H]ACh was released from hippocampal slices by field stimulation (15 Hz) in the absence of vesamicol, but as expected [3H]ACh synthesized during exposure to vesamicol was not released significantly by stimulation. Treatment of slices with the PKC activator phorbol myristate acetate (PMA) decreased the inhibitory effect of vesamicol on [3H]ACh release. The effect of PMA was dose-dependent, was sensitive to calphostin C, a PKC-selective inhibitor, and could not be mimicked by alpha-PMA, an inactive phorbol ester. PMA did not alter the release of [3H]ACh in the absence of vesamicol, suggesting that the site of PKC action could be related to the VAChT. In agreement with this observation, immunoprecipitation of VAChT from 32P-labeled synaptosomes showed that phosphorylation occurs and that incorporation of 32P in the VAChT protein increases in the presence of PMA. We suggest that PKC alters the output of [3H]ACh formed in the presence of vesamicol and also provide circumstantial evidence for a role of phosphorylation of VAChT in this process.