The E3 ubiquitin protein ligase HERC2 modulates the activity of tumor protein p53 by regulating its oligomerization.

The tumor suppressor p53 is a transcription factor that coordinates the cellular response to several kinds of stress. p53 inactivation is an important step in tumor progression. Oligomerization of p53 is critical for its posttranslational modification and its ability to regulate the transcription of target genes necessary to inhibit tumor growth. Here we report that the HECT E3 ubiquitin ligase HERC2 interacts with p53. This interaction involves the CPH domain of HERC2 (a conserved domain within Cul7, PARC, and HERC2 proteins) and the last 43 amino acid residues of p53. Through this interaction, HERC2 regulates p53 activity. RNA interference experiments showed how HERC2 depletion reduces the transcriptional activity of p53 without affecting its stability. This regulation of p53 activity by HERC2 is independent of proteasome or MDM2 activity. Under these conditions, up-regulation of cell growth and increased focus formation were observed, showing the functional relevance of the HERC2-p53 interaction. This interaction was maintained after DNA damage caused by the chemotherapeutic drug bleomycin. In these stressed cells, p53 phosphorylation was not impaired by HERC2 knockdown. Interestingly, p53 mutations that affect its tetramerization domain disrupted the HERC2-p53 interaction, suggesting a role for HERC2 in p53 oligomerization. This regulatory role was shown using cross-linking assays. Thus, the inhibition of p53 activity after HERC2 depletion can be attributed to a reduction in p53 oligomerization. Ectopic expression of HERC2 (residues 2292-2923) confirmed these observations. Together, these results identify HERC2 as a novel regulator of p53 signaling.

Safety and effectiveness of sucroferric oxyhydroxide in Spanish patients on dialysis: sub-analysis of the VERIFIE study.

BACKGROUND AND AIMS: In this study, we show the results of the subset of Spanish patients of the VERIFIE study, the first post-marketing study assessing the long-term safety and effectiveness of sucroferric oxyhydroxide (SFOH) in patients with hyperphosphatemia undergoing dialysis during clinical practice. PATIENTS AND METHODS: Patients undergoing hemodialysis and peritoneal dialysis with indication of SFOH treatment were included. Follow-up duration was 12-36 months after SFOH initiation. Primary safety variables were the incidence of adverse drug reactions (ADRs), medical events of special interest (MESIs), and variations in iron-related parameters. SFOH effectiveness was evaluated by the change in serum phosphorus levels. RESULTS: A total of 286 patients were recruited and data from 282 were analyzed. Among those 282 patients, 161 (57.1%) withdrew the study prematurely and 52.5% received concomitant treatment with other phosphate binders. ADRs were observed in 35.1% of patients, the most common of which were gastrointestinal disorders (77.1%) and mild/moderate in severity (83.7%). MESIs were reported in 14.2% of patients, and 93.7% were mild/moderate. An increase in ferritin (386.66ng/mL vs 447.55ng/mL; p=0.0013) and transferrin saturation (28.07% vs 30.34%; p=0.043) was observed from baseline to the last visit (p=0.0013). Serum phosphorus levels progressively decreased from 5.69mg/dL at baseline to 4.84mg/dL at the last visit (p<0.0001), increasing by 32.2% the proportion of patients who achieved serum phosphorus levels ≤5.5mg/dL, with a mean daily SFOH dose of 1.98 pills/day. CONCLUSIONS: SFOH showed a favorable effectiveness profile, a similar safety profile to that observed in the international study with most adverse events of mild/moderate severity, and a low daily pill burden in Spanish patients in dialysis.

Mismatch repair system decreases cell survival by stabilizing the tetraploid G1 arrest in response to SN-38.

The role of the mismatch repair (MMR) system in correcting base-base mismatches is well established; its involvement in the response to DNA double strand breaks, however, is less clear. We investigated the influence of the essential component of MMR, the hMLH1 protein, on the cellular response to DNA-double strand breaks induced by treatment with SN-38, the active metabolite of topoisomerase I inhibitor irinotecan, in a strictly isogenic cell system (p53(wt), hMLH1(+)/p53(wt), hMLH1(-)). By using hMLH1 expressing clones or cells transduced with the hMLH1-expressing adenovirus as well as siRNA technology, we show that in response to SN-38-induced DNA damage the MMR proficient (MMR(+)) cells make: (i) a stronger G2/M arrest, (ii) a subsequent longer tetraploid G1 arrest, (iii) a stronger activation of Chk1 and Chk2 kinases than the MMR deficient (MMR(-)) counterparts. Both Cdk2 and Cdk4 kinases contribute to the basal tetraploid G1 arrest in MMR(+) and MMR(-) cells. Although the Chk1 kinase is involved in the G2/M arrest, neither Chk1 nor Chk2 are involved in the enhancement of the tetraploid G1 arrest. The long-lasting tetraploid G1 arrest of MMR(+) cells is associated with their lower clonogenic survival after SN-38 treatment, the abrogation of the tetraploid G1 arrest resulted in their better clonogenic survival. These data show that the stabilization of the tetraploid G1 arrest in response to double strand breaks is a novel function of the MMR system that contributes to the lesser survival of MMR(+) cells.

The chemopreventive agent ursodeoxycholic acid inhibits proliferation of colon carcinoma cells by suppressing c-Myc expression.

Ursodeoxycholic acid (UDCA) can prevent chemical and colitis-associated colon carcinogenesis by unknown mechanism(s). One of the processes underlying the chemopreventive action could be the inhibition of proliferation by UDCA. To clarify the antiproliferative mechanism of UDCA, we used p53 wt colon carcinoma cell lines HCT8 and HCT116. UDCA-induced inhibition of proliferation was reversible and was associated with a decrease of the S-phase and an increase of G1 phase population, but not with apoptosis or senescence. The treatment suppressed the expression of c-Myc protein and, as a consequence, of several cell cycle regulatory molecules, including CDK4 and CDK6. Using the HCT8 cell line as a model, we show that UDCA suppresses c-Myc at the protein level. The suppression of c-Myc alone or a simultaneous suppression of CDK4 and of CDK6 kinase is sufficient to inhibit cell proliferation. In sum, we identified c-Myc as a primary UDCA target in colon carcinoma cells. The degradation of c-Myc protein decreases the expression of the cell cycle regulators CDK4 and CDK6, which reversibly slows down the cell cycle. The suppression of these proproliferatory molecules is the likely initial mechanism of antiproliferatory action of UDCA on colon cancer cells.