DNA methyltransferase 1 (DNMT1) is the primary enzyme that maintains DNA methylation. We describe a previously unknown mode of regulation of DNMT1 protein stability through the coordinated action of an array of DNMT1-associated proteins. DNMT1 was destabilized by acetylation by the acetyltransferase Tip60, which triggered ubiquitination by the E3 ligase UHRF1, thereby targeting DNMT1 for proteasomal degradation. In contrast, DNMT1 was stabilized by histone deacetylase 1 (HDAC1) and the deubiquitinase HAUSP (herpes virus–associated ubiquitin-specific protease). Analysis of the abundance of DNMT1 and Tip60, as well as the association between HAUSP and DNMT1, suggested that during the cell cycle the initiation of DNMT1 degradation was coordinated with the end of DNA replication and the need for DNMT activity. In human colon cancers, the abundance of DNMT1 correlated with that of HAUSP. HAUSP knockdown rendered colon cancer cells more sensitive to killing by HDAC inhibitors both in tissue culture and in tumor xenograft models. Thus, these studies provide a mechanism-based rationale for the development of HDAC and HAUSP inhibitors for combined use in cancer therapy.

Protein tyrosine phosphatase receptor-type T (PTPRT) is the most frequently mutated tyrosine phosphatase in human cancers. However, the cell signaling pathways regulated by PTPRT largely remain to be elucidated. Here, we show that paxillin is a direct substrate of PTPRT and that PTPRT specifically regulates paxillin phosphorylation at tyrosine residue 88 (Y88) in colorectal cancer (CRC) cells. We engineered CRC cells homozygous for a paxillin Y88F knock-in mutant and found that these cells exhibit significantly reduced cell migration and impaired anchorage-independent growth, fail to form xenograft tumors in nude mice, and have decreased phosphorylation of p130CAS, SHP2, and AKT. PTPRT knockout mice that we generated exhibit increased levels of colonic paxillin phosphorylation at residue Y88 and are highly susceptible to carcinogen azoxymethane-induced colon tumor, providing critical in vivo evidence that PTPRT normally functions as a tumor suppressor. Moreover, similarly increased paxillin pY88 is also found as a common feature of human colon cancers. These studies reveal an important signaling pathway that plays a critical role in colorectal tumorigenesis.colorectal cancer R eversible tyrosine phosphorylation, which is coordinately controlled by protein tyrosine kinases (PTKs) and phosphatases (PTPs), governs numerous signaling pathways that regulate cell proliferation, apoptosis, adhesion, and migration. Over the last two decades, many PTKs have been found to be mutated in a variety of different tumor types (reviewed in ref. 1). In contrast to PTKs, the role of PTPs in tumorigenesis is underexplored. To systematically evaluate possible roles of PTPs in tumorigenesis, we used a high throughput molecular and bioinformatics approach to detect genetic alterations of the tyrosine phosphatase gene family in colorectal cancers (CRCs) (2). Among the six mutated PTPs that we identified, protein tyrosine phosphatase receptor-type T (PTPRT), also known as PTPρ, was the most frequently mutated (2). In addition, we and others found that PTPRT is also mutated in lung, stomach, and skin cancers (2, 3). The spectrum of mutations, which includes nonsense mutations and frameshifts, suggested that these mutations were inactivating (2). Biochemical analyses demonstrated that missense mutations in the catalytic domains of PTPRT diminished its phosphatase activity, whereas overexpression of PTPRT inhibited CRC cell growth (2). Taken together, these studies suggest that PTPRT normally acts as a tumor suppressor gene. In light of these data, it is important to identify the functionally significant substrates of PTPRT as well as to elucidate the signal transduction pathways regulated by this phosphatase. Here, we report that paxillin, an adaptor protein involved in cell adhesion, migration, proliferation, and apoptosis (4, 5), is a direct substrate of PTPRT and that phospho-paxillin has oncogenic properties. We further demonstrate that in an in vivo model, PTPRT is both a potent tumor suppressor gene and a key regulator of colonic ph...

Background: The hippocampus is an important limbic structure closely related to memory function. However, few studies have focused on the association between hippocampal subfields and age-related memory decline. We investigated the volume alterations of hippocampal subfields at different ages and assessed the correlations with Immediate and Delayed recall abilities.Materials and Methods: A total of 275 participants aged 20–89 years were classified into 4 groups: Young, 20–35 years; Middle-early, 36–50 years; Middle-late, 51–65 years; Old, 66–89 years. All data were acquired from the Dallas Lifespan Brain Study (DLBS). The volumes of hippocampal subfields were obtained using Freesurfer software. Analysis of covariance (ANCOVA) was performed to analyze alterations of subfield volumes among the 4 groups, and multiple comparisons between groups were performed using the Bonferroni method. Spearman correlation with false discovery rate correction was used to investigate the relationship between memory recall scores and hippocampal subfield volumes.Results: Apart from no significant difference in the left parasubiculum (P = 0.269) and a slight difference in the right parasubiculum (P = 0.022), the volumes of other hippocampal subfields were significantly different across the adult lifespan (P < 0.001). The hippocampal fissure volume was increased in the Old group, while volumes for other subfields decreased. In addition, Immediate recall scores were associated with volumes of the bilateral molecular layer, granule cell layer of the dentate gyrus (GC-DG), cornus ammonis (CA) 1, CA2/3, CA4, left fimbria and hippocampal amygdala transition area (HATA), and right fissure (P < 0.05). Delayed recall scores were associated with the bilateral molecular layer, GC-DG, CA2/3 and CA4; left tail, presubiculum, CA1, subiculum, fimbria and HATA (P < 0.05).Conclusion: The parasubiculum volume was not significantly different across the adult lifespan, while atrophy in dementia patients in some studies. Based on these findings, we speculate that volume changes in this region might be considered as a biomarker for dementia disorders. Additionally, several hippocampal subfield volumes were significantly associated with memory scores, further highlighting the key role of the hippocampus in age-related memory decline. These regions could be used to assess the risk of memory decline across the adult lifespan.

PIK3CA encodes the p110α catalytic subunit of PI3K and is frequently mutated in human cancers, including ∼30% of colorectal cancer. Oncogenic mutations in PIK3CA render colorectal cancers more dependent on glutamine. Here we report that the glutaminase inhibitor CB-839 preferentially inhibits xenograft growth of PIK3CA-mutant, but not wild-type (WT), colorectal cancers. Moreover, the combination of CB-839 and 5-fluorouracil (5-FU) induces PIK3CA-mutant tumor regression in xenograft models. CB-839 treatment increased reactive oxygen species and caused nuclear translocation of Nrf2, which in turn upregulated mRNA expression of uridine phosphorylase 1 (UPP1). UPP1 facilitated the conversion of 5-FU to its active compound, thereby enhancing the inhibition of thymidylate synthase. Consistently, knockout of UPP1 abrogated the tumor inhibitory effect of combined CB-839 and 5-FU administration. A phase I clinical trial showed that the combination of CB-839 and capecitabine, a prodrug of 5-FU, was well tolerated at biologically-active doses. Although not designed to test efficacy, an exploratory analysis of the phase I data showed a trend that PIK3CA-mutant patients with colorectal cancer might derive greater benefit from this treatment strategy as compared with PIK3CA WT patients with colorectal cancer. These results effectively demonstrate that targeting glutamine metabolism may be an effective approach for treating patients with PIK3CA-mutant colorectal cancers and warrants further clinical evaluation. Significance: Preclinical and clinical trial data suggest that the combination of CB-839 with capecitabine could serve as an effective treatment for PIK3CA-mutant colorectal cancers.