No abstract

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.

MicroRNAs (miRNA/miR) are a class of small non-coding RNAs implicated in the pathogenesis of various malignancies. In the current study, using micro(RNA)arrays, we found a ubiquitous loss of miR-126 expression in colon cancer lines when compared to normal human colon epithelia. Reconstitution of miR-126 in colon cancer cells resulted in a significant growth reduction as evidenced in clonogenic assays. A search for miR-126 gene targets revealed p85β, a regulatory subunit involved in stabilizing and propagating the PI3K signal, as one of the potential substrates. Restoration of miR-126 in cancer cells induced a ≥3-fold reduction in p85β protein levels, with no concomitant change in p85α, a gene that is functionally related to p85β but not a supposed target of miR-126. Additionally, using reporter constructs, we show that the p85β-3′ UTR is directly targeted by miR-126. Furthermore, this miR-126 mediated reduction of p85β was accompanied by a substantial reduction in phosphorylated AKT levels in the cancer cells, suggesting an impairment in PI3K signaling. Finally, in a panel of matched normal colon and primary colon tumors, each of the tumors demonstrated miR-126 down-regulation together with an increase in the p85β protein level. Taken together, we propose that miR-126 regulates PI3K signaling partly by targeting p85β, and that the loss of miR-126 may provide a selective growth advantage during colon carcinogenesis.

15-Hydroxyprostaglandin dehydrogenase (15-PGDH)is a prostaglandin-degrading enzyme that is highly expressed in normal colon mucosa but is ubiquitously lost in human colon cancers. Herein, we demonstrate that 15-PGDH is active in vivo as a highly potent suppressor of colon neoplasia development and acts in the colon as a required physiologic antagonist of the prostaglandin-synthesizing activity of the cyclooxygenase 2 (COX-2) oncogene. We first show that 15-PGDH gene knockout induces a marked 7.6-fold increase in colon tumors arising in the Min (multiple intestinal neoplasia) mouse model. Furthermore, 15-PGDH gene knockout abrogates the normal resistance of C57BL͞6J mice to colon tumor induction by the carcinogen azoxymethane (AOM), conferring susceptibility to AOM-induced adenomas and carcinomas in situ. Susceptibility to AOM-induced tumorigenesis is mediated by a marked induction of dysplasia, proliferation, and cyclin D1 expression throughout microscopic aberrant crypt foci arising in 15-PGDH null colons and is concomitant with a doubling of prostaglandin E 2 in 15-PGDH null colonic mucosa. A parallel role for 15-PGDH loss in promoting the earliest steps of colon neoplasia in humans is supported by our finding of a universal loss of 15-PGDH expression in microscopic colon adenomas recovered from patients with familial adenomatous polyposis, including adenomas as small as a single crypt. These models thus delineate the in vivo significance of 15-PGDH-mediated negative regulation of the COX-2 pathway and moreover reveal the particular importance of 15-PGDH in opposing the neoplastic progression of colonic aberrant crypt foci.colon cancer ͉ prostaglandin E2 T he first and rate-limiting step in the inactivation and degradation of prostaglandins is catalyzed by the enzyme 15-hydroxyprostaglandin dehydrogenase (15-PGDH) (1). Studies by our group and by others have demonstrated that 15-PGDH is highly expressed by normal colonic epithelial cells residing in the luminal regions of colonic crypts but that transcription of 15-PGDH mRNA is ubiquitously lost in colon cancers (2, 3). These findings have suggested the hypothesis that 15-PGDH could be a candidate tumor suppressor gene (2, 3) that might, in the normal colon, act to antagonize the prostaglandingenerating activity of the cyclooxygenase 2 (COX-2) oncogene (4). Transcriptional up-regulation of COX-2 is thought to contribute to the genesis of up to 85% of all human colon cancers (4), with the oncogenic activity of COX-2 having been demonstrated in multiple different in vivo models (5-7) and by the activity of COX-2-inhibitory drugs in shrinking premalignant human colonic adenomas (4, 8). Although negative regulation of the COX-2 pathway by 15-PGDH could thus be of clear potential significance to colon carcinogenesis, the hypothesized tumor suppressor activity of 15-PGDH has thus far not been tested in vivo. We therefore embarked on a series of studies designed to test the in vivo potency of 15-PGDH as a colon tumor suppressor by using the 15-PGDH knockout mouse as ...