DNA methyltransferase 3b contributes to oncogenic transformation induced by SV40T antigen and activated Ras

Abstract

Transcriptional silencing of tumor suppressor genes in association with DNA methylation contributes to malignant transformation. However, the specific DNA methyltransferases that initiate this process are unknown. Here we show that a de novo DNA methyltransferase, DNMT3b, substantially contributes to the oncogenic phenotype in a lung cancer model. Normal human bronchial epithelial (NHBE) cells expressing telomerase, SV40 large T antigen, and activated Ras were immortal, formed colonies in soft agar, and expressed DNMT3b. Antisense suppression of DNMT3b prevented soft agar growth. Furthermore, mouse embryo fibroblasts expressing T antigen and Ras formed soft agar colonies and large tumors, but fibroblasts from Dnmt3b À/À mice did not grow in soft agar and were much less tumorigenic in vivo. The tumor suppressor genes, FHIT, TSLC1, and RASSF1A were downregulated in transformed NHBE cells, and antisense DNMT3b treatment resulted in reexpression of FHIT and TSLC1. While expression of TSCL1 correlated with methylation of CpG dinucleotides in its promoter region, the expression of FHIT did not, suggesting that DNMT3b may silence genes by several mechanisms including direct DNA methylation or recruitment of proteins that modify chromatin. Regardless of mechanism, our data indicate that DNMT3b plays an important role in transformation.

Presence of simian virus 40 DNA sequences in diffuse large B‐cell lymphomas in Tunisia correlates with aberrant promoter hypermethylation of multiple tumor suppressor genes

“…SV40 is a potent tumorigenic DNA virus that causes various tumors when injected into hamsters, including leukemias and lymphomas, [1][2][3][4]54 and is a unique carcinogen that is shown to promote in vitro malignancy transformation simultaneously by inactivating many TSGs, 1,25 inducing telomerase activity, 26 and activating several oncogenes and growth factors. 27 Several previous studies have detected SV40 DNA sequences in a subset of hematological malignancies lymphomas mainly in DLBCLs.…”

“…1 In vitro, the infection of human cells by SV40 has showed that SV40 Tag can promote malignancy transformation by blocking the products of several tumor suppressor genes (TSGs), 1,25 inducing telomerase activity, 26 and stimulating other oncogenes and growth factors. 27 Hypermethylation of the DNA promoter and the related phenomenon of histone deacetylation are epigenetic changes in chromatin structure that do not alter the DNA sequence and can cause gene inactivation.…”

Ras transformation requires metabolic control by 6-phosphofructo-2-kinase

“…Ras and 6-phosphofructo-2-kinase S Telang et al PFKFB mRNA expression and F2,6BP in normal human bronchial epithelial (NHBE) cells that had been sequentially immortalized and transformed using the telomerase catalytic subunit (hT), SV40 LT and an oncogenic allele of ras (Soejima et al, 2003). We observed simultaneous expression of three isozymes, PFKFB2-4, but not PFKFB1 (i.e., the liver isozyme), as well as a modest increase in PFKFB3 protein product expression and a large decrease in intracellular F2,6BP with immortalization (Figure 5a-e), which was similar to our findings with the mouse lung fibroblasts.…”

Epigenetic changes in virus-associated human cancers

“…ICF syndrome, a rare disorder characterized by immunodeficiency, centromeric instability, and facial abnormalities, is caused by germ-line mutations in dnmt3b [22,23]. Interestingly, mouse embryo fibroblasts deficient of DNMT3B are resistant to transformation by SV40 large T antigen and contain active Ras oncogenes [24], suggesting that epigenetic and genetic mechanisms likely act in concert during cellular transformation. It is also worth noting that DNMT1 and DNMT3B are overexpressed in certain human cancers, although only at moderate levels [25][26][27].…”

“…In addition to inactivate tumor suppressor gene, SV40 T antigen has been shown to act in concert with activated Ras and teleomerase to form colonies in soft agar assay by using normal human bronchial cells (NHBC) [24]. The expression of one of the DNMT 3B isoforms was increased in these transformed cells.…”