Sequential loss of cell cycle checkpoint control contributes to malignant transformation of murine embryonic fibroblasts induced by 20-methylcholanthrene.
Definitive information about the number and nature of discrete steps of tumorigenesis is enigmatic. To understand the multistep nature of carcinogenesis, an in vitro model of 20-Methylcholanthrene-treated primary fibroblast cells CNCI-PM-20, from 20-day old Swiss mouse embryo was used. Visible neoplastic changes with distinct morphological variations along with specific chromosomal aberrations like Robertsonian metacentrics, double and single-minute chromosomes and aneuploidy were observed from Passage-20 onwards. The cell cycle profile showed gradual increase in G(2)/M population till P-32, followed by evasion of block from P-36 onwards. Gradual increase in expression of C-myc, CyclinD1 and a decrease in expression of P21 was observed from P-20 onwards. CDC25A expression was significantly increased at P-27 and remained more or less constant in subsequent passages. Additionally, an increased P16 and P53 expression were seen at P-20 followed by their significant down-regulation at P-32. An increased level of phosphorylated retinoblastoma (ppRb) was observed from P-27, probably responsible for a compromised G(1)/S checkpoint. The inactivation of p21 and p16 might be due to their promoter hyper-methylation as suggested through de-methylation experiment by 5-aza-deoxycytidine at P-42. G(2)/M checkpoint abrogation was marked by gradual increase in expression of CyclinB1 and Cdc20, and a significant increase of Mad2 at P-20. Interestingly, increased expression of phospho-ATM, ATR and phospho-Chk1 were also seen at P-20 followed by their down-regulation at subsequent passages, indicating a perturbation of DNA damage response pathway at early passages. Our findings therefore dramatize the multiple genetic events that can cooperate to promote tumorigenesis.
Department of Oncogene Regulation, Chittaranjan National Cancer Institute, Kolkata, India.
Sudeshna
S
Mukherjee
Sugata
S
Manna
Debolina
D
Pal
Pratima
P
Mukherjee
Chinmay K
CK
Panda
D000782:Aneuploidy; D000818:Animals; D000963:Antimetabolites / Q000494:pharmacology; D001374:Azacitidine / Q000031:analogs & derivatives / Q000494:pharmacology; D002273:Carcinogens / Q000633:toxicity*; D002453:Cell Cycle / Q000187:drug effects* / Q000235:genetics; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D049109:Cell Proliferation / Q000187:drug effects*; D002471:Cell Transformation, Neoplastic / Q000139:chemically induced* / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D002869:Chromosome Aberrations / Q000139:chemically induced; D004249:DNA Damage; D019175:DNA Methylation / Q000187:drug effects; D000077209:Decitabine; D004622:Embryo, Mammalian / Q000187:drug effects / Q000378:metabolism; D044127:Epigenesis, Genetic / Q000187:drug effects; D005347:Fibroblasts / Q000187:drug effects* / Q000378:metabolism / Q000473:pathology; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D005838:Genotype; D005865:Gestational Age; D008748:Methylcholanthrene / Q000633:toxicity*; D051379:Mice; D010641:Phenotype; D010766:Phosphorylation; D015398:Signal Transduction
D000963:Antimetabolites; D002273:Carcinogens; D018797:Cell Cycle Proteins; D008748:Methylcholanthrene; D000077209:Decitabine; D001374:Azacitidine