Re-expression of endogenous p16ink4a in oral squamous cell carcinoma lines by 5-aza-2′-deoxycytidine treatment induces a senescence-like state

Abstract

We have previously reported that a set of oral squamous cell carcinoma lines express speci®cally elevated cdk6 activity. One of the cell lines, SCC4, contains a cdk6 ampli®cation and expresses functional p16 ink4a , the other cell lines express undetectable levels of p16 ink4a , despite a lack of coding-region mutations. Two of the cell lines, SCC15 and SCC40 have a hypermethylated p16 ink4A promoter and a third cell line, SCC9, has a mutation in the p16 ink4a promoter. Using the demethylation agent 5-aza-2'-deoxycytidine, we showed that the p16 ink4a protein was re-expressed after a 5-day treatment with this chemical. One cell line, SCC15 expressed high levels of p16 ink4a . In this line, cdk6 activity was decreased after 5-aza-2'deoxycytidine treatment, and the hypophosphorylated, growth suppressive form of the retinoblastoma tumor suppressor protein pRB was detected. Expression of p16 ink4a persisted, even after the drug was removed and the cells expressed senescence-associated b-galactosidase activity. Ectopic expression of p16 ink4a with a recombinant retrovirus in this cell line also induced a similar senescence-like phenotype. Hence, it was possible to restore a functional pRB pathway in an oral squamous cell carcinoma line by inducing re-expression of endogenous p16 ink4a in response to treatment with a demethylating agent.

CDK6 blocks differentiation: coupling cell proliferation to the block to differentiation in leukemic cells

“…CDK6 is the earliest CDK to be induced during human T-cell activation and high levels of CDK6 expression were found in 100% (N ¼ 25) of cases of T-cell lymphoblastic lymphoma/leukemia (T-LBL/ALL) (Chilosi et al, 1998). Elevated levels of CDK6 also were reported in human squamous cell carcinomas (Timmermann et al, 1997) and in human neuroblastomas (Easton et al, 1998) and gliomas (Costello et al, 1997). It will be important to investigate the relative roles of CDK6 and CDK4 in driving proliferation of other types of malignant cells, as well as in the corresponding normal cells at various stages of differentiation.…”

Cytokine expression and signaling in drug-induced cellular senescence

“…Chronic subtoxic doses of stress-inducing compounds such as ethanol or H 2 O 2 can cause stress-induced senescence (Toussaint et al, 2002). Drug-induced senescence can be promoted by a variety of chemically and functionally unrelated DNA-damaging anticancer agents, such as doxorubicin (Chang et al, 1999), camptothecin (Han et al, 2002), 5-aza-2 0 -deoxycytidine (Timmermann et al, 1998;Kulaeva et al, 2003), aphidicolin (APH) and hydroxyurea (HU) (Yogev et al, 2006), or halogenated nucleotide analogs such as 5-bromo-2 0 -deoxyuridine (BrdU) (Michishita et al, 1999;Minagawa et al, 2005). Despite the fact that oncogenic or stress stimuli do not promote telomere shortening, prematurely senescent cells share similar characteristics with cells undergoing replicative senescence.…”

Cooperation between p53 and p130(Rb2) in induction of cellular senescence

“…Accordingly, ectopic p16INK4a is a potent inducer of cellular senescence, provided pRb is present. [5][6][7][8] However, ectopic p53 activated cellular senescence in H1299 lung tumor cells despite the absence of p16INK4a. 2 It suggests that the inhibitor is dispensible for p53-mediated cellular senescence.…”