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Mol. Cell. Biol., Jan 1998, 378-387, Vol 18, No. 1
GI Shapiro, CD Edwards, ME Ewen and BJ Rollins
Members of the INK4 protein family specifically inhibit cyclin- dependent
kinase 4 (cdk4) and cdk6-mediated phosphorylation of the retinoblastoma
susceptibility gene product (Rb). p16INK4A, a prototypic INK4 protein, has
been identified as a tumor suppressor in many human cancers. Inactivation
of p16INK4A in tumors expressing wild-type Rb is thought to be required in
order for many malignant cell types to enter S phase efficiently or to
escape senescence. Here, we demonstrate another mechanism of tumor
suppression by implicating p16INK4A in a G1 arrest checkpoint in response
to DNA damage. Calu-1 non-small cell lung cancer cells, which retain Rb and
lack p53, do not arrest in G1 following DNA damage. However, engineered
expression of p16INK4A at levels compatible with cell proliferation
restores a G1 arrest checkpoint in response to treatment with
gamma-irradiation, topoisomerase I and II inhibitors, and cisplatin. A
similar checkpoint can be demonstrated in p53-/- fibroblasts that express
p16INK4A. DNA damage-induced G1 arrest, which requires the expression of
pocket proteins such as Rb, can be abrogated by overexpression of cdk4,
kinase- inactive cdk4 variants capable of sequestering p16INK4A, or a cdk4
variant incapable of binding p16INK4A. After exposure to DNA-damaging
agents, there was no change either in overall levels of p16INK4A or in
amounts of p16INK4A found in complex with cdks 4 and 6. Nonetheless,
p16INK4A expression is required for the reduction in cdk4- and cdk6-
mediated Rb kinase activity observed in response to DNA damage. During
tumor progression, loss of p16INK4A expression may be necessary for cells
with wild-type Rb to bypass this G1 arrest checkpoint and attain a fully
transformed phenotype.
Copyright © 1998, American Society for Microbiology
p16INK4A participates in a G1 arrest checkpoint in response to DNA damage
Department of Adult Oncology, Dana-Farber Cancer Institute, Harvard Medical School, Boston, Massachusetts 02115, USA.
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