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It is now believed that loss of p16 is an early and often critical event in
tumor progression. Consequently, p16 is a major tumor suppressor gene whose
frequent loss occurs early in many human cancers, and by even the strictest
of measures, p16 fulfills all the criteria necessary to be labeled a bona
fide tumor suppressive gene. Despite these advances, some confusion still
surrounds p16's exact tumor suppressor role in cancer. At the genetic level,
this lack of clarity can be attributed to the disparities between human molecular
genetic data and the cancer-prone phenotype of mouse knockout models at the
INK4a/ARF locus. From a functional viewpoint, there is a true lack of understanding
of the critical signals that regulate p16 function.
Apart from cell cycle control and potentially Anoikis, p16 has also been implicated
in another fundamental cellular process, senescence. Primary epithelial cells
grown in culture will eventually stop dividing and develop a flattened morphology
referred to as senescence. The senescent phenotype is not well understood,
although characteristic features are present. One identifying feature is
the accumulation of elevated levels of p16 protein coinciding with permanent
cell cycle arrest. In addition, cells that have escaped replicative senescence
often have lost p16 function.
To identify potential upstream mediators of the p16 transcriptional response,
we have developed a senescent model based on the expression of oncogenic
ras from an inducible promoter in human oral keratinocytes immortalized with
retroviral TERT. In this system, ras expression elicits a strong p16 transcriptional
response that is easily assayed using a p16-promoter-luciferase construct.
We are now assaying a number of ras mutant constructs to identify the specific
pathways involved. Parallel studies based on ras expressed from a replication-defective
adenovirus in primary oral keratinocytes are also in progress.
Identification of p16 upstream regulators
Principal Investigator: James W. Rocco M.D., Ph.D.
Group Members: William Michaud, MD
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