Authors
Lee AC. Fenster BE. Ito H. Takeda K. Bae NS. Hirai T. Yu ZX. Ferrans
VJ. Howard BH. Finkel T.
Institution
Cardiology Branch, National Institutes of Health, Bethesda, Maryland 20892,
USA.
Title
Ras proteins induce
senescence by altering the intracellular levels of reactive oxygen species.
Source
Journal of Biological Chemistry. 274(12):7936-40, 1999 Mar 19.
Abstract
Human diploid fibroblasts eventually lose the capacity to replicate in
culture and enter a viable but nonproliferative state of senescence.
Recently, it has been demonstrated that retroviral-mediated gene transfer
into primary fibroblasts of an activated ras gene
(V12ras) rapidly accelerates development of the senescent
phenotype. Using this in vitro system, we have sought to define the mediators
of Ras-induced senescence. We demonstrate
that expression of V12Ras results in an increase in
intracellular and in particular, mitochondrial reactive oxygen species. The
ability of V12Ras to induce growth arrest
and senescence is shown to be partially inhibited by coexpression of an
activated rac1 gene. A more dramatic rescue of
V12Ras-expressing cells is demonstrated when the cells are
placed in a low oxygen environment, a condition in which reactive oxygen
species production is inhibited. In addition, in a 1% oxygen environment,
Ras is unable to trigger an increase in the level of the
cyclin-dependent kinase inhibitor p21 or to activate the senescent program.
Under normoxic (20% O2) conditions, the V12Ras senescent
phenotype is demonstrated to be unaffected by scavengers of superoxide but
rescued by scavengers of hydrogen peroxide. These results suggest that in
normal diploid cells, Ras proteins regulate
oxidant production and that a rise in intracellular H2O2 represents a
critical signal mediating replicative senescence.