Cm. Coopersmith et al., BI-TRANSGENIC MICE REVEAL THAT K-RAS(VAL12) AUGMENTS A P53-INDEPENDENT APOPTOSIS WHEN SMALL-INTESTINAL VILLUS ENTEROCYTES REENTER THE CELL-CYCLE, The Journal of cell biology, 138(1), 1997, pp. 167-179
Studies in cell culture systems have indicated that oncogenic forms of
Ras can affect apoptosis. Activating mutations of Ras occur in simila
r to 30% of all human tumors and 50% of colorectal carcinomas, Since t
hese mutations appear at early or intermediate stages in multistep jou
rneys to neoplasia, an effect on apoptosis may help determine whether
initiated cells progress towards a more neoplastic state, We have test
ed the effects of K-ras(Val12) on apoptosis in transgenic mice, A line
age-specific promoter was used to direct expression of human K-ras(Val
12), with or without wild-type (wt) or mutant SV-40 T antigens (TAg),
in postmitotic villus enterocytes, the principal cell type of the smal
l intestinal epithelium, Enterocytes can be induced to reenter the cel
l cycle by TAgWt. Reentry is dependent upon the ability of TAg to bind
pRB and is associated with a p53-independent apoptosis. Analyses of K
-ras(Val12) x TAgWt bi-transgenic animals indicated that K-ras(Val12)
can enhance this apoptosis threefold but only in cycling cells; increa
sed apoptosis does not occur when K-ras(Val12) is expressed alone or w
ith a TAg containing Glu(107,108) --> Lys(107,108) substitutions that
block its ability to bind pRB. Analysis of bi-transgenic K-ras(Val12)
x TAgWt mice homozygous for wild-type or null p53 alleles established
that the enhancement of apoptosis occurs through a p53-independent mec
hanism, is not attributable to augmented proliferation or to an increa
se in abortive cell cycle reentry (compared to TAgWt mice), and is not
associated with detectable changes in the crypt-villus patterns of ex
pression of apoptotic regulators (Bcl-2, Bcl-x(L), Bak, and Bar) or me
diators of epithelial cell-matrix interactions and survival (e.g., alp
ha(5) beta(1) integrin and its ligand, fibronectin), Coexpression of K
-ras(Val12) and TAgWt produces dysplasia. The K-ras(Val12)-augmented a
poptosis is unrelated to this dysplasia; enhanced apoptosis is also ob
served in cycling nondysplastic enterocytes that produce K-ras(Val12)
and a TAg with a COOH-terminal truncation The dysplastic epithelium of
K-ras(Val12) x TAgWt mice does not develop neoplasms. Our results are
consistent with this finding: (a) When expressed in initiated enteroc
ytes with a proliferative abnormality, K-ras(Val12) facilitates progre
ssion to a dysplastic phenotype; (b) by diminishing cell survival on t
he villus, the oncoprotein may impede further progression; and (c) add
itional mutations may be needed to suppress this proapoptotic response
to K-ras(Val12).