DECREASED STABILITY OF TRANSFORMING-GROWTH-FACTOR-BETA TYPE-II RECEPTOR MESSENGER-RNA IN RER-CARCINOMA CELLS( HUMAN COLON)

Transforming growth factor beta (TGF-beta) is a potent inhibitor of ce ll growth and tumor progression. Previous work has shown that loss of functional TGF-beta type II receptor (RII) due to a frameshift mutatio n in the 5' half of the RII gene leads to TGF-beta resistance in a hig hly progressed, RER+ human colon carcinoma cell line designated HCT116 . Expression of this mutated RII gene was highly regressed in RER+ cel l lines such as HCT116 and RKO, as analyzed by RNase protection assays . Nuclear Nn-on and RII promoter-reporter (CAT) assays showed that the transcriptional levels of the RII gene in these RER+ cells were not r educed, compared to RII-expressing cells. However, the half-lives of t he RII mRNA, as analyzed by RNase protection assays following actinomy cin D treatment, were significantly decreased. This suggested that the decreased expression of the RII gene mutant was due to decreased mRNA stability, Furthermore, RII mRNA from HCT116 transfected with wild-ty pe RII had a longer half-life than the endogenous mutated RII mRNA. A dominant negative RII mutant, which encodes a similarly truncated RII protein as HCT116 but lacks the extensive 3' untranslated region of RI I mRNA, gave the same half-life as endogenous wild-type RII mRNA; We c onclude that the frameshift mutation which results in a premature stop codon in the 5' half of the mRNA transcript accounts for the reduced RII mRNA levels in RER+ cells.