Transforming growth factor β (TGF-β), a potent ubiquitous endogenous inhibitor of epithelial cell growth, is secreted as a latent molecule (LTGF-β) requiring activation for function. TGF-β signals through the type I (TβRI) and type II (TβRII) receptors, which cooperate to phosphorylate/activate Smad2/3, the transcriptional regulators of genes that induce cell cycle arrest. That carcinomas grow in vivo suggests that they are refractory to TGF-β. However, this has been difficult to prove because of an inability to analyze the functional status of TGF-β in vivo as well as lack of close physiological paradigms for carcinoma cells in vitro. The current studies demonstrate that whereas primary cultures of endometrial epithelial cells derived from normal proliferative endometrium (PE; n = 10) were dose-dependently and maximally growth inhibited by 55% ± 5.3% with 10 pM TGF-β1, endometrial epithelial cells derived from endometrial carcinomas (ECAs; n = 10) were unresponsive (P ≤ 0.0066). To determine the mechanism of TGF-β resistance in ECAs, we analyzed the TGF-β signaling pathway in vivo by immunohistochemistry using specific antibodies to TβRI and TβRII, Smads, and to the phosphorylated form of Smad2 (Smad2P), an indicator of cells responding to bioactive TGF-β. Smad2P was expressed in all of the normal endometria (n = 25), and was localized to the cytoplasm and nucleus in PE, and only nuclear in the secretory endometrium. In marked contrast, Smad2P immunostaining was weak or undetectable in ECA (n = 22; P ≤ 0.001) and reduced in glandular hyperplasia (n = 25) compared with normal endometrium. However, total Smad2 and Smad7 (which inhibits Smad2 activation) levels were identical in ECA and normal tissue. Consistent with loss of downstream signaling, both TβRI (n = 19) and TβRII (n = 22) protein expression were significantly reduced In ECA compared with PE (n = 11; P ≤ 0.05). By in situ hybridization, the mRNA levels of TβRI and TβRII were decreased in the carcinoma cells compared with normal PE glands, suggesting that receptor down-regulation occurs at the transcriptional level. Furthermore, a somatic frameshift mutation in the polyadenine tract at the 5′ end of the TβR-II gene was detected in two of six cases examined. Finally, the ability of explants of ECA to activate endogenous LTGF-β was deficient compared with normal tissue (23.5% versus 7.4%). Therefore, our results suggest that loss of Smad2 signaling in ECA may be because of down-regulation of TβRI and TβRII, and/or decreased activation of LTGF-β. Because disruption of TGF-β signaling occurred independent of grade or degree of invasion and was evident in premalignant hyperplasia, we conclude that inactivation of TGF-β signaling leading to escape from normal growth control occurs at an early stage in endometrial carcinogenesis, thereby defining novel molecular targets for cancer prevention.
|Number of pages||13|
|Publication status||Published - May 15 2002|
ASJC Scopus subject areas
- Cancer Research