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It is well established that loss of a growth inhibitory response to transforming growth factor-beta (TGF-beta) is a common feature of epithelial cancers including esophageal cancer. However, the molecular basis for the abrogation of this key homeostatic mechanism is poorly understood. In esophageal cancer cell lines that are resistant to TGF-beta-induced growth inhibition, TGF-beta also fails to decrease transcription of c-myc despite the presence of functional signaling components. Consequently, to gain a better understanding of the mechanisms leading to resistance to TGF-beta-induced growth arrest, the basis for the inability to decrease c-myc transcription was investigated. Regardless of sensitivity to TGF-beta-induced growth arrest, TGF-beta enhanced the ability of Smad3-protein complexes to bind c-myc regulatory elements. However, in a growth inhibition-resistant esophageal cancer cell line, the Smad3-protein complexes contained the SnoN oncoprotein. Furthermore, in esophageal cancer cell lines that are resistant to TGF-beta-induced growth arrest, TGF-beta does not cause degradation of SnoN. Analyses of the effect of modulating SnoN expression in both growth inhibition-sensitive and growth inhibition-resistant cell lines showed that degradation of SnoN is a prerequisite for both TGF-beta-induced repression of c-myc transcription and growth arrest. The data indicate that SnoN-Smad3 complexes do not cause repression of c-myc transcription but rather prevent functionality of active repressor complexes. Thus, these studies reveal a novel mechanism for resistance to TGF-beta-induced growth inhibition in esophageal cancer, namely the failure to degrade SnoN. In addition, they show that SnoN can block TGF-beta repression of gene transcription.

The role of the transforming growth factor beta (TGFbeta) pathway in the development and progression of esophageal cancers is poorly understood. As an initial step in clarification of this issue, the functional status of the TGFbeta pathway was evaluated in a panel of esophageal cancer derived cell lines. Both adenocarcinoma and squamous cell carcinoma derived lines were represented. Although the TGFbeta pathway was intact and functional in four of the five cell lines, only one of them was growth inhibited by TGFbeta. In one cell line, the loss of a growth inhibitory response to TGFbeta could be explained by decreased expression of Smad 4 and a general inability to activate TGFbeta responsive promoters. In the other three cell lines, TGFbeta was able to activate transcription of TGFbeta responsive promoters, but unable to downregulate transcription of c-myc. Taken together these findings indicate that a selective loss in the ability of TGFbeta to regulate expression of a key component of the growth inhibitory pathway may contribute to the poor prognosis of esophageal cancers.