Proliferation but Not Apoptosis Is Associated with Distinct β-Catenin Expression Patterns in Non-Small-Cell Lung Carcinomas

摘要

β-catenin (β-cat) is a versatile component of homotypic cell adhesion and signaling. Its subcellular localization and cytoplasmic levels are tightly regulated by the adenomatous polyposis coli (APC) protein. Mutations in >β-cat (exon 3) or >APC (MCR) result in β-cat aberrant overexpression that is associated with its nuclear accumulation and improper gene activation. Data from experimental models have shown that β-cat overexpression has a multitude of effects on cell-cycle behavior. In many of these aspects its function depends on major G1 phase regulators. To the best of our knowledge, most of these issues have never been addressed concurrently in tumors. For this reason we investigated in a panel of 92 non-small-cell lung carcinomas, β-cat and APC expression, and their relationship with cell-cycle kinetics (PI and AI) and ploidy status. Moreover, the above correlations were examined in relation to the main G1/S-phase checkpoint regulators. Four β-cat immunohistochemical expression patterns [membranous (11.1%), membranous-cytoplasmic (54.3%), cytoplasmic (9.9%), cytoplasmic-nuclear (24.7%)] and three APC immunohistochemical expression patterns [cytoplasmic (37.7%), cytoplasmic-nuclear (58%), nuclear (4.3%)] were observed, which were further confirmed by Western blot analysis on subcellular fractions in representative samples. The frequent presence of β-cat in the cytoplasm is an indication of aberrant expression, whereas membranous and nuclear localization were inversely related. Absence of mutations in β-cat (exon 3) and APC (MCR) suggest that β-cat destruction mechanisms may be functional. However, expression analysis revealed attenuated levels for APC, indicating a residual ability to degrade β-cat. Decreased levels were associated with loss of heterozygosity at the >APC region in 24% of the cases suggesting that additional silencing mechanisms may be involved. Interestingly, the 90-kd APC isoform associated with apoptosis, was found to be the predominant isoform in normal and cancerous lung tissues. The most important finding in our study, was the correlation of nuclear β-cat immunohistochemical localization with increased proliferation, overexpression of E2F1 and MDM2, aberrant p53, and low expression of p27^KIP, providing for the first time >in vivo evidence that β-cat-associated proliferation correlates with release of E2F1 activity and loss of p53- and p27^KIP-dependent cell-cycle checkpoints. Loss of these checkpoints is accompanied by low levels of APC, which possibly reflects a diminished ability to degrade β-cat. Taken together our data indicate that cases with nuclear β-cat immunohistochemical expression represent a subset of non-small-cell lung carcinomas that have gained an increased proliferation advantage in contrast to the other β-cat immunohistochemical expression profiles.