Tumor suppressor p53 down-regulates expression of human leukocyte marker CD43 in non-hematopoietic tumor cells.

CD43 (leukosialin, sialophorin), a cell surface protein on most hematopoietic cells, is an important regulator of immune cell function and is involved in regulation of cell adhesion and proliferation. Aberrant expression of CD43 is a common event observed in human tumors of non-hematopoietic origin suggesting a role in tumor development. We have previously shown that overexpression of CD43 causes activation of the ARF-p53 tumor-suppressor pathway and results in cell death. In a non-functional ARF-p53 background, the cells overexpressing CD43 display an increased cell growth rate due to higher survival. Here we show that p53 specifically downregulates the expression of CD43 at the protein and mRNA level. Transactivating properties of p53 are necessary to affect the expression of exogenous CD43. The downregulation of CD43 mRNA is caused by p53-dependent transrepression, at least in part, via a histone deacetylation mechanism. These studies establish that under certain conditions there exists a negative feedback loop between p53 and CD43: CD43-dependent signaling activates p53, which in turn downregulates the expression of CD43.

Leukocyte marker CD43 promotes cell growth in co-operation with ß-catenin in non-hematopoietic cancer cells.

The Wnt/ß-catenin pathway regulates key cellular processes such as differentiation, proliferation, apoptosis; and its activation promotes development of several cancer types. Expression of CD43 (leukosialin), the predominant leukocyte transmembrane sialoglycoprotein, has been detected in many tumors of non-hematopoietic origin. CD43 participates in cell adhesion and regulates intracellular signal transduction pathways involved in cell proliferation and survival. The cytoplasmic domain of CD43 has been reported to translocate to the nucleus, interact with ß-catenin and affect its target gene expression, but the impact of this action on cell fate is still unknown. We demonstrate, here, by colony formation assay and siRNA-mediated gene silencing that CD43 and ß-catenin co-operate in promoting cell growth. Moreover, in cells with down-regulated ß-catenin expression the activation of p53 in response to CD43 overexpression is significantly impaired. In addition, the presence of both CD43 and ß-catenin is required for the TCF/LEF-mediated transcription. Presumably, the full-length CD43 participates in this transcriptional regulation. We show that the mature CD43 localizes to the nucleus, where it binds chromatin, co-localizes and co-immunoprecipitates with ß-catenin, and enhances the reporter gene expression regulated by ß-catenin. These observations provide clear evidence linking CD43 to the Wnt/APC/ß-catenin signaling pathway and supporting our hypothesis according to which CD43 plays a role in tumor development.