CD9 expression and its putative role in prostate cancer progression

The mechanisms and associated genes implicated in prostate tumorigenesis and cancer progression are still not fully known. Thus, we characterized the expression profile and the role of an attractive candidate, the motility-related protein-1/cluster-of-differentiation antigen 9 (MRP1/CD9) gene (abbreviated as CD9).We first performed immunohistochemistry studies in a panel of prostate cancer tissues including 107 localized, 60 advanced prostate cancers, 25 bone tissues and 65 lymph nodes with prostate cancer metastasis. CD9 protein, whose expression is inversely correlated with many clinicopathological parameters such as PSA level, differentiation, stage and metastasis status, is inactivated during prostate cancer progression and metastasis.We then cloned and sequenced CD9 cDNA from prostate cancer cell lines and tissues. This analysis revealed the presence of three major deletions and four commonly occurring point mutations in prostate cancer. All of these genetic aberrations affect the large extracellular and intracellular domains of CD9 protein. We also showed that the overexpression of wild type CD9 protein gives rise to phenotypic changes and loss of replicative potential in an advanced prostate cancer cell line (PC-3). These dying cells demonstrated remarkably enlarged cell surface and prominent cellular protrusions. By carefully defining the time course of these dying cells, we found one third of CD9 overexpressed PC-3 clones died after one and a half months on average after a limited number of cell divisions. In contrast, the other two third of CD9-transfected clones did not die by virtue of loss of exogenous CD9 expression. Furthermore, overexpression of mutated forms of the CD9 protein with one of the major deletions or two of the point mutations did not result in cell mortality and profound morphological changes.Finally, we compared the RNA expression profile of dying and growing clones using cDNA microarray. Among the differentially expressed genes, the most interesting candidate is MARK1, which is under-expressed by a factor of five in dying clones as compared to growing clones. MAP/microtubule Affinity-Regulating Kinase 1, abbreviated as MARK1, is a member of a family of serine/threonine protein kinases and is able to phosphorylate microtubule-associated proteins (MAPs). The published phenotypic changes reported from overexpression of MARK1 in CHO cells are very similar to what we observed for CD9-overexpressed PC-3 dying cells. Further analysis to confirm the linkage of CD9 protein to cytoskeleton proteins such as MARK1 is compulsory.These data underline the significance of CD9 expression in prostate cancer progression, uncover major mutations in prostate cancer, and for the first time argue for the connection between CD9 and cytoskeleton-associated proteins. Further studies to confirm the role of CD9 in vivo and to find out the pathway(s) involved in loss of replicative potential caused by CD9 overexpression will definitely improve our understanding of the cell biology of prostate cancer progression