The progression of epithelial tumors to invasive carcinomas involves changes in cell polarity, adhesion and motility that permit the detachment of cancer cells from the epithelial layer, their invasion into adjacent tissue layers and eventually their spread throughout the body. These processes require reorganization of the cellular cytoskeleton and altered expression of proteins that connect it to the cell membrane as well as remodelling of the extracellular matrix including changes in the composition and processing of its constituents.
The 4.1 proteins, encoded by the EPB41 (erythrocyte protein band 4.1) genes, are components of the cortical cytoskeleton underlying the cell membrane [1, 2]. The family of 4.1 proteins consists of the eponymous 4.1R protein first identified in erythrocytes (gene: EPB41), 4.1N (EPB41L1), 4.1G (EPB41L2), 4.1B (EPB41L3) as well as the less closely related members NBL4 (EPB41L4A), EHM2 (EPB41L4B) and EPB41L5 (EPB41L5). They form nodes in the cell cortex connecting further components of the cortical cytoskeleton like spectrins, actin and transmembrane adhesion proteins, receptors and transporters with each other. In this fashion 4.1 proteins contribute to the organization of cell polarity, adhesion and motility, and affect transport through the membrane and responses to growth factors.
The 4.1B protein is most strongly expressed in neurons and is enriched in the basal cells of certain epithelia [2, 3]. In addition to spectrins and actin, known binding partners comprise the adhesion molecule CD44 that binds hyaluronic acid in the extracellular matrix  and the candidate tumor suppressor disc large 1/DLG1 . The 4.1B protein is downregulated in several carcinomas, including prostate cancer , likely by deletion or promoter hypermethylation of the EPB41L3 gene promoter . Mouse models of prostate cancer progression suggest that it acts as a metastasis suppressor . In contrast, EHM2 is conspicuous in tumor cells with high migratory potential, such as metastatic melanoma and fibrosarcoma cells [8, 9]. In prostate cancer, EHM2 has been reported to be overexpressed [6, 10] and to diminish adhesion of prostate cancer cells to collagen . The most recently discovered 4.1 family member, the product of the EPB41L5 gene (also called Limulus), regulates cell adhesion during development , but has not yet been investigated in the context of human cancer.
The 4.1 proteins are part of a larger protein family characterized by FERM domains, of which many have related functions. For instance, the FERM domain protein ezrin, encoded by the VIL2 gene, connects CD81 at the cell membrane to the actin cytoskeleton. Ezrin has been shown to mediate invasion of prostate cancer cells [12, 13], but whether it is overexpressed in prostate cancer is not known. The more distantly related protein dematin, too, interacts with the actin cytoskeleton and growth factor receptors. It is encoded by the EPB49 gene on 8p21.1, a region frequently deleted in prostate cancer. Overexpression of dematin in PC3 prostate cancer cells changed their morphology towards a more epithelial phenotype , but no investigations of EPB49 expression in prostate cancer tissues have been published.
The fundamental reorganization of the cytoskeleton, its attachment to the cell membrane and the extracellular matrix during cancer progression are orchestrated by transcription factors that activate cellular programs for cell migration and invasion, which are physiologically employed during embryogenesis, wound healing and tissue regeneration [15, 16]. In cancer, such transcription factors, like Snail/SNAI1, Slug/SNAI2 and ZEB1, often become deregulated and promote tumor progression.
In prostate carcinoma, transcription factors of the ETS family are prominent candidates for oncogenes driving this facet of tumor progression. Specific members of this protein family are activated towards oncogenes by chromosomal translocations  placing a structural ETS transcription factor gene under the control of an androgen-responsive promoter, resulting in its deregulation and overexpression. The most common translocation, found in 30-70% of all cases, creates a fusion gene placing the androgen-responsive promoter of TMPRSS2 in control of the ERG structural gene encoding an ETS family transcription factor. This genetic aberration results in the androgen-driven overexpression of intact or amino-terminally truncated ERG proteins in prostate epithelial cells. ERG oncoproteins influence tumor cell proliferation, but exert a more pronounced effect on migration and invasion through broad changes in gene expression [18–20]. In accord with a function in promoting tumor progression, TMPRSS2-ERG translocations are observed in a significantly lower fraction of high-grade prostate intraepithelial neoplasias, a non-invasive precursor stage, than in invasive carcinomas .
We have previously reported downregulation of EPB41L3 encoding protein band 4.1B and upregulation of EPB41L4B encoding EHM2, respectively, in prostate cancer , in accord with observations by other groups [7, 10]. In the present study, we have investigated the expression of further members of the family as well as selected genes encoding related or interacting proteins such as disc large 1, ezrin and dematin, and the relation of the changes to activation of oncogenic ERG.