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Cancer is a histologically and genetically heterogeneous population of tumor cells that exhibits distinct molecular profiles determined by epigenetic alterations. P-element-induced wimpy testis (PIWI) proteins in complex with PIWI-interacting RNA (piRNA) have been previously demonstrated to be involved in epigenetic regulation in germline cells. Recently, reactivation of PIWI expression, primarily PIWI-like protein 1 and 2, through aberrant DNA methylation resulting in genomic silencing has been identified in various types of tumors. It has been suggested that the PIWI-piRNA complex contributes to cancer development and progression by promoting a stem-like state of cancer cells, or cancer stem cells (CSCs). It has been identified that CSCs represent the cells that have undergone epithelial-mesenchymal transition (EMT) and acquired metastatic capacities. However, the molecular association between the EMT process and the stem-cell state remains unclear. Further extensive characterization of CSCs in individual types of tumors is required to identify specific markers for the heterogeneous population of CSCs and therefore selectively target CSCs. Previous studies indicate a reciprocal regulation between PIWI proteins and a complex signaling network linking markers characterized for CSCs and transcription factors involved in EMT. In the present review, studies of PIWI function are summarized, and the potential involvement of PIWI proteins in cancer development and progression is discussed.

Background/aim P-Element-induced wimpy testis (PIWI) proteins in complex with PIWI-interacting RNA (piRNA) are involved in epigenetic regulation of gene expression in germline cells. Aberrant expression of piRNA and PIWI proteins have been identified in various types of tumour cells. The aim of this study was to evaluate the expression profiles of PIWI-like protein-1, -2 (PIWIL1 and PIWIL2), their immunohistochemical (IHC) characteristics in ductal breast cancer, and determine their correlation with clinicopathological parameters of this type of cancer. Materials and methods Material for IHC studies comprised of 101 invasive ductal carcinoma (IDC) cases and 31 mastopathy tissues. Frozen fragments of paired tissue specimens (tumour and adjacent non-malignant breast tissue) taken from 55 patients with IDC and 18 samples of mastopathy were used for molecular studies using real-time polymerase chain reaction (RT-PCR). Results A statistically significantly higher level of PIWIL1 and PIWIL2 was found in IDC compared to mastopathy samples (p≤0.0001). Increased expression of PIWIL1 was correlated with increased PIWIL2 expression in breast cancer tissue. Surprisingly, PIWIL1 mRNA was detected only in cancer and mastopathy, but was not found in most normal breast tissues, although it is noteworthy that the PIWIL2 mRNA level was statistically significantly lower in mastopathy and IDC samples compared to normal breast tissues. Conclusion Our results affirm the hypothesis that reactivation of PIWI expression in various caner types is crucial for cancer development.

Cell migration is a multistep process initiated by extracellular matrix components that leads to cytoskeletal changes and formation of different protrusive structures at the cell periphery. Lumican, a small extracellular matrix leucine-rich proteoglycan, has been shown to inhibit human melanoma cell migration by binding to α2β1 integrin and affecting actin cytoskeleton organization. The aim of this study was to determine the effect of lumican overexpression on the migration ability of human colon adenocarcinoma LS180 cells. The cells stably transfected with plasmid containing lumican cDNA were characterized by the increased chemotactic migration measured on Transwell filters. Lumican-overexpressing cells presented the elevated filamentous to monomeric actin ratio and gelsolin up-regulation. This was accompanied by a distinct cytoskeletal actin rearrangement and gelsolin subcellular relocation, as observed under laser scaning confocal microscope. Moreover, LS180 cells overexpressing lumican tend to form podosome-like structures as indicated by vinculin redistribution and its colocalization with gelsolin and actin at the submembrane region of the cells. In conclusion, the elevated level of lumican secretion to extracellular space leads to actin cytoskeletal remodeling followed by an increase in migration capacity of human colon LS180 cells. These data suggest that lumican expression and its presence in ECM has an impact on colon cancer cells motility and may modulate invasiveness of colon cancer.