The expression profiles of piRNAs and their interacting Piwi proteins in cellular model of renal development: Focus on Piwil1 in mitosis

Piwi proteins and Piwi interacting RNAs, piRNAs, presented in germline cells play a role in transposon silencing during germline development. In contrast, the role of somatic Piwi proteins and piRNAs still remains obscure. Here, we characterize the expression pattern and distribution of piRNAs in human renal cells in terms of their potential role in kidney development. Further, we show that all PIWI genes are expressed at the RNA level, however, only PIWIL1 gene is detected at the protein level by western blotting in healthy and cancerous renal cells. So far, the expression of human Piwil1 protein has only been shown in testes and cancer cells, but not in healthy somatic cell lines. Since we observe only Piwil1 protein, the regulation of other PIWI genes is probably more intricated, and depends on environmental conditions. Next, we demonstrate that downregulation of Piwil1 protein results in a decrease in the rate of cell proliferation, while no change in the level of apoptotic cells is observed. Confocal microscopy analysis reveals that Piwil1 protein is located in both cellular compartments, cytoplasm and nucleus in renal cells. Interestingly, in nucleus region Piwil1 is observed close to the spindle during all phases of mitosis in all tested cell lines. It strongly indicates that Piwil1 protein plays an essential role in proliferation of somatic cells. Moreover, involvement of Piwil1 in cell division could, at least partly, explain invasion and metastasis of many types of cancer cells with upregulation of PIWIL1 gene expression. It also makes Piwil1 protein as a potential target in the anticancer therapy.