Critical Roles of PIWIL1 in Human Tumors: Expression, Functions, Mechanisms, and Potential Clinical Implications

P-element-induced wimpy testis (PIWI)-interacting RNAs (piRNAs) are a class of small non-coding RNA molecules that are 24-31 nucleotides in length. PiRNAs are thought to bind to PIWI proteins (PIWL1-4, a subfamily of Argonaute proteins), forming piRNA/PIWI complexes that influence gene expression at the transcriptional or post-transcriptional levels. However, it has been recently reported that the interaction of PIWI proteins with piRNAs does not encompass the entire function of PIWI proteins in human tumor cells. PIWIL1 (also called HIWI) is specifically expressed in the testis but not in other normal tissues. In tumor tissues, PIWIL1 is frequently overexpressed in tumor tissues compared with normal tissues. Its high expression is closely correlated with adverse clinicopathological features and shorter patient survival. Upregulation of PIWIL1 drastically induces tumor cell proliferation, epithelial-mesenchymal transition (EMT), invasion, cancer stem-like properties, tumorigenesis, metastasis and chemoresistance, probably via piRNA-independent mechanisms. In this article, we summarize the current existing literature on PIWIL1 in human tumors, including its expression, biological functions and regulatory mechanisms, providing new insights into how the dysregulation of PIWIL1 contributes to tumor initiation, development and chemoresistance through diverse signaling pathways. We also discuss the most recent findings on the potential clinical applications of PIWIL1 in cancer diagnosis and treatment.

Manipulations in HIWI level exerts influence on the proliferation of human non-small cell lung cancer cells

Lung cancer is the leading cause of cancer-associated mortality worldwide, although molecular imaging techniques, including fludeoxyglucose positron emission tomography, have markedly improved the diagnosis of lung cancer. HIWI is a member of the human piwi family, members of which are known for their roles in RNA silencing. HIWI has been shown to serve a crucial function in stem cell self-renewal, and previous studies have reported HIWI overexpression in lung cancers. Furthermore, HIWI has been proposed to regulate the maintenance of cancer stem cell populations in lung cancers. The present study investigated the mRNA and protein expression levels of HIWI in non-small cell lung cancer (NSCLC) specimens harvested from 57 patients, using reverse transcription-quantitative polymerase chain reaction and western blot analysis, respectively. Subsequently, the HIWI expression level was manipulated using gain-of-function and loss-of-function strategies, and the role of HIWI in the proliferation of human A549 NSCLC cells was investigated using Cell Counting Kit-8 and colony formation assays. The mRNA and protein expression levels of HIWI were significantly upregulated in the intratumor NSCLC specimens, as compared with the peritumor specimens. Furthermore, the mRNA and protein expression levels of HIWI in A549 cells were successfully manipulated using the two strategies. Overexpression and knockout of HIWI were associated with the promotion and inhibition of A549 cell proliferation, respectively. The results of the present study suggested that HIWI is overexpressed in NSCLC tissues and demonstrated that upregulation of HIWI may promote the growth of lung cancer cells; thus suggesting that HIWI may have an oncogenic role in lung cancer.

Correlation of HIWI and HILI Expression with Cancer Stem Cell Markers in Colorectal Cancer

BACKGROUND/AIM

Cancer stem cells (CSCs) constitute a sub-population of tumor cells that possess stem cell properties, such as self-renewal and the ability of differentiation. The presence of CSCs is associated with metastatic potential, treatment resistance and poor patient prognosis. Recently, aberrant expression of P-element induced wimpy testis proteins-PIWI (HIWI and HILI) has been identified in various types of tumors. The aim of the study was to evaluate the clinical significance of the HIWI and HILI expression and its relationship with cancer stem cells markers in 72 patients with colorectal carcinoma (CRC).

MATERIALS AND METHODS

The expression level of HIWI and HILI and cancer stem cells markers in paired cancerous and non-cancerous tissues was measured by real-time reverse transcription-polymerase chain reaction (RT-PCR) assay. Immunohistochemistry was performed to confirm the observed changes on mRNA level and detect tissue localization of PIWI proteins.

RESULTS

Significantly higher mRNA levels of HIWI and decreased HILI mRNA were measured in colorectal cancer tissues compared to corresponding non-cancerous samples. The changes in HIWI mRNA level in cancer tissues were correlated with OCT4 expression. Positive correlations between HILI level and SOX2 were also observed in cancerous tissues.

CONCLUSION

Our results indicate a reciprocal regulation between HIWI, HILI and some CSCs markers in colorectal cancer.