Expression profiles of PIWIL2 short isoforms differ in testicular germ cell tumors of various differentiation subtypes

Selective degradation of PL2L60 by metabolic stresses‑induced autophagy suppresses multi‑cancer growth

It has been reported that PL2L60 proteins, a product of PIWIL2 gene which might be activated by an intragenic promoter, could mediate a common pathway specifically for tumorigenesis. In the present study, it was further identified by using western blot assay that the PL2L60 proteins could be degraded in cancer cells through a mechanism of selective autophagy in response to oxidative stress. The PL2L60 was downregulated in various types of cancer cells under the hypoxic condition independently of HIF‑1α, resulting in apoptosis of cancer cells. Inhibition of autophagy by small interfering RNA targeting of either Beclin‑1 (BECN1) or Atg5 resulted in restoration of PL2L60 expression in hypoxic cancer cell. The hypoxic degradation of PL2L60 was also blocked by the attenuation of the autophagosome membrane protein Atg8/microtubule‑associated protein 1 light chain 3 (LC3) or autophagy cargo protein p62 expression. Surprisingly, Immunofluorescence analysis demonstrated that LC3 could be directly bound to PL2L60 and was required for the transport of PL2L60 from the nucleus to the cytoplasm for lysosomal flux under basal or activated autophagy in cancer cells. Moreover, flow cytometric analysis displayed that knocking down of PL2L60 mRNA but not PIWIL2 mRNA effectively inhibited cancer cell proliferation and promoted apoptosis of cancer cells. The similar results were obtained from in vivo tumorigenic experiment, in which PL2L60 downregulation in necroptosis areas was confirmed by immunohistochemistry. These results suggested that various cancer could be suppressed by promoting autophagy. The present study revealed a key role of autophagic degradation of PL2L60 in hypoxia‑induced cancer cell death, which could be used as a novel therapeutic target of cancer.

piRNAs and PIWI Proteins as Diagnostic and Prognostic Markers of Genitourinary Cancers

piRNAs (PIWI-interacting RNAs) are small non-coding RNAs capable of regulation of transposon and gene expression. piRNAs utilise multiple mechanisms to affect gene expression, which makes them potentially more powerful regulators than microRNAs. The mechanisms by which piRNAs regulate transposon and gene expression include DNA methylation, histone modifications, and mRNA degradation. Genitourinary cancers (GC) are a large group of neoplasms that differ by their incidence, clinical course, biology, and prognosis for patients. Regardless of the GC type, metastatic disease remains a key therapeutic challenge, largely affecting patients’ survival rates. Recent studies indicate that piRNAs could serve as potentially useful biomarkers allowing for early cancer detection and therapeutic interventions at the stage of non-advanced tumour, improving patient’s outcomes. Furthermore, studies in prostate cancer show that piRNAs contribute to cancer progression by affecting key oncogenic pathways such as PI3K/AKT. Here, we discuss recent findings on biogenesis, mechanisms of action and the role of piRNAs and the associated PIWI proteins in GC. We also present tools that may be useful for studies on the functioning of piRNAs in cancers.

A novel tumor-specific broad-spectral monoclonal antibody to PL2L60 is highly effective for the treatment of various types of cancers from human and mouse

There are numerous antibodies used for cancer therapy in clinic, but they are essentially less efficacy than expected. None of them has tumor-specific and broad-spectral properties. PIWIL2-like (PL2L) protein 60 (PL2L60) is a product of alienated activation of PIWIL2 gene, and has been found to be specifically and widely expressed in various types of cancers, including hematopoietic and solid ones. Current study aims to investigate whether a monoclonal antibody (mAb) to PL2L60 has both tumor-specific and broad-spectral properties, which can be used universally to treat various types of cancers. The expression of PL2L60 protein in the cell surface and cytoplasm were determined in a panel of human and mouse tumor cell lines by flow cytometry, immunofluorescent microscopy and Western Blotting. The apoptosis and the cell cycle arrest of the tumor cells treated with mAb KAO3 were evaluated by flow cytometry. The tumorigenesis of the mAb KAO3-pretreated tumor cells was determined by tumor incidence and tumor size, and the efficacy of mAb KAO3 treatment on tumor growth in tumors-bearing mice were kinetically evaluated. Complement-dependent cytotoxicity (CDC) assay was used to determine the capacity of mAb KAO3 to kill tumor cells. Treatment of human or mouse tumor cells from hematopoietic or solid tumors with mAb KAO3 at the time of inoculation efficiently inhibited tumorigenesis in the severe combined immunodeficient (SCID) mice. Moreover, injection of mAb KAO3 into established tumors significantly inhibited their growth, and prolonged survival of the tumor-bearing mice, including lymphoma, breast cancer, lung cancer and cervical cancer. The efficacy of mAb KAO3 treatment is likely associated with its binding to PL2L60 expressed on tumor cell surface, which may lead to cancer cell death through blocking cell cycling and/or activation of complement. In conclusion, we have identified a tumor-specific mAb to PL2L60 (KAO3), which may be used potentially to treat all the types of human cancers including from both hematopoietic and solid ones.

Non-Coding RNAs and Splicing Activity in Testicular Germ Cell Tumors

Testicular germ cell tumors (TGCTs) are the most common tumors in adolescent and young men. Recently, genome-wide studies have made it possible to progress in understanding the molecular mechanisms underlying the development of tumors. It is becoming increasingly clear that aberrant regulation of RNA metabolism can drive tumorigenesis and influence chemotherapeutic response. Notably, the expression of non-coding RNAs as well as specific splice variants is deeply deregulated in human cancers. Since these cancer-related RNA species are considered promising diagnostic, prognostic and therapeutic targets, understanding their function in cancer development is becoming a major challenge. Here, we summarize how the different expression of RNA species repertoire, including non-coding RNAs and protein-coding splicing variants, impacts on TGCTs’ onset and progression and sustains therapeutic resistance. Finally, the role of transcription-associated R-loop misregulation in the maintenance of genomic stability in TGCTs is also discussed.

Human-specific subcellular compartmentalization of P-element induced wimpy testis-like (PIWIL) granules during germ cell development and spermatogenesis

STUDY QUESTION

What is the dynamics of expression of P-element induced wimpy testis-like (PIWIL) proteins in the germline during human fetal development and spermatogenesis?

SUMMARY ANSWER

PIWIL1, PIWIL2, PIWIL3 and PIWIL4 were expressed in a sex-specific fashion in human germ cells (GC) during development and adulthood. PIWILs showed a mutually exclusive pattern of subcellular localization. PIWILs were present in the intermitochondrial cement and a single large granule in meiotic GC and their expression was different from that observed in mice, highlighting species-differences.

WHAT IS KNOWN ALREADY

In mice, PIWIL proteins play prominent roles in male infertility. PIWIL mouse mutants show either post-meiotic arrest at the round spermatid stage (PIWIL1) or arrest at the zygotene-pachytene stage of meiosis I (PIWIL2 and PIWIL4) in males, while females remain fertile. Recent studies have reported a robust piRNA pool in human fetal ovary.

STUDY DESIGN, SIZE, DURATION

This is a qualitative analysis of PIWILs expression in paraffin-embedded fetal human male (N = 8), female gonads (N = 6) and adult testes (N = 5), and bioinformatics analysis of online available single-cell transcriptomics data of human fetal germ cells (n = 242).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Human fetal gonads from elective abortion without medical indication and adult testes biopsies were donated for research with informed consent. Samples were fixed, paraffin-embedded and analyzed by immunofluorescence to study the temporal and cellular localization of PIWIL1, PIWIL2, PIWIL3 and PIWIL4.

MAIN RESULTS AND THE ROLE OF CHANCE

PIWIL1, PIWIL2 and PIWIL4 showed a mutually exclusive pattern of subcellular localization, particularly in female oocytes. To our surprise, PIWIL1 immunostaining revealed the presence of a single dense paranuclear body, resembling the chromatoid body of haploid spermatocytes, in meiotic oocytes. Moreover, in contrast to mice, PIWIL4, but not PIWIL2, localized to the intermitochondrial cement. PIWIL3 was not expressed in GC during development. The upregulation of PIWIL transcripts correlated with the transcription of markers associated with piRNAs biogenesis like the TDRDs and HENMT1 in fetal GC.

LARGE SCALE DATA

Non-applicable.

LIMITATIONS, REASONS FOR CAUTION

This study is limited by the restricted number of samples and consequently stages analyzed.

WIDER IMPLICATIONS OF THE FINDINGS

In the germline, PIWILs ensure the integrity of the human genome protecting it from ‘parasitic sequences’. This study offers novel insights on the expression dynamics of PIWILs during the window of epigenetic remodeling and meiosis, and highlights important differences between humans and mice, which may prove particularly important to understand causes of infertility and improve both diagnosis and treatment in humans.

STUDY FUNDING/COMPETING INTEREST(S)

M.G.F. was funded by Fundação para a Ciência e Tecnologia (FCT) [SFRH/BD/78689/2011]; N.H. by China Scholarship Council (CSC) [No. 201307040026] and F.W. by Medical Personnel Training Abroad Project of Henan Province [No. 2015022] and S.M.C.d.S.L. by the Netherlands Organization of Scientific Research (NWO) [ASPASIA 015.007.037] and the Interuniversity Attraction Poles-Phase VII [IUAP/PAI P7/14]. The authors have no conflicts of interest to declare.

Differential Regulation of PIWI-LIKE 2 Expression in Primordial Germ Cell Tumor Cell Lines by Promoter Methylation

PIWI-LIKE 2, a member of the ARGONAUTE protein family, is exclusively expressed in pre-pachytene and pachytene stages of spermatogenesis. PIWI-LIKE 2 acts in the germ cell development and the silencing of retrotransponsons to maintain the genomic integrity and stem cell character. In the present study we investigated DNA methylation as potential mechanism for the regulation of human PIWI-LIKE 2 expression in cell lines related to spermatozoa precursor cells. We detected a high methylation of the PIWI-LIKE 2 promoter in TCam-2 cells, while in NT2/D1 cells the promoter was hypomethylated. Concordantly, PIWI-LIKE 2 expression is higher in NT2/D1 cells than in TCam-2 cells. By demethylation of the promoter with 5'-Aza-2'-deoxycytidine, PIWI-LIKE 2 expression in TCam-2 was increased, while in NT2/D1 no alterations in PIWI-LIKE 2 expression could be detected. In conclusion, we analyzed the DNA methylation driving PIWI-LIKE 2 expression in undifferentiated germ cell tumors and demonstrated an epigenetic basis for PIWI-LIKE 2 expression in this cell type.

An unusual intragenic promoter of PIWIL2 contributes to aberrant activation of oncogenic PL2L60

PIWIL2-like (PL2L) protein 60 (PL2L60), a product of aberrantly activated PIWIL2 gene, is widely expressed in various types of tumors and may promote tumorigenesis. However, the mechanisms underlying the activation of expression of PL2L60 remain unknown. In this study, an intragenic promoter responsible for the activation of PL2L60 within the human PIWIL2 gene has been identified, cloned and characterized. The promoter of PL2L60 is located in the intron 10 of the host gene PIWIL2. Bioinformatic and mutagenic analysis reveals that this intragenic promoter within the sequence of 50 nucleotides contains two closely arranged cis-acting elements specific for the hepatic leukemia factor (HLF) in the positive strand and signal transducer and activator of transcription 3 (STAT3) in the negative strand. Chromatin immunoprecipitation analysis demonstrates that both the HLF and polymerase II (Pol II), a hallmark of active promoters, directly bind to the sequence, although STAT3 does not. Knockdown of HLF and STAT3 alone or both by RNA interference significantly reduced both promoter activity and the PL2L60 protein expression, although there is no additive effect. The expression of PL2L60 proteins was enhanced when host gene Piwil2 was genetically disrupted in a murine cell model. Taken together, we have identified a PL2L60-specific intragenic promoter in the host gene of PIWIL2, which is interdependently activated by HLF and STAT3 through steric interaction. This activation is dependent on cellular milieu rather than the integrity of host gene PIWIL2, highlighting a novel, important mechanism for a cancer-causing gene to be activated during tumorigenesis.

Piwil2 is reactivated by HPV oncoproteins and initiates cell reprogramming via epigenetic regulation during cervical cancer tumorigenesis

The human papillomavirus (HPV) oncoproteins E6 and E7 are risk factors that are primarily responsible for the initiation and progression of cervical cancer, and they play a key role in immortalization and transformation by reprogramming differentiating host epithelial cells. It is unclear how cervical epithelial cells transform into tumor-initiating cells (TICs). Here, we observed that the germ stem cell protein Piwil2 is expressed in pre-cancerous and malignant lesions of the cervix and cervical cancer cell lines with the exception of the non-HPV-infected C33a cell line. Knockdown of Piwil2 by shRNA led to a marked reduction in proliferation and colony formation, in vivo tumorigenicity, chemo-resistance, and the proportion of cancer stem-like cells. In contrast, Piwil2 overexpression induced malignant transformation of HaCaT cells and the acquisition of tumor-initiating capabilities. Gene-set enrichment analysis revealed embryonic stem cell (ESC) identity, malignant biological behavior, and specifically, activation targets of the cell reprogramming factors c-Myc, Klf4, Nanog, Oct4, and Sox2 in Piwil2-overexpressing HaCaT cells. We further confirmed that E6 and E7 reactivated Piwil2 and that E6 and E7 overexpression resulted in a similar gene-set enrichment pattern as Piwil2 overexpression in HaCaT cells. Moreover, Piwil2 overexpression or E6 and E7 activation induced H3K9 acetylation but reduced H3K9 trimethylation, which contributed to the epigenetic reprogramming and ESC signature maintenance, as predicted previously. Our study demonstrates that Piwil2, reactivated by the HPV oncoproteins E6 and E7, plays an essential role in the transformation of cervical epithelial cells to TICs via epigenetics-based cell reprogramming.

Differential expression of PIWIL2 in papillary thyroid cancers

Thyroid cancer is the most common type of endocrine malignancy and a leading cause of death among endocrine organ-related cancers. Similar to other types of cancers, early diagnosis of thyroid cancer is important to increase the survival and treatment of this disease. Several immunohistochemical markers are used in the differential diagnosis of thyroid papillary carcinoma. Also, increasing evidence indicates that P-element induced wimpy testis like 2 (PIWIL2) is an RNA-binding protein involved in the induction and progression of numerous types of human malignancies such as lung, breast, colon, prostate and cervix cancers. However, the role of PIWIL2 was poorly investigated in thyroid cancers. Accordingly, aim of the present study was to elucidate the relationship between PIWIL2 and thyroid cancers. The expression level of PIWIL2 was determined by analyzing both protein and mRNA levels in papillary and micropapillary carcinoma tissues by using immunohistochemistry and real-time PCR methods, respectively. Immunohistochemical analysis of HBME-1, galectin-3 and CK-19 was also performed. Similar to other immune markers of HBME-1, galectin-3 and CK-19, protein expression levels of PIWIL2 was significantly up-regulated in both papillary and micropapillary thyroid cancers (p < 0.01). Moreover, consistent with protein expression levels, mRNA expression levels of PIWIL2 was elevated in both papillary and micropapillary thyroid cancer tissues. Yet, mRNA expression changes were statistically insignificant. In conclusion, results of the current study suggest that PIWIL2 can be involved in thyroid cancer tumorigenesis and can be used as a novel predictive biomarker and/or therapeutic target.

Assessment of piRNA biogenesis and function in testicular germ cell tumors and their precursor germ cell neoplasia in situ

BACKGROUND

Aberrant overexpression of PIWI/piRNA pathway proteins is shown for many types of tumors. Interestingly, these proteins are downregulated in testicular germ cell tumors (TGCTs) compared to normal testis tissues. Here, we used germline and TGCT markers to assess the piRNA biogenesis and function in TGCTs and their precursor germ cell neoplasia in situ (GCNIS).

METHODS

We used small RNA deep sequencing, qRT-PCR, and mining public RNAseq/small RNA-seq datasets to examine PIWI/piRNA gene expression and piRNA biogenesis at four stages of TGCT development: (i) germ cells in healthy testis tissues, (ii) germ cells in testis tissues adjacent to TGCTs, (iii) GCNIS cells and (iv) TGCT cells. To this end, we studied three types of samples: (a) healthy testis, (b) testis tissues adjacent to two types of TGCTs (seminomas and nonseminomas) and containing both germ cells and GCNIS cells, as well as (c) matching TGCT samples.

RESULTS

Based on our analyses of small RNA-seq data as well as the presence/absence of expression correlation between PIWI/piRNA pathway genes and germline or TGCT markers, we can suggest that piRNA biogenesis is intact in germ cells present in healthy adult testes, and adjacent to TGCTs. Conversely, GCNIS and TGCT cells were found to lack PIWI/piRNA pathway gene expression and germline-like piRNA biogenesis. However, using an in vitro cell line model, we revealed a possible role for a short PIWIL2/HILI isoform expressed in TGCTs in posttranscriptional regulation of the youngest members of LINE and SINE classes of transposable elements. Importantly, this regulation is also implemented without involvement of germline-like biogenesis of piRNAs.

CONCLUSIONS

Though further studies are warranted, these findings suggest that the conventional germline-like PIWI/piRNA pathway is lost in transition from germ cells to GCNIS cells.

Distinguishing epigenetic features of preneoplastic testis tissues adjacent to seminomas and nonseminomas

PIWI pathway proteins are expressed during spermatogenesis where they play a key role in germ cell development. Epigenetic loss of PIWI proteins expression was previously demonstrated in testicular germ cell tumors (TGCTs), implying their involvement in TGCT development. In this work, apart from studying only normal testis and TGCT samples, we also analyzed an intermediate stage, i.e. preneoplastic testis tissues adjacent to TGCTs. Importantly, in this study, we minimized the contribution of patient-to-patient heterogeneity by using matched preneoplastic/TGCT samples. Surprisingly, expression of germ cell marker DDX4 suggests that spermatogenesis is retained in premalignant testis tissues adjacent to nonseminoma, but not those adjacent to seminoma. Moreover, this pattern is followed by expression of PIWI pathway genes, which impacts one of their functions: DNA methylation level over LINE-1 promoters is higher in preneoplastic testis tissues adjacent to nonseminomas than those adjacent to seminomas. This finding might imply distinct routes for development of the two types of TGCTs and could be used as a novel diagnostic marker, possibly, noninvasively. Finally, we studied the role of CpG island methylation in expression of PIWI genes in patient samples and using in vitro experiments in cell line models: a more complex interrelation between DNA methylation and expression of the corresponding genes was revealed.

The meaning of PIWI proteins in cancer development

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.

Intragenic Locus in Human PIWIL2 Gene Shares Promoter and Enhancer Functions

Recently, more evidence supporting common nature of promoters and enhancers has been accumulated. In this work, we present data on chromatin modifications and non-polyadenylated transcription characteristic for enhancers as well as results of in vitro luciferase reporter assays suggesting that PIWIL2 alternative promoter in exon 7 also functions as an enhancer for gene PHYHIP located 60Kb upstream. This finding of an intragenic enhancer serving as a promoter for a shorter protein isoform implies broader impact on understanding enhancer-promoter networks in regulation of gene expression.

Identification of PIWIL1 Isoforms and Their Expression in Bovine Testes, Oocytes, and Early Embryos

PIWI proteins are members of the larger Argonaute family and bind to specific 24-32 nucleotide RNAs called PIWI-interacting RNAs (piRNAs). PIWI-interacting RNAs direct PIWI-mediated suppression of retrotransposon expression in the male germline in humans and mice, but their roles in bovine reproduction and embryogenesis are unknown. Although the majority of research in mammals has focused on the functions of PIWI proteins during spermatogenesis, this family of proteins and their associated piRNAs have recently been identified in early embryos. The goals of this study were to characterize the expression of PIWIL1 in bovine testis, oocytes, and early embryos. A full-lengthPIWIL1transcript and protein was found in the testis, specifically in the germs cells of mature seminiferous tubules. RNA-immunoprecipitation demonstrated the presence of putative piRNAs with a mean length of 30 nucleotides bound to PIWIL1 in testes. 3'-Rapid amplification of cDNA ends analysis ofPIWIL1transcripts in testes and oocytes revealed two shorter isoforms in addition to the full-length transcript that was only present in testes. TruncatedPIWIL1isoforms in oocytes and testes were confirmed through amplification of their unique intronic fragments. Expression profiling ofPIWIL1through early embryogenesis demonstrated peak mRNA expression at the 2-cell stage with decreasing levels through to the blastocyst. PIWIL1-YFP fusion plasmids were produced for each isoform and expressed in HEK 293 cells, demonstrating nuclear exclusion and size-specific banding of the different isoforms. These data represent the first comprehensive characterization of PIWIL1 in bovine, revealing functional similarities with PIWIL1 in other species and suggest tissue-specific expression of several isoforms.

Piwil 2 expression is correlated with disease-specific and progression-free survival of chemotherapy-treated bladder cancer patients

Piwi-like 2 (Piwil 2) belongs to the family of Argonaute genes/proteins. The expression of Piwil 2 is associated with stem cells. A role in tumorigenesis and/or tumor progression is proposed for different cancers but not yet for bladder cancer (BCa). We investigated the Piwil 2 expression by immunohistochemistry in a cohort of 202 BCa patients treated by cystectomy and adjuvant chemotherapy. The association between Piwil 2 expression and disease-specific (DSS) or progression-free survival (PFS) was calculated using Kaplan Meier analyses and univariate/multivariate Cox's regression hazard models.In a multivariate Cox's regression, Piwil 2 expression, either in the cytoplasm or the nucleus, was significantly associated with DSS and PFS. A weak cytoplasmic staining pattern was associated with poor DSS and tumor progression (RR=2.7; P=0.004 and RR=2.4; P=0.027). Likewise,, absent nuclear Piwil 2 immunoreactivity was associated with poor DSS and tumor progression (RR=2.3; P=0.023 and RR=2.2; P=0.022). BCa patients whose tumors exhibited a combination of weak cytoplasmic and absent nuclear immunoreactivity had a 6-fold increased risk of tumor-related death (P=0.005) compared to patients with strong expression. Considering only patients with high grade G3 tumors, a 7.8-fold risk of tumor-associated death and a 3.6-fold risk of tumor progression were detected independently of the histologic tumor subtype or the chemotherapy regimen. In summary, a combination of weak cytoplasmic and absent nuclear expression of Piwil 2 is significantly associated with an increased risk of DSS and tumor progression. This implicates that Piwil 2 could be a valuable prognostic marker for high-risk BCa patients.

Emerging roles for PIWI proteins in cancer

It is generally accepted that PIWI proteins are predominately expressed in the germline but absent in somatic tissues. Their best-characterized role is to suppress transposon expression, which ensures genomic stability in the germline. However, increasing evidence has suggested that PIWI proteins are linked to the hallmarks of cancer defined by Weinberg and Hanahan, such as cell proliferation, anti-apoptosis, genomic instability, invasion and metastasis. This provides new possibilities for anticancer therapies through the targeting of PIWI proteins, which may have fewer side effects due to their potential classification as a CTA (cancer/testis antigen). Furthermore, PIWI has been proposed to act as a diagnostic and prognostic marker for many types of cancer, and even to differentiate early- and late-stage cancers. We herein summarize the latest progress in this exciting field, hoping to encourage new investigations of PIWIs in cancer biology that will help to develop new therapeutics for clinical application.