Previously we found that elevated levels of
Piwi-like 1 in STS and high as well as low levels of
Piwi-like 1 in pancreatic adenocarcinomas were significantly associated with worse patient’s survival. [
18,
19]. In our study, the levels of
Piwi-like 2–4 mRNA in STS were analyzed and correlated with each other, with clinical and histopathologic parameters and with tumor-specific survival. We found a significant correlation between tumor-specific survival and
Piwi-like 2 and Piwi-like 4 mRNA expression levels, but not with
Piwi-like 3 gene expression. There were correlations between the mRNA expressions of the
Piwi-like genes and other stem cell-associated genes, such as
NanogOct3/4 and
survivin. Surprisingly, a low expression of
Piwi-like 2 or
Piwi-like 3 was significantly correlated with a high expression level of
Nanog and
Piwi-like-2 in addition with an increased transcript level of
Oct3/4, but with a decreased level of the
survivin 2B splice variant. Furthermore, low
Piwi-like 4 mRNA expression was correlated with a high level of the survivin Delta 3 splice variant. However, the survivin 2B splice variant is considered to be a pro-apoptotic protein, whereas the
survivin Delta 3 splice variant has an anti-apoptotic function [
22]. PIWIL2 protein may inhibit apoptosis via activation of Stat3/Bcl-X signaling [
20].
A somewhat surprising result of our study was that a low mRNA expression level of all three
Piwi-like genes was correlated with a poor prognosis, although higher protein levels have been reported to be associated with a poor survival [
8-
11]. There are several possible explanations for this phenomenon. Low mRNA transcription levels may characterize more aggressive tumors. For example, low mRNA levels for the oncogenes
mdm2 and
c-myc have been correlated with a poor prognosis of patients with STS and squamous carcinomas of the tongue, respectively [
23,
24]. Another possibility is that there could be a negative autoregulatory feedback loop between protein and RNA levels in
Piwi-like genes. Such a feedback loop has been described for other genes, for example for Snail1. Snail1 is a transcriptional repressor that binds to its own promoter and controls its expression [
25]. Analogously, one may speculate that Piwi-like proteins, as the main effector component of the RNA-induced silencing complex (RISC), may regulate their own expression by post-transcriptional gene silencing via an unknown small RNA binding partner. Furthermore, different alternative transcripts leading to shortened protein products of PIWIL2 have been described recently [
12]. The predominant form, PL2L60, is mainly expressed in tumor cells and promotes cell survival and proliferation of a human breast cancer cell line (MDA-MB-231), possibly by the upregulation of Bcl2 and Stat3. Because our primers targeted the 5′ end of the different mRNA transcripts for a better discrimination between the homologous genes, we were not able to measure the shortened transcript. Further studies are needed to determine if shortened alternative mRNA transcripts and protein isoforms for Piwi-like 3 and Piwi-like 4 also exist. Additionally, shorter transcripts and/or their protein products could decrease the levels of their full-length transcripts.
Piwi-like proteins bind exclusively to Piwi-interacting RNA (piRNA), a class of small RNAs that differ structurally from the classic siRNA and miRNA in several ways, including length (24–30 nt instead of 18–23 nt), the carriage of a 2′O-methyl modification at the 3′ end and their low conservation among even closely related species [
26-
28]. piRNAs are transcribed from a limited set of clusters located in pericentromeric or telomeric heterochromatic regions by an as yet unknown mechanism [
29]. Their maturation is involved in a set of degradation steps involving transposon transcripts. Uncontrolled transposon activation in
Piwi gene mutants has been observed in
Drosophila and in mice [
30,
31], which points to a role of
Piwi-like genes in transposon control during spermatogenesis. However, the mechanisms and consequences of
Piwi-like gene re-initiation in tumors are not yet fully understood. Given our observation that a low mRNA expression of
Piwi-like genes is correlated with increased tumor size and worse survival, one may speculate that a decrease in the mRNA of
Piwi-like genes can derepress transposon activity and enhance tumor cell selection to a more proliferative, active and aggressive phenotype.
PIWI proteins appear to have sex-specific functions, especially in vertebrate male germ cell maturation [
30-
32]. We were able to show for the first time a sex-specific effect of
Piwi-like gene expression on the survival of tumor patients. The presence of androgen and estrogen receptors has been reported in soft tissue sarcomas [
33]. An in-silico search for binding sites for sex-steroid receptors in the putative promoter regions of the
Piwi-like genes showed an androgen receptor binding site in the
Piwi-like 2 gene promoter (unpublished results). However, the transcriptional activation of the
Piwi-like genes may also be initiated indirectly by transcription factors regulated by sex-steroid binding factors. This question should be addressed in further studies.
7,8,10
publication history
=
