Evidence of a novel role for Pygopus in rRNA transcription. Biochem J. 2013;453:61-70. [PMID: 23517060 DOI: 10.1042/BJ20121667]

Augmentation of Myc-Dependent Mitotic Gene Expression by the Pygopus2 Chromatin Effector

Mitotic segregation of chromosomes requires precise coordination of many factors, yet evidence is lacking as to how genes encoding these elements are transcriptionally controlled. Here, we found that the Pygopus (Pygo)2 chromatin effector is indispensable for expression of the MYC-dependent genes that regulate cancer cell division. Depletion of Pygo2 arrested SKOV-3 cells at metaphase, which resulted from the failure of chromosomes to capture spindle microtubules, a critical step for chromosomal biorientation and segregation. This observation was consistent with global chromatin association findings in HeLa S3 cells, revealing the enrichment of Pygo2 and MYC at promoters of biorientation and segmentation genes, at which Pygo2 maintained histone H3K27 acetylation. Immunoprecipitation and proximity ligation assays demonstrated MYC and Pygo2 interacting in nuclei, corroborated in a heterologous MYC-driven prostate cancer model that was distinct from Wnt/β-catenin signaling. Our evidence supports a role for Pygo2 as an essential component of MYC oncogenic activity required for mitosis.

HMGB proteins involved in TOR signaling as general regulators of cell growth by controlling ribosome biogenesis

The number of ribosomes and their activity need to be highly regulated because their function is crucial for the cell. Ribosome biogenesis is necessary for cell growth and proliferation in accordance with nutrient availability and other external and intracellular signals. High-mobility group B (HMGB) proteins are conserved from yeasts to human and are decisive in cellular fate. These proteins play critical functions, from the maintenance of chromatin structure, DNA repair, or transcriptional regulation, to facilitation of ribosome biogenesis. They are also involved in cancer and other pathologies. In this review, we summarize evidence of how HMGB proteins contribute to ribosome-biogenesis control, with special emphasis on a common nexus to the target of rapamycin (TOR) pathway, a signaling cascade essential for cell growth and proliferation from yeast to human. Perspectives in this field are also discussed.

Of Wnts and Ribosomes

Wnt proteins are secreted glycoproteins that activate different intracellular signal transduction pathways. They regulate cell proliferation and are required for proper embryonic development. Misregulation of Wnt signaling can result in various diseases including cancer. In most circumstances, cell growth is essential for cell division and thus cell proliferation. Therefore, several reports have highlighted the key role of Wnt proteins for cell growth. Ribosomes represent the cellular protein synthesis machinery and cells need to be equipped with an appropriate number of ribosomes to allow cell growth. Recent findings suggest a role for Wnt proteins in regulating ribosome biogenesis and we here summarize these findings representing a previously unknown function of Wnt proteins. Understanding this role of Wnt signaling might open new avenues to slow down proliferation by drugs for instance in cancer therapy.

PYGOPUS2 expression in prostatic adenocarcinoma is a potential risk stratification marker for PSA progression following radical prostatectomy

AIMS

Prostate cancer (PrCa) is the most frequently diagnosed non-cutaneous cancer in men. Without clear pathological indicators of disease trajectory at diagnosis, management of PrCa is challenging, given its wide-ranging manifestation from indolent to highly aggressive disease. This study examines the role in PrCa of the Pygopus (PYGO)2 chromatin effector protein as a risk stratification marker in PrCa.

METHODS

RNA expression was performed in PrCa cell lines using Northern and RT-PCR analyses. Protein levels were assessed using immunoblot and immunofluorescence. Immunohistochemistry was performed on tissue microarrays constructed from radical prostatectomies with 5-year patient follow-up data including Gleason score tumour staging, margin and lymph node involvement and prostate serum antigen (PSA) levels. Biochemical recurrence (BR) was defined as a postoperative PSA level of >0.2 nL. Univariate and multivariate analyses were performed using SAS and Kaplan-Meier curves using graphPad (Prism).

RESULTS

In vitro depletion of PYGO2 by RNAi in both androgen receptor positive and negative PrCa cell lines attenuated growth and reduced Ki67 and 47S rRNA expression, while PYGO2 protein was localised to the nuclei of tumours as determined by immunohistochemistry. High expression levels of PYGO2 in tumours (n=156) were correlated with BR identified as PSA progression, after 7-year follow-up independent of other traditional risk factors. Most importantly, high PYGO2 levels in intermediate grade tumours suggested increased risk of recurrence over those with negative or weak expression.

CONCLUSION

Our data suggest that elevated PYGO2 expression in primary prostate adenocarcinoma is a potential risk factor for BR.

Role of Pygo2 in tumors

Pygo2, a vital component of the Wnt signaling pathway that was recently discovered, has been reported to be closely related to the tumorigenesis of several types of malignant tumors. There are two highly conservative domains named NHD in the N terminus and PHD in the C terminus of Pygo2. Previous studies have shown that the Pygo2 PHD domain can act as a protein code reader to link the chromatin remodeling complex to specific changes in gene transcription, as demonstrated for the Wnt target genes. Furthermore, the activity of the chromatin remodeling is further facilitated by the recruiting of histone methyltransferase and acetyltransferase through the interaction with the Pygo2 NHD domain. However, the molecular mechanism of Pygo2 in the tumor development is still poorly understood. In the present study, we intend to review the structure and role of Pygo2 in tumor progression.

Wnt/β-catenin-dependent acetylation of Pygo2 by CBP/p300 histone acetyltransferase family members

Pygopus 2 (Pygo2) is a chromatin effector that plays an essential role in canonical Wnt signaling associated with development and stem cell growth. Its function is to facilitate histone acetylation by recruitment of histone acetyltransferases (HATs) at active sites of β-catenin-mediated transcription. In the present study, we report that Pygo2 itself is transiently acetylated when bound to the activated TCF/β-catenin transcription complex, which correlated with β-catenin binding and Axin2 gene activation. The HAT CBP/p300, but not GCN5/PCAF, targeted specific lysine residues of the N-terminal homology domain of Pygo2 for acetylation. Functional analyses revealed that the presence of CBP and p300 increased the association of Pygo2 with GCN5, independent of Pygo2 acetylation status. Finally, while acetylation of Pygo2 had little effect on active β-catenin complex formation, p300-mediated Pygo2 acetylation resulted in the displacement of Pygo2 from the nucleus to the cytoplasm by targeting specific lysine residues in the Pygo2 nuclear localization sequence. Taken together, these findings are consistent with a model in which acetylation of Pygo2 by CBP/p300 family members in the active TCF/β-catenin complex occurs coincident with histone acetylation and may be required for the recycling of Pygo2 away from the complex subsequent to target gene activation.

Selective estrogen receptor modulators and betulinic acid act synergistically to target ERα and SP1 transcription factor dependent Pygopus expression in breast cancer

AIMS

Estrogen and progesterone hormone receptor (ER and PR) expression in invasive breast cancer predicts response to hormone disruptive therapy. Pygopus2 (hPYGO2) encodes a chromatin remodelling protein important for breast cancer growth and cell cycle progression. The aims of this study were to determine the mechanism of expression of hPYGO2 in breast cancer and to examine how this expression is affected therapeutically.

METHODS

hPYGO2 and ER protein expression was examined in a breast tumour microarray by immunohistochemistry. hPYGO2 RNA and protein expression was examined in ER+ and ER- breast cancer cell lines in the presence of selective estrogen hormone receptor modulator drugs and the specificity protein-1 (SP1) inhibitor, betulinic acid (BA). The effects of these drugs on the ability for ER and SP1 to bind the hPYGO2 promoter and affect cell cycle progression were studied using chromatin immunoprecipitation assays.

RESULTS

hPYGO2 was expressed in seven of eight lines and in nuclei of 98% of 65 breast tumours, including 3 Ductal carcinoma in situ and 62 invasive specimens representing ER-negative (22%) and ER-positive (78%) cases. Treatment with either 4-Hydroxytamoxifen (OHT) or fulvestrant reduced hPYGO2 mRNA 10-fold and protein 5-10-fold within 4 h. Promoter analysis indicated an ER/SP1 binding site at nt -225 to -531 of hPYGO2. SP1 RNA interference and BA reduced hPYGO2 protein and RNA expression by fivefold in both ER- and ER+ cells. Further attenuation was achieved by combining BA and 4-OHT resulting in eightfold reduction in cell growth.

CONCLUSIONS

Our findings reveal a mechanistic link between hormone signalling and the growth transcriptional programme. The activation of its expression by ERα and/or SP1 suggests hPYGO2 as a theranostic target for hormone therapy responsive and refractory breast cancer.

Pygo2 functions as a prognostic factor for glioma due to its up-regulation of H3K4me3 and promotion of MLL1/MLL2 complex recruitment

Pygo2 has been discovered as an important Wnt signaling component contributing to the activation of Wnt-target gene transcription. In the present study, we discovered that Pygo2 mRNA and protein levels were up-regulated in the majority of (152/209) human brain glioma tissues and five glioma cell lines, and significantly correlated with the age, the WHO tumor classification and poor patient survival. The histone methyltransferase complex components (WDR5, Ash2, and menin, but not CXCC1 or NCOA6) were down-regulated at the promoter loci of Wnt target genes after Pygo2 knockdown, and this was accompanied by the down-regulation of Wnt/β-catenin pathway activity. Further, we demonstrated that the involvement of Pygo2 in the activation of the Wnt pathway in human glioma progression is through up-regulation of the H3K4me3 (but not H3K4me2) by promoting the recruitment of the histone methyltransferase MLL1/MLL2 complex to Wnt target gene promoters. Thus, our study provided evidence that Pygo2 functions as a novel prognostic marker and represents a potential therapeutic target.

Wnt/β-catenin signaling integrates patterning and metabolism of the insect growth zone

Wnt/β-catenin and hedgehog (Hh) signaling are essential for transmitting signals across cell membranes in animal embryos. Early patterning of the principal insect model, Drosophila melanogaster, occurs in the syncytial blastoderm, where diffusion of transcription factors obviates the need for signaling pathways. However, in the cellularized growth zone of typical short germ insect embryos, signaling pathways are predicted to play a more fundamental role. Indeed, the Wnt/β-catenin pathway is required for posterior elongation in most arthropods, although which target genes are activated in this context remains elusive. Here, we use the short germ beetle Tribolium castaneum to investigate two Wnt and Hh signaling centers located in the head anlagen and in the growth zone of early embryos. We find that Wnt/β-catenin signaling acts upstream of Hh in the growth zone, whereas the opposite interaction occurs in the head. We determine the target gene sets of the Wnt/β-catenin and Hh pathways and find that the growth zone signaling center activates a much greater number of genes and that the Wnt and Hh target gene sets are essentially non-overlapping. The Wnt pathway activates key genes of all three germ layers, including pair-rule genes, and Tc-caudal and Tc-twist. Furthermore, the Wnt pathway is required for hindgut development and we identify Tc-senseless as a novel hindgut patterning gene required in the early growth zone. At the same time, Wnt acts on growth zone metabolism and cell division, thereby integrating growth with patterning. Posterior Hh signaling activates several genes potentially involved in a proteinase cascade of unknown function.