Nucleolar localization of Small G protein RhoA is associated with active RNA synthesis in human carcinoma HEp-2 cells

CX-5461 causes nucleolar compaction, alteration of peri- and intranucleolar chromatin arrangement, an increase in both heterochromatin and DNA damage response

In this study, we characterize the changes in nucleolar morphology and its dynamics induced by the recently introduced compound CX-5461, an inhibitor of ribosome synthesis. Time-lapse imaging, immunofluorescence and ultrastructural analysis revealed that exposure of cells to CX-5461 has a profound impact on their nucleolar morphology and function: nucleoli acquired a compact, spherical shape and display enlarged, ring-like masses of perinucleolar condensed chromatin. Tunnels consisting of chromatin developed as transient structures running through nucleoli. Nucleolar components involved in rRNA transcription, fibrillar centres and dense fibrillar component with their major constituents ribosomal DNA, RNA polymerase I and fibrillarin maintain their topological arrangement but become reduced in number and move towards the nucleolar periphery. Nucleolar changes are paralleled by an increased amount of the DNA damage response indicator γH2AX and DNA unwinding enzyme topoisomerase I in nucleoli and the perinucleolar area suggesting that CX-5461 induces torsional stress and DNA damage in rDNA. This is corroborated by the irreversibility of the observed altered nucleolar phenotypes. We demonstrate that incubation with CX-5461, apart from leading to specific morphological alterations, increases senescence and decreases cell replication. We discuss that these alterations differ from those observed with other drugs interfering with nucleolar functions.

VPRBP Functions Downstream of the Androgen Receptor and OGT to Restrict p53 Activation in Prostate Cancer

Androgen receptor (AR) is a major driver of prostate cancer initiation and progression. O-GlcNAc transferase (OGT), the enzyme that catalyzes the covalent addition of UDP-N-acetylglucosamine (UDP-GlcNAc) to serine and threonine residues of proteins, is often highly expressed in prostate cancer with its expression correlated with high Gleason score. In this study, we have identified an AR and OGT coregulated factor, Vpr (HIV-1) binding protein (VPRBP) also known as DDB1 and CUL4 Associated Factor 1 (DCAF1). We show that VPRBP is regulated by the AR at the transcript level, and stabilized by OGT at the protein level. VPRBP knockdown in prostate cancer cells led to a significant decrease in cell proliferation, p53 stabilization, nucleolar fragmentation, and increased p53 recruitment to the chromatin. In human prostate tumor samples, VPRBP protein overexpression correlated with AR amplification, OGT overexpression, a shorter time to postoperative biochemical progression and poor clinical outcome. In clinical transcriptomic data, VPRBP expression was positively correlated with the AR and also with AR activity gene signatures.

IMPLICATIONS

In conclusion, we have shown that VPRBP/DCAF1 promotes prostate cancer cell proliferation by restraining p53 activation under the influence of the AR and OGT.

RhoA activation and nuclearization marks loss of chondrocyte phenotype in crosstalk with Wnt pathway

De-differentiation comprises a major drawback for the use of autologous chondrocytes in cartilage repair. Here, we investigate the role of RhoA and canonical Wnt signaling in chondrocyte phenotype. Chondrocyte de-differentiation is accompanied by an upregulation and nuclear localization of RhoA. Effectors of canonical Wnt signaling including β-catenin and YAP/TAZ are upregulated in de-differentiating chondrocytes in a Rho-dependent manner. Inhibition of Rho activation with C3 transferase inhibits nuclear localization of RhoA, induces expression of chondrogenic markers on 2D and enhances the chondrogenic effect of 3D culturing. Upregulation of chondrogenic markers by Rho inhibition is accompanied by loss of canonical Wnt signaling markers in 3D or on 2D whereas treatment of chondrocytes with Wnt-3a abrogates this effect. However, induction of canonical Wnt signaling inhibits chondrogenic markers on 2D but enhances chondrogenic re-differentiation on 2D with C3 transferase or in 3D. These data provide insights on the context-dependent role of RhoA and Wnt signaling in de-differentiation and on mechanisms to induce chondrogenic markers for therapeutic approaches.