Foxo-dependent Par-4 Upregulation Prevents Long-term Survival of Residual Cells Following PI3K-Akt Inhibition

Prostate apoptosis response-4 and tumor suppression: it's not just about apoptosis anymore

The tumor suppressor prostate apoptosis response-4 (Par-4) has recently turned ‘twenty-five’. Beyond its indisputable role as an apoptosis inducer, an increasing and sometimes bewildering, new roles for Par-4 are being reported. These roles include its ability to regulate autophagy, senescence, and metastasis. This growing range of responses to Par-4 is reflected by our increasing understanding of the various mechanisms through which Par-4 can function. In this review, we summarize the existing knowledge on Par-4 tumor suppressive mechanisms, and discuss how the interaction of Par-4 with different regulators influence cell fate. This review also highlights the new secretory pathway that has emerged and the likely discussion on its clinical implications.

Roles of the HOX Proteins in Cancer Invasion and Metastasis

Invasion and metastasis correspond to the foremost cause of cancer-related death, and the molecular networks behind these two processes are extremely complex and dependent on the intra- and extracellular conditions along with the prime of the premetastatic niche. Currently, several studies suggest an association between the levels of HOX genes expression and cancer cell invasion and metastasis, which favour the formation of novel tumour masses. The deregulation of HOX genes by HMGA2/TET1 signalling and the regulatory effect of noncoding RNAs generated by the HOX loci can also promote invasion and metastasis, interfering with the expression of HOX genes or other genes relevant to these processes. In this review, we present five molecular mechanisms of HOX deregulation by which the HOX clusters products may affect invasion and metastatic processes in solid tumours.

Expression of PAWR predicts prognosis of ovarian cancer

Background

Ovarian cancer greatly threatens the general health of women worldwide. Implementation of predictive prognostic biomarkers aids in ovarian cancer management.

Methods

Using online databases, the general expression profile, target-disease associations, and interaction network of PAWR were explored. To identify the role of PAWR in ovarian cancer, gene correlation analysis, survival analysis, and combined analysis of drug responsiveness and PAWR expression were performed. The predictive prognostic value of PAWR was further validated in clinical samples.

Results

PAWR was widely expressed in normal and cancer tissues, with decreased expression in ovarian cancer tissues compared with normal tissues. PAWR was associated with various cancers including ovarian cancer. PAWR formed a regulatory network with a group of proteins and correlated with several genes, which were both implicated in ovarian cancer and drug responsiveness. High PAWR expression denoted better survival in ovarian cancer patients (OS: HR = 0.84, P = 0.0077; PFS, HR = 0.86, P = 0.049). Expression of PAWR could predict platinum responsiveness in ovarian cancer and there was a positive correlation between PAWR gene effect and paclitaxel sensitivity. In 12 paired clinical samples, the cancerous tissues exhibited significantly lower PAWR expression than matched normal fallopian tubes. The predictive prognostic value of PAWR was maintained in a cohort of 50 ovarian cancer patients.

Conclusions

High PAWR expression indicated better survival and higher drug responsiveness in ovarian cancer patients. PAWR could be exploited as a predictive prognostic biomarker in ovarian cancer.

Adaptation and selection shape clonal evolution of tumors during residual disease and recurrence

The survival and recurrence of residual tumor cells following therapy constitutes one of the biggest obstacles to obtaining cures in breast cancer, but it remains unclear how the clonal composition of tumors changes during relapse. We use cellular barcoding to monitor clonal dynamics during tumor recurrence in vivo. We find that clonal diversity decreases during tumor regression, residual disease, and recurrence. The recurrence of dormant residual cells follows several distinct routes. Approximately half of the recurrent tumors exhibit clonal dominance with a small number of subclones comprising the vast majority of the tumor; these clonal recurrences are frequently dependent upon Met gene amplification. A second group of recurrent tumors comprises thousands of subclones, has a clonal architecture similar to primary tumors, and is dependent upon the Jak/Stat pathway. Thus the regrowth of dormant tumors proceeds via multiple routes, producing recurrent tumors with distinct clonal composition, genetic alterations, and drug sensitivities.

The cellular composition of recurrent tumors can provide insight into resistance to therapy and inform on second line therapies. Here, using a genetically modified mouse, the authors perform barcoding experiments of the primary tumors to allow them to study the clonal dynamics of tumor recurrence.

Tetramer formation by the caspase-activated fragment of the Par-4 tumor suppressor

The prostate apoptosis response-4 (Par-4) tumor suppressor can selectively kill cancer cells via apoptosis while leaving healthy cells unharmed. Full length Par-4 has been shown to be predominantly intrinsically disordered in vitro under neutral conditions. As part of the apoptotic process, cellular Par-4 is cleaved at D131 by caspase-3, which generates a 24 kDa C-terminal activated fragment (cl-Par-4) that enters the nucleus and inhibits pro-survival genes, thereby preventing cancer cell proliferation. Here, the structure of cl-Par-4 was investigated using CD spectroscopy, dynamic light scattering, intrinsic tyrosine fluorescence, and size exclusion chromatography with mutli-angle light scattering. Biophysical characterization shows that cl-Par-4 aggregates and is disordered at low ionic strength. However, with increasing ionic strength, cl-Par-4 becomes progressively more helical and less aggregated, ultimately forming largely ordered tetramers at high NaCl concentration. These results, together with previous results showing induced folding at acidic pH, suggest that the in vivo structure and self-association state of cl-Par-4 may be strongly dependent upon cellular environment.

PAR-4 overcomes chemo-resistance in breast cancer cells by antagonizing cIAP1

Most deaths from breast cancer result from tumour recurrence, which is typically an incurable disease. Down-regulation of the pro-apoptotic tumour suppressor protein prostate apoptosis response-4 (PAR-4) is required for breast cancer recurrence and resistance to chemotherapy. Recent advances in the analysis of apoptotic signalling networks have uncovered an important role for activation of caspase-8 following DNA damage by genotoxic drugs. DNA damage induces depletion of IAP proteins and causes caspase-8 activation by promoting the formation of a cytosolic cell death complex. We demonstrate that loss of PAR-4 in triple negative breast cancer cell lines (TNBC) mediates resistance to DNA damage-induced apoptosis and prevents activation of caspase-8. Moreover, loss of PAR-4 prevents DNA damage-induced cIAP1 depletion. PAR-4 functions downstream of caspase-8 by cleavage-induced nuclear translocation of the C-terminal part and we demonstrate that nuclear translocation of the C-terminal PAR-4 fragment leads to depletion of cIAP1 and subsequent caspase-8 activation. Specifically targeting cIAP1 with RNAi or Smac mimetics (LCL161) overcomes chemo-resistance induced by loss of PAR-4 and restores caspase-8 activation. Our data identify cIAP1 as important downstream mediator of PAR-4 and we provide evidence that combining Smac mimetics and genotoxic drugs creates vulnerability for synthetic lethality in TNBC cells lacking PAR-4.

CCL5 promotes breast cancer recurrence through macrophage recruitment in residual tumors

Over half of breast-cancer-related deaths are due to recurrence 5 or more years after initial diagnosis and treatment. This latency suggests that a population of residual tumor cells can survive treatment and persist in a dormant state for many years. The role of the microenvironment in regulating the survival and proliferation of residual cells following therapy remains unexplored. Using a conditional mouse model for Her2-driven breast cancer, we identify interactions between residual tumor cells and their microenvironment as critical for promoting tumor recurrence. Her2 downregulation leads to an inflammatory program driven by TNFα/NFκB signaling, which promotes immune cell infiltration in regressing and residual tumors. The cytokine CCL5 is elevated following Her2 downregulation and remains high in residual tumors. CCL5 promotes tumor recurrence by recruiting CCR5-expressing macrophages, which may contribute to collagen deposition in residual tumors. Blocking this TNFα-CCL5-macrophage axis may be efficacious in preventing breast cancer recurrence.

TERT and Akt Are Involved in the Par-4-Dependent Apoptosis of Islet β Cells in Type 2 Diabetes

Islet β cell apoptosis plays an important role in type 2 diabetes. We previously reported that Par-4-mediated islet β cell apoptosis is induced by high-glucose/fatty acid levels. In the present study, we show that Par-4, which is induced by high-glucose/fatty acid levels, interacts with and inhibits TERT in the cytoplasm and then translocates to the nucleus. Par-4 also inhibited Akt phosphorylation, leading to islet β cell apoptosis. We inhibited Par-4 in islet β cells under high-glucose/fatty acid conditions and knocked out Par-4 in diabetic mice, which led to the up-regulation of TERT and an improvement in the apoptosis rate. We inhibited Akt phosphorylation in islet β cells and diabetic mice, which led to aggressive apoptosis. In addition, the biological film interference technique revealed that Par-4 bound to TERT via its NLS and leucine zipper domains. Our research suggests that Par-4 activation and binding to TERT are key steps required for inducing the apoptosis of islet β cells under high-glucose/fatty acid conditions. Inhibiting Akt phosphorylation aggravated apoptosis by activating Par-4 and inhibiting TERT, and Par-4 inhibition may be an attractive target for the treatment of islet β cell apoptosis.