Ubiquitination is absolutely required for the degradation of hypoxia-inducible factor - 1 alpha protein in hypoxic conditions

Responses of Porcupine and Wntless proteins to oxidative, hypoxic and endoplasmic reticulum stresses

The WNT (Wingless and Int-1) proteins play a role in stem cell development and cell differentiation. Mutations in the WNT proteins lead to the development of various tumours, including gastric tumours. Porcupine (PORCN) is a palmitoyltransferase and Wntless (WLS) is a dedicated WNT transport protein that modify and fold the WNT proteins respectively and are involved in their proper secretion and binding to the frizzled (FZD) receptor and the lipoprotein receptor-related protein 5 or 6 (LRP5/6). We investigated how modifications of PORCN and WLS result in changes in WNT expression and secretion from cells under stress conditions that occur in the tumour microenvironment (hypoxia, oxidative stress, endoplasmic reticulum (ER) stress). In the present study, we found the mRNA expression of both PORCN and WLS were significantly increased with treatments inducing oxidative stress (antimycin A) and proteasome inhibition (MG-132), in human colon cancer (HCT116) and human intestinal epithelial cell-6 (HIEC-6) cells. Treatment with ER stressors thapsigargin, tunicamycin, and dithiolthreitol significantly increased PORCN gene expression, while treatment with thapsigargin and dithiolthreitol increased WLS gene expression. The expression of PORCN and WLS proteins increased with hypoxia and ER stressor treatments in both HCT116 and HIEC-6 cells. All stressors used in this study increased beta-catenin (β-catenin) expression in HCT116 cells. Our results suggest that these stressors alter PORCN, WLS and β-catenin expression and function which may, in turn, alter WNT secretion. Silencing the expression of PORCN and WLS with siRNA expression reduced the expression of WLS and WNT3A in HCT116 cells. The possibility exists that PORCN specifically may be involved in a novel signaling pathway, independent of its palmitoleation of the WNT proteins and its role in their secretion, that is rate-limiting for cancer cell growth and tumorigenesis, within the tumour microenvironment.

The Oxygen-Generating Calcium Peroxide-Modified Magnetic Nanoparticles Attenuate Hypoxia-Induced Chemoresistance in Triple-Negative Breast Cancer

Cancer response to chemotherapy is regulated not only by intrinsic sensitivity of cancer cells but also by tumor microenvironment. Tumor hypoxia, a condition of low oxygen level in solid tumors, is known to increase the resistance of cancer cells to chemotherapy. Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer. Due to lack of target in TNBC, chemotherapy is the only approved systemic treatment. We evaluated the effect of hypoxia on chemotherapy resistance in TNBC in a series of in vitro and in vivo experiments. Furthermore, we synthesized the calcium peroxide-modified magnetic nanoparticles (CaO₂-MNPs) with the function of oxygen generation to improve and enhance the therapeutic efficiency of doxorubicin treatment in the hypoxia microenvironment of TNBC. The results of gene set enrichment analysis (GSEA) software showed that the hypoxia and autophagy gene sets are significantly enriched in TNBC patients. We found that the chemical hypoxia stabilized the expression of hypoxia-inducible factor 1α (HIF-1α) protein and increased doxorubicin resistance in TNBC cells. Moreover, hypoxia inhibited the induction of apoptosis and autophagy by doxorubicin. In addition, CaO₂-MNPs promoted ubiquitination and protein degradation of HIF-1α. Furthermore, CaO₂-MNPs inhibited autophagy and induced apoptosis in TNBC cells. Our animal studies with an orthotopic mouse model showed that CaO₂-MNPs in combination with doxorubicin exhibited a stronger tumor-suppressive effect on TNBC, compared to the doxorubicin treatment alone. Our findings suggest that combined with CaO₂-MNPs and doxorubicin attenuates HIF-1α expression to improve the efficiency of chemotherapy in TNBC.

HAHmiR.DB: a server platform for high-altitude human miRNA-gene coregulatory networks and associated regulatory circuits

Around 140 million people live in high-altitude (HA) conditions! and even a larger number visit such places for tourism, adventure-seeking or sports training. Rapid ascent to HA can cause severe damage to the body organs and may lead to many fatal disorders. During induction to HA, human body undergoes various physiological, biochemical, hematological and molecular changes to adapt to the extreme environmental conditions. Several literature references hint that gene-expression-regulation and regulatory molecules like miRNAs and transcription factors (TFs) control adaptive responses during HA stress. These biomolecules are known to interact in a complex combinatorial manner to fine-tune the gene expression and help in controlling the molecular responses during this stress and ultimately help in acclimatization. High-Altitude Human miRNA Database (HAHmiR.DB) is a unique, comprehensive and curated collection of miRNAs that have been experimentally validated to be associated with HA stress, their level of expression in different altitudes, fold change, experiment duration, biomarker association, disease and drug association, tissue-specific expression level, Gene Ontology (GO) and Kyoto Encyclopaedia of Gene and Genomes (KEGG) pathway associations. As a server platform, it also uniquely constructs and analyses interactive miRNA-TF-gene coregulatory networks and extracts regulatory circuits/feed-forward loops (FFLs). These regulatory circuits help to offer mechanistic insights into complex regulatory mechanisms during HA stress. The server can also build these regulatory networks between two and more miRNAs of the database and also identify the regulatory circuits from this network. Hence, HAHmiR.DB is the first-of-its-kind database in HA research, which is a reliable platform to explore, compare, analyse and retrieve miRNAs associated with HA stress, their coregulatory networks and FFL regulatory-circuits. HAHmiR.DB is freely accessible at http://www.hahmirdb.in.

Comparative microRNA Transcriptomes in Domestic Goats Reveal Acclimatization to High Altitude

High-altitude acclimatization is a representative example of vertebrates' acclimatization to harsh and extreme environments. Previous studies reported sufficient evidence for a molecular genetic basis of high-altitude acclimatization, and genomic patterns of genetic variation among populations and species have been widely elucidated in recent years. However, understanding of the miRNA role in high-altitude acclimatization have lagged behind, especially in non-model species. To investigate miRNA expression alterations of goats that were induced by high-altitude stress, we performed comparative miRNA transcriptome analysis on six hypoxia-sensitive tissues (heart, kidney, liver, lung, skeletal muscle, and spleen) in two goat populations from distinct altitudes (600 and 3000 m). We obtained the expression value of 1391 mature miRNAs and identified 138 differentially expressed (DE) miRNAs between high and low altitudes. Combined with tissue specificity analysis, we illustrated alterations of expression levels among altitudes and tissues, and found that there were coexisting tissue-specific and -conserved mechanisms for hypoxia acclimatization. Notably, the interplay between DE miRNA and DE target genes strongly indicated post-transcriptional regulation in the hypoxia inducible factor 1, insulin, and p53 signaling pathways, which might play significant roles in high-altitude acclimatization in domestic goats. It's also worth noting that we experimentally confirmed miR-106a-5p to have a negative regulation effect on angiogenesis by directly targeting FLT-1. These results provide insight into the complicated miRNA expression patterns and regulatory mechanisms of high-altitude acclimatization in domestic goats.

Ubiquitin carboxyl-terminal hydrolase L1 promotes hypoxia-inducible factor 1-dependent tumor cell malignancy in spheroid models

Hypoxia-inducible factor 1 (HIF-1) is a critical heterodimeric transcription factor for tumor malignancy. Recently, ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) has been reported to function as a deubiquitinating enzyme for the stabilization of its α subunit (HIF-1α). In the present study, we showed that UCHL1 inhibition can be an effective therapeutic strategy against HIF-1-dependent tumor malignancy. In 2D monolayer culture, a UCHL1 inhibitor suppressed HIF activity and decreased the transcription of HIF downstream genes by inhibiting the UCHL1-mediated accumulation of HIF-1α. Phenotypically, UCHL1 inhibition remarkably blocked cell migration. In 3D spheroid culture models, ectopic expression of UCHL1 significantly upregulated malignancy-related factors such as solidity, volume, as well as viable cell number in an HIF-1α-dependent manner. Conversely, inhibition of the UCHL1-HIF-1 pathway downregulated these malignancy-related factors and also abolished UCHL1-mediated cell proliferation and invasiveness. Finally, inhibition of UCHL1 promoted HIF-1α degradation and lowered the expression of HIF-1 target genes in the 3D model, as also observed in 2D monolayer culture. Our research indicates that the UCHL1-HIF-1 pathway plays a crucial role in tumor malignancy, making it a promising therapeutic target for cancer chemotherapy.

ΔNp73 enhances HIF-1α protein stability through repression of the ECV complex

Cellular responses to low oxygen conditions are mainly regulated by the Hypoxia-inducible factors (HIFs). Induction of HIF-1α in tumor cells activates the angiogenic switch and allows for metabolic adaptations. HIF-1α protein levels are tightly regulated through ubiquitin-mediated proteosomal degradation; however, high levels of HIF-1α is a common feature in many solid tumors and is thought to enhance cancer cell proliferation, migration, and survival. Here, we report that the oncogenic p73 isoform, ∆Np73, increases HIF-1α protein stability. We found that ∆Np73 represses expression of genes encoding subunits of the ECV complex, in particular Elongin C, Elongin B, Cullin 2, and Rbx1. The ECV complex is an E3 ligase complex responsible for polyubiquitinating HIF-1α. Loss of ∆Np73 increases ubiquitination of HIF-1α, leading to its degradation via the proteosomal pathway, and subsequent decrease of HIF-1α target genes. Taken together, our data demonstrates that high levels of ∆Np73 stabilize HIF-1α protein, allowing for it to accumulate and further potentiating its transcriptional activity and supporting tumor progression.

Splice variants of cytosolic polyadenylation element-binding protein 2 (CPEB2) differentially regulate pathways linked to cancer metastasis

The translational regulator cytosolic polyadenylation element-binding protein 2 (CPEB2) has two isoforms, CPEB2A and CPEB2B, derived by alternative splicing of RNA into a mature form that either includes or excludes exon 4. Previously, we reported that this splicing event is highly dysregulated in aggressive forms of breast cancers, which overexpress CPEB2B. The loss of CPEB2A with a concomitant increase in CPEB2B was also required for breast cancer cells to resist cell death because of detachment (anoikis resistance) and metastasize in vivo To examine the mechanism by which CPEB2 isoforms mediate opposing effects on cancer-related phenotypes, we used next generation sequencing of triple negative breast cancer cells in which the isoforms were specifically down-regulated. Down-regulation of the CPEB2B isoform inhibited pathways driving the epithelial-to-mesenchymal transition and hypoxic response, whereas down-regulation of the CPEB2A isoform did not have this effect. Examining key nodes of these pathways showed that CPEB2B induced the expression of regulatory DNA trans-factors (e.g. HIF1α and TWIST1). Specifically, CPEB2B functioned as a translational activator of TWIST1 and HIF1α. Functional studies showed that specific down-regulation of either HIF1α or TWIST1 inhibited the ability of CPEB2B to induce the acquisition of anoikis resistance and drive metastasis. Overall, this study demonstrates that CPEB2 alternative splicing is a major regulator of key cellular pathways linked to anoikis resistance and metastasis.

The functional interplay between the HIF pathway and the ubiquitin system - more than a one-way road

The hypoxia inducible factor (HIF) pathway and the ubiquitin system represent major cellular processes that are involved in the regulation of a plethora of cellular signaling pathways and tissue functions. The ubiquitin system controls the ubiquitination of proteins, which is the covalent linkage of one or several ubiquitin molecules to specific targets. This ubiquitination is catalyzed by approximately 1000 different E3 ubiquitin ligases and can lead to different effects, depending on the type of internal ubiquitin chain linkage. The best-studied function is the targeting of proteins for proteasomal degradation. The activity of E3 ligases is antagonized by proteins called deubiquitinases (or deubiquitinating enzymes), which negatively regulate ubiquitin chains. This is performed in most cases by the catalytic removal of these chains from the targeted protein. The HIF pathway is regulated in an oxygen-dependent manner by oxygen-sensing hydroxylases. Covalent modification of HIFα subunits leads to the recruitment of an E3 ligase complex via the von Hippel-Lindau (VHL) protein and the subsequent polyubiquitination and proteasomal degradation of HIFα subunits, demonstrating the regulation of the HIF pathway by the ubiquitin system. This unidirectional effect of an E3 ligase on the HIF pathway is the best-studied example for the interplay between these two important cellular processes. However, additional regulatory mechanisms of the HIF pathway through the ubiquitin system are emerging and, more recently, also the reciprocal regulation of the ubiquitin system through components of the HIF pathway. Understanding these mechanisms and their relevance for the activity of each other is of major importance for the comprehensive elucidation of the oxygen-dependent regulation of cellular processes. This review describes the current knowledge of the functional bidirectional interplay between the HIF pathway and the ubiquitin system on the protein level.