Subcellular dynamics of the VHL tumor suppressor: on the move for HIF degradation

Nuclear-cytoplasmatic shuttling of proteins in control of cellular oxygen sensing

In order to pass through the nuclear pore complex, proteins larger than ∼40 kDa require specific nuclear transport receptors. Defects in nuclear-cytoplasmatic transport affect fundamental processes such as development, inflammation and oxygen sensing. The transcriptional response to O2 deficiency is controlled by hypoxia-inducible factors (HIFs). These are heterodimeric transcription factors of each ∼100-120 kDa proteins, consisting of one out of three different O2-labile α subunits (primarily HIF-1α) and a more constitutive 1β subunit. In the presence of O2, the α subunits are hydroxylated by specific prolyl-4-hydroxylase domain proteins (PHD1, PHD2, and PHD3) and an asparaginyl hydroxylase (factor inhibiting HIF-1, FIH-1). The prolyl hydroxylation causes recognition by von Hippel-Lindau tumor suppressor protein (pVHL), ubiquitination, and proteasomal degradation. The activity of the oxygen sensing machinery depends on dynamic intracellular trafficking. Nuclear import of HIF-1α and HIF-1β is mainly mediated by importins α and β (α/β). HIF-1α can shuttle between nucleus and cytoplasm, while HIF-1β is permanently inside the nucleus. pVHL is localized to both compartments. Nuclear import of PHD1 relies on a nuclear localization signal (NLS) and uses the classical import pathway involving importin α/β receptors. PHD2 shows an atypical NLS, and its nuclear import does not occur via the classical pathway. PHD2-mediated hydroxylation of HIF-1α occurs predominantly in the cell nucleus. Nuclear export of PHD2 involves a nuclear export signal (NES) in the N-terminus and depends on the export receptor chromosome region maintenance 1 (CRM1). Nuclear import of PHD3 is mediated by importin α/β receptors and depends on a non-classical NLS. Specific modification of the nuclear translocation of the three PHD isoforms could provide a promising strategy for the development of new therapeutic substances to tackle major diseases.

Role of Ran-regulated nuclear-cytoplasmic trafficking of pVHL in the regulation of microtubular stability-mediated HIF-1α in hypoxic cardiomyocytes

Our previous study suggested that microtubule network alteration affects the process of glycolysis in cardiomyocytes (CMs) via the regulation of hypoxia-inducible factor (HIF)-1α during the early stages of hypoxia. However, little is known regarding the underlying mechanisms of microtubule network alteration-induced changes of HIF-1α. The von Hippel–Lindau tumor suppressor protein (pVHL) has been shown to mediate the ubiquitination of HIF-1α in the nuclear compartment prior to HIF-1α exportation to the cytoplasm, and pVHL dynamic nuclear-cytoplasmic trafficking is indicated to be involved in the process of HIF-1α degradation. In this study, by administering different microtubule-stabilizing and -depolymerizing interventions, we demonstrated that microtubule stabilization promoted pVHL nuclear export and drove the translocation of pVHL to the cytoplasm, while microtubule disruption prevented pVHL nuclear export in hypoxic CMs. Moreover, the ratio between nuclear and cytoplasmic pVHL was associated with HIF-1α regulation. Importantly, microtubule network alteration also affected the subcellular localization of Ran, which was involved in the regulation of pVHL nuclear-cytoplasmic trafficking. The above results suggest that the subcellular translocation of pVHL plays an important role in microtubular structure alteration-induced HIF-1α regulation. Interestingly, Ran is involved in the process of pVHL nuclear-cytoplasmic trafficking following microtubule network alteration in hypoxic CMs.

Gerometabolites: the pseudohypoxic aging side of cancer oncometabolites

Oncometabolites are defined as small-molecule components (or enantiomers) of normal metabolism whose accumulation causes signaling dysregulation to establish a milieu that initiates carcinogenesis. In a similar manner, we propose the term "gerometabolites" to refer to small-molecule components of normal metabolism whose depletion causes signaling dysregulation to establish a milieu that drives aging. In an investigation of the pathogenic activities of the currently recognized oncometabolites R(-)-2-hydroxyglutarate (2-HG), fumarate, and succinate, which accumulate due to mutations in isocitrate dehydrogenases (IDH), fumarate hydratase (FH), and succinate dehydrogenase (SDH), respectively, we illustrate the fact that metabolic pseudohypoxia, the accumulation of hypoxia-inducible factor (HIFα) under normoxic conditions, and the subsequent Warburg-like reprogramming that shifts glucose metabolism from the oxidative pathway to aerobic glycolysis are the same mechanisms through which the decline of the "gerometabolite" nicotinamide adenine dinucleotide (NAD)(+) reversibly disrupts nuclear-mitochondrial communication and contributes to the decline in mitochondrial function with age. From an evolutionary perspective, it is reasonable to view NAD(+)-driven mitochondrial homeostasis as a conserved response to changes in energy supplies and oxygen levels. Similarly, the natural ability of 2-HG to significantly alter epigenetics might reflect an evolutionarily ancient role of certain metabolites to signal for elevated glutamine/glutamate metabolism and/or oxygen deficiency. However, when chronically altered, these responses become conserved causes of aging and cancer. Because HIFα-driven pseudohypoxia might drive the overproduction of 2-HG, the intriguing possibility exists that the decline of gerometabolites such as NAD(+) could promote the chronic accumulation of oncometabolites in normal cells during aging. If the sole activation of a Warburg-like metabolic reprogramming in normal tissues might be able to significantly increase the endogenous production of bona fide etiological determinants in cancer, such as oncometabolites, this undesirable trade-off between mitochondrial dysfunction and activation of oncometabolites production might then pave the way for the epigenetic initiation of carcinogenesis in a strictly metabolic-dependent manner. Perhaps it is time to definitely adopt the view that aging and aging diseases including cancer are governed by a pivotal regulatory role of metabolic reprogramming in cell fate decisions.

CHIP/Stub1 functions as a tumor suppressor and represses NF-κB-mediated signaling in colorectal cancer

The carboxyl terminus of Hsc70-interacting protein (CHIP, also named Stub1), a U-box containing E3 ubiquitin ligase, is involved in degradation of certain oncogenic proteins. Recent studies indicated that CHIP suppresses tumor progression in human cancers by targeting Src-3, hypoxia inducible factor 1α, NF-κB, ErbB2 and c-Myc. Here, we report that CHIP was downregulated, predominantly, in the late stages of human colorectal cancer (CRC), and that the CHIP promoter was hypermethylated in CRC specimens. Overexpression of CHIP in HCT-116 cells resulted in impaired tumor growth in nude mice and decreased abilities of tumor cell migration and invasion. Conversely, depletion of CHIP in HCT-116 cells promoted tumor growth and increased tumor cell migration and invasion. CHIP was further found to negatively regulate NF-κB signaling in HCT-116 cells by promoting ubiquitination and degradation of p65, a subunit of the NF-κB complex. The suppressive effect of CHIP led to decreased expression of NF-κB-targeted oncogenes including Cyclin D1, c-Myc, MMP-2, VEGF and IL-8. We proposed that CHIP inhibits the malignancy of CRC cells, possibly through targeting NF-κB signaling. This study provides functional evidence for CHIP as a potential tumor suppressor in CRC, and CHIP expression may be a marker for stages of CRC.