Sprouty2 Protein Regulates Hypoxia-inducible Factor-α (HIFα) Protein Levels and Transcription of HIFα-responsive Genes

14-3-3σ-NEDD4L axis promotes ubiquitination and degradation of HIF-1α in colorectal cancer

A hypoxic microenvironment contributes to tumor progression, with hypoxia-inducible factor-1α (HIF-1α) being a critical regulator. We have reported that 14-3-3σ is negatively associated with HIF-1α expression; however, its role in hypoxia-induced tumor progression remains poorly characterized. Here we show that 14-3-3σ suppresses cancer hypoxia-induced metastasis and angiogenesis in colorectal cancer (CRC). 14-3-3σ opposes HIF-1α expression by regulating the protein stability of HIF-1α, thereby decreasing HIF-1α transcriptional activity and suppressing tumor progression. Mechanistic studies show that the 14-3-3σ-interacting protein neural precursor cell-expressed developmentally down-regulated 4-like (NEDD4L) is an E3 ligase that targets HIF-1α. 14-3-3σ promotes the binding of S448-phosphorylated NEDD4L to HIF-1α, thereby enhancing HIF-1α poly-ubiquitination and subsequent proteasome-mediated degradation. Consistent with this anti-tumorigenic function for 14-3-3σ, low 14-3-3σ expression levels correlate with poor CRC patient survival, and 14-3-3σ enhances the response of CRC to bevacizumab. These results reveal an important mechanism for 14-3-3σ in tumor suppression through HIF-1α regulation.

PKD phosphorylation and COP9/Signalosome modulate intracellular Spry2 protein stability

Spry2 is a molecular modulator of tyrosine kinase receptor signaling pathways that has cancer-type-specific effects. Mammalian Spry2 protein undergoes tyrosine and serine phosphorylation in response to growth factor stimulation. Spry2 expression is distinctly altered in various cancer types. Inhibition of the proteasome functionality results in reduced intracellular Spry2 degradation. Using in vitro and in vivo assays, we show that protein kinase D (PKD) phosphorylates Spry2 at serine 112 and interacts in vivo with the C-terminal half of this protein. Importantly, missense mutation of Ser112 decreases the rate of Spry2 intracellular protein degradation. Either knocking down the expression of all three mammalian PKD isoforms or blocking their kinase activity with a specific inhibitor contributes to the stabilization of Spry2 wild-type protein. Downregulation of CSN3, a component of the COP9/Signalosome that binds PKD, significantly increases the half-life of Spry2 wild-type protein but does not affect the stability of a Spry2 after mutating Ser112 to the non-phosphorylatable residue alanine. Our data demonstrate that both PKD and the COP9/Signalosome play a significant role in control of Spry2 intracellular stability and support the consideration of the PKD/COP9 complex as a potential therapeutic target in tumors where Spry2 expression is reduced.

Dysregulation of Oxygen Sensing/Response Pathways in Pregnancies Complicated by Idiopathic Intrauterine Growth Restriction and Early-Onset Preeclampsia

Preeclampsia (PE) and intrauterine growth restriction (IUGR) are the leading causes of maternal and fetal morbidity/mortality. The central deficit in both conditions is impaired placentation due to poor trophoblast invasion, resulting in a hypoxic milieu in which oxidative stress contributes to the pathology. We examine the factors driving the hypoxic response in severely preterm PE (n = 19) and IUGR (n = 16) placentae compared to the spontaneous preterm (SPT) controls (n = 13) using immunoblotting, RT-PCR, immunohistochemistry, proximity ligation assays, and Co-IP. Both hypoxia-inducible factor (HIF)-1α and HIF-2α are increased at the protein level and functional in pathological placentae, as target genes prolyl hydroxylase domain (PHD)2, PHD3, and soluble fms-like tyrosine kinase-1 (sFlt-1) are increased. Accumulation of HIF-α-subunits occurs in the presence of accessory molecules required for their degradation (PHD1, PHD2, and PHD3 and the E3 ligase von Hippel–Lindau (VHL)), which were equally expressed or elevated in the placental lysates of PE and IUGR. However, complex formation between VHL and HIF-α-subunits is defective. This is associated with enhanced VHL/DJ1 complex formation in both PE and IUGR. In conclusion, we establish a significant mechanism driving the maladaptive responses to hypoxia in the placentae from severe PE and IUGR, which is central to the pathogenesis of both diseases.

Interplay between MAPK/ERK signaling pathway and MicroRNAs: A crucial mechanism regulating cancer cell metabolism and tumor progression

Mitogen-activated protein kinase (MAPK) signal transduction, as a highly conserved signaling pathway, is reported to be involved in various biological events, including metabolic reprogramming, cell proliferation, survival, and differentiation. Mutations in key molecules involved in MAPK/ERK signaling and dysregulation of this pathway are very common events in various human malignancies, which make the MAPK signaling a crucial signaling pathway participating in the regulation of glucose uptake by malignant cells and tumorigenesis. MicroRNAs (miRNAs), as small non-coding RNAs, are critical regulators of gene expression that play key roles in cancer initiation and progression. On the other hand, these small RNAs mutually regulate the MAPK signaling which is often overexpressed in the case of cancer progression; suggesting that crosstalk between miRNAs and this signaling pathway plays a pivotal role in the development of human cancers. Some miRNAs such as miR-20b, miR-34c-3p, miR-152, miR-181a, and miR-302b through inhibiting MAPK signaling, and miR-193a-3p, miR-330-3p, and miR-592 by activating this signaling pathway, play imperative roles in tumorigenesis. Therefore, in this review, we aimed to focus on the interplay between miRNAs and MAPK signaling in the various steps of tumorigenesis, including metabolic regulation, cell proliferation, apoptosis, metastasis, angiogenesis, and drug resistance.

Specification of Sprouty2 functions in osteogenesis in in vivo context

Sprouty proteins are modulators of the MAPK/ERK pathway. Amongst these, Sprouty2 (SPRY2) has been investigated as a possible factor that takes part in the initial phases of osteogenesis. However, the in vivo context has not yet been investigated and the underlying mechanisms taking place in vitro remain unknown. Therefore, in this study, the impact of Spry2 deficiency was examined in the developing tibias of Spry2 deficient (-/-) mouse. The investigation was performed when the osteogenic zone became clearly visible and when all three basic bone cells types were present. The main markers of osteoblasts, osteocytes and osteoclasts were evaluated by immunohistochemistry and RT-PCR. RT-PCR showed that the expression of Sost was 3.5 times higher in Spry2-/- than in the wild-type bone, which pointed to a still unknown mechanism of action of SPRY2 on the differentiation of osteocytes. The up-regulation of Sost was independent of Hif-1α expression and could not be related to its positive regulator, Runx2, since none of these factors showed an increased expression in the bone of Spry2-/- mice. Regarding the RANK/RANKL/OPG pathway, the Spry2-/- showed an increased expression of Rank, but no significant change in the expression of Rankl and Opg. Thanks to these results, the impact of Spry2 deletion is shown for the first time in the developing bone as a complex organ including, particularly, an effect on osteoblasts (Runx2) and osteocytes (Sost). This might explain the previously reported decrease in bone formation in postnatal Spry2-/- mice.

The uremic toxin p-cresyl sulfate induces proliferation and migration of clear cell renal cell carcinoma via microRNA-21/ HIF-1α axis signals

p-Cresyl sulfate (pCS), a uremic toxin, can cause renal damage and dysfunction. Studies suggest that renal dysfunction increases the prevalence of renal cancer. However, the effect of pCS on the proliferation and migration of renal cancer is unclear. Clear cell renal cell carcinoma (ccRCC) expresses mutant von Hippel-Lindau gene and is difficult to treat. Hypoxia-inducible factor-1α and 2-α (HIF-1α and HIF-2α) as well as microRNA-21 (miR-21) can regulate the proliferation and migration of ccRCC cells. However, the association between HIF-α and miR-21 in ccRCC remains unclear. Therefore, the effects of pCS on ccRCC cells were investigated for HIF-α and miR-21 signals. Our results showed that pCS induced overexpression of HIF-1α and promoted the proliferation and regulated epithelial-mesenchymal transition-related proteins, including E-cadherin, fibronectin, twist and vimentin in ccRCC cells. pCS treatment increased miR-21 expression. Specifically, inhibition of miR-21 blocked pCS-induced proliferation and migration. Taken together, the present results demonstrate that pCS directly induced the proliferation and migration of ccRCC cells through mechanisms involving miR-21/HIF-1α signaling pathways.

Synaptic activity-induced glycolysis facilitates membrane lipid provision and neurite outgrowth

The formation of neurites is an important process affecting the cognitive abilities of an organism. Neurite growth requires the addition of new membranes, but the metabolic remodeling necessary to supply lipids for membrane expansion is poorly understood. Here, we show that synaptic activity, one of the most important inducers of neurite growth, transcriptionally regulates the expression of neuronal glucose transporter Glut3 and rate-limiting enzymes of glycolysis, resulting in enhanced glucose uptake and metabolism that is partly used for lipid synthesis. Mechanistically, CREB regulates the expression of Glut3 and Siah2, the latter and LDH activity promoting the normoxic stabilization of HIF-1α that regulates the expression of rate-limiting genes of glycolysis. The expression of dominant-negative HIF-1α or Glut3 knockdown blocks activity-dependent neurite growth in vitro while pharmacological inhibition of the glycolysis and specific ablation of HIF-1α in early postnatal mice impairs the neurite architecture. These results suggest that the manipulation of neuronal glucose metabolism could be used to treat some brain developmental disorders.

A compendium of proteins that interact with HIF-1α

Hypoxia-inducible factor 1 (HIF-1) is the founding member of a family of transcription factors that function as master regulators of oxygen homeostasis. HIF-1 is composed of an O₂-regulated HIF-1α subunit and a constitutively expressed HIF-1β subunit. This review provides a compendium of proteins that interact with the HIF-1α subunit, many of which regulate HIF-1 activity in either an O₂-dependent or O₂-independent manner.

Hypoxia inducible factors regulate the transcription of the sprouty2 gene and expression of the sprouty2 protein

Receptor Tyrosine Kinase (RTK) signaling plays a major role in tumorigenesis and normal development. Sprouty2 (Spry2) attenuates RTK signaling and inhibits processes such as angiogenesis, cell proliferation, migration and survival, which are all upregulated in tumors. Indeed in cancers of the liver, lung, prostate and breast, Spry2 protein levels are markedly decreased correlating with poor patient prognosis and shorter survival. Thus, it is important to understand how expression of Spry2 is regulated. While prior studies have focused on the post-translation regulation of Spry2, very few studies have focused on the transcriptional regulation of SPRY2 gene. Here, we demonstrate that in the human hepatoma cell line, Hep3B, the transcription of SPRY2 is inhibited by the transcription regulating hypoxia inducible factors (HIFs). HIFs are composed of an oxygen regulated alpha subunit (HIF1α or HIF2α) and a beta subunit (HIF1β). Intriguingly, silencing of HIF1α and HIF2α elevates SPRY2 mRNA and protein levels suggesting HIFs reduce the transcription of the SPRY2 promoter. In silico analysis identified ten hypoxia response elements (HREs) in the proximal promoter and first intron of SPRY2. Using chromatin immunoprecipitation (ChIP), we show that HIF1α/2α bind near the putative HREs in the proximal promoter and intron of SPRY2. Our studies demonstrated that not only is the SPRY2 promoter methylated, but silencing HIF1α/2α reduced the methylation. ChIP assays also showed DNA methyltransferase1 (DNMT1) binding to the proximal promoter and first intron of SPRY2 and silencing HIF1α/2α decreased this association. Additionally, silencing of DNMT1 mimicked the HIF1α/2α silencing-mediated increase in SPRY2 mRNA and protein. While simultaneous silencing of HIF1α/2α and DNMT1 increased SPRY2 mRNA a little more, the increase was not additive suggesting a common mechanism by which DNMT1 and HIF1α/2α regulate SPRY2 transcription. Together these data suggest that the transcription of SPRY2 is inhibited by HIFs, in part, via DNMT1- mediated methylation.