SMAD family member 3 (SMAD3) and SMAD4 repress HIF2α-dependent iron-regulatory genes

Abstract

Hypoxia-inducible factor 2␣ (HIF2␣) directly regulates a battery of genes essential for intestinal iron absorption. Interestingly, iron deficiency and overload disorders do not result in increased intestinal expression of glycolytic or angiogenic HIF2␣ target genes. Similarly, inflammatory and tumor foci can induce a distinct subset of HIF2␣ target genes in vivo. These observations indicate that different stimuli activate distinct subsets of HIF2␣ target genes via mechanisms that remain unclear. Here, we conducted a high-throughput siRNA-based screen to identify genes that regulate HIF2␣'s transcriptional activity on the promoter of the iron transporter gene divalent metal transporter-1 (DMT1). SMAD family member 3 (SMAD3) and SMAD4 were identified as potential transcriptional repressors. Further analysis revealed that SMAD4 signaling selectively represses iron-absorptive gene promoters but not the inflammatory or glycolytic HIF2␣ or HIF1␣ target genes. Moreover, the highly homologous SMAD2 did not alter HIF2␣ transcriptional activity. During iron deficiency, SMAD3 and SMAD4 expression was significantly decreased via proteasomal degradation, allowing for derepression of iron target genes. Several iron-regulatory genes contain a SMAD-binding element (SBE) in their proximal promoters; however, mutation of the putative SBE on the DMT1 promoter did not alter the repressive function of SMAD3 or SMAD4. Importantly, the transcription factor forkhead box protein A1 (FOXA1) was critical in SMAD4-induced DMT1 repression, and DNA binding of SMAD4 was essential for the repression of HIF2␣ activity, suggesting an indirect repressive mechanism through DNA binding. These results provide mechanistic clues to how HIF signaling can be regulated by different cellular cues.

A genetic mouse model of severe iron deficiency anemia reveals tissue-specific transcriptional stress responses and cardiac remodeling

“…Interestingly, HIF-2␣ transcriptional targets involved in inflammation were not active despite activation of iron-absorptive genes. This finding is in line with recent work showing that HIF-2␣ functions with cofactors and other transcriptional partners to regulate subsets of target genes when the protein is stabilized (28,29). Future work will need to define how specific HIF target genes are regulated during unique environmental ques as well as the relative contribution of HIF-2␣ oxygen and iron sensing in different cells and tissues.…”

SMAD4 activates Wnt signaling pathway to inhibit granulosa cell apoptosis

“…In mechanistic terms, we discovered two important and unexpected ways that SMAD4 promotes FZD4 transcription in ovarian GCs. First, SMAD4 directly controls FZD4 transcription by acting as a transcription factor, the best-characterized manner in which SMAD4 regulates its target genes 12,39,54 . The SMAD4 protein affects the transcriptional activity of target genes, usually by interactions between its DNA-binding domain and SBE motifs within the promoter region of target genes 55 .…”

“…The SMAD4 protein affects the transcriptional activity of target genes, usually by interactions between its DNA-binding domain and SBE motifs within the promoter region of target genes 55 . Intriguingly, the regulatory effect of SMAD4 on target genes can act in one of two opposing ways: it can promote target gene transcription by acting as a transcriptional activator 6,12 , or inhibit the transcription of target genes by acting as a transcriptional repressor 54,56 . In this study, we showed that SMAD4 is a new transcriptional activator of FZD4, and that the two factors form a novel regulatory axis that mediates crosstalk between the TGF-β and Wnt signaling pathways in porcine GCs.…”

Hypoxia inhibits ferritinophagy, increases mitochondrial ferritin, and protects from ferroptosis

“…Under hypoxia, which occurs in tumors or during inflammation, hypoxia inducible factors (HIF) are known to control iron regulatory proteins. HIF-regulated proteins are not only the transferrin receptor (TfR) and the divalent metal transporter 1 (DMT1), which are responsible for iron import, but also ferroportin (FPN), which is the only established iron exporter [ [4] , [5] , [6] , [7] ]. Iron storage is equally important and may prevent cells from oxidative stress caused by an overload with redox active free iron.…”