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Permanent URL to this publication: http://dx.doi.org/10.5167/uzh-56116

Wollenick, K. HIF dependent and independent transcriptional regulation of the human PHD2 promoter. 2011, University of Zurich, Faculty of Science.



A disproportion between oxygen delivery and consumption leads to a restricted oxygenation of tissues. In a variety of pathologies like ischemia, stroke, inflammation and cancer this shortage of oxygen is a key feature. It enrolls a unique response that is based on the transcriptional regulation of hundreds of downstream target genes that promote eventually the adaption of cell metabolism. The maintenance of the oxygen homeostasis is centrally governed by hypoxia-inducible transcription factors (HIFs). Direct target genes of these transcription factors are increasingly expressed by binding of the HIF-complex to the cis-acting highly conserved consensus sequence 5‘-RCGTG-3’; also referred to as hypoxia response elements (HRE). Heterodimeric HIFs consist of a tightly O2-regulated a-subunit (in humans HIF-1a, HIF-2a or HIF-3a) and a constitutively expressed b-subunit (HIF-1b). In oxygenated conditions HIF a-subunits are continuously marked for proteasomal degradation through hydroxylation of two key prolyl-residues by prolyl-4-hydroxylase domain (PHD) oxygen sensor proteins. In hypoxic conditions the HIF-� subunit is stabilized and translocates to the nucleus where it forms the heterodimer HIF. Biochemically, the stabilization of the HIF-� subunit is explained by a reduced hydroxylation that is required for the interaction with the ubiquitin-ligase von-Hippel-Lindau protein (pVHL). Two of three PHDs are transcriptionally regulated by HIFs. As a consequence HIF causes increased expression of PHD2/3 that compensates for their decreased enzymatic activity in hypoxia. In this negative HIF-PHD2/3 feedback loop we decided to focus on the oxygen sensor PHD2. PHD2 is widely considered as the main cellular oxygen sensor since, amongst other evidences, only the knockout of the PHD2 (EGLN1) gene shows prenatal lethality in mice. We regard the transcriptional regulation of the PHD2 gene as important since the abundance of the PHD2 enzyme determines the above mentioned negative feedback loop. Therefore, we aimed to profoundly understand the transcriptional regulation by studying the PHD2 promoter architecture and to elucidate further regulatory mechanisms of its activity. We carried out consecutive truncations of the PHD2 promoter and defined the minimal promoter region. By chromosome immunoprecipitation we could confirm on SUMMARY X an endogenous level that hypoxic PHD2 expression is predominantly mediated through HIF-1� rather than HIF-2�. Additionally we identified and cloned a 95 and 55 nucleotide PHD2 promoter region encompassing a single HBS as highly conserved in several organisms and demonstrated high hypoxia-inducibility. To date, HIF is the only known transcription factor influencing PHD2 gene transcription. However, various putative transcription factor binding sites were predicted in this conserved PHD2 promoter region. By a mutation approach we could exclude the ubiquitous transcription factor Sp1 to be involved in basal or hypoxia-induced regulation of the PHD2 gene although numerous predicted Sp1-consensus motifs suggested so. When motifs located 5' or 3' to the HBS were mutated, total abrogation of the hypoxic response was observed, but binding of the HIF-1 complex remained unaffected. This suggests that other transcription factors might contribute to hypoxic activation of the PHD2 promoter. In order to find out which other (co-) transcription factors might influence the PHD2 promoter activity we established a synthetic transactivation screening where 704 arrayed transcription factors were analyzed for their influence on the PHD2 HBS (Wollenick et al., Nucleic Acid Res., in press). We found several family members of the activator protein-1 (AP-1) transcription factors, such as JUN and FOSB, and three ETS-transcription factors to be involved in the activation of the PHD2 promoter. Most strikingly, the ETS-transcription factor ETS variant 4 (ETV4) showed, when overexpressed, not only impact on hypoxic PHD2 expression but also on other wellknown hypoxic target genes such as PHD3 (EGLN3) and carbonic anhydrase 9 (CA9). We hypothesize that ETV4 potentially increases the hypoxic activation of those promoters or elements that contain a distinct sequence architecture surrounding the HBS. HBSs that are similar to the PHD2 HBS seem to be preferentially super-induced by ETV4. By mammalian two-hybrid and fluorescence resonance energy transfer (FRET) analysis we found evidence for formation of a complex between ETV4 and HIF-1/2a. Chromatin immunoprecipitation confirmed the recruitment of HIF-1� and ETV4 to the PHD2 locus. Additionally, we could provide evidence that the co-activation of hypoxic target genes by ETV4 also has relevance for clinical data. In vivo data underlined that ETV4 expression strongly correlates with PHD2, HIF-1/2� and other hypoxic marker genes in 282 human tissues of breast cancer patients. SUMMARY XI Although FRET data suggest a direct interaction, we hypothesize a trimeric complex composed of HIF:p300/CBP:ETV4. We carried out a thorough HIF-1a domain mapping and found that during the hypoxically induced HIF-1�:ETV4 interaction mainly the C-terminal activation domain is involved. Additionally, overexpression of CBP/p300-interacting transactivator 2 (CITED2), a competitor of HIF for the p300/CBP interaction, disrupted the ETV4:HIF complex pointing towards the involvement of p300/CBP. Factor inhibiting HIF (FIH) depletion provoked unregulated binding of HIF to p300/CBP and as a result the loss of oxygen-dependent suppression of the interaction between HIF and ETV4. Taken together, these experiments provided evidence for the cooperation between HIF-1a and p300/CBP in ETV4 binding. Recent data provide indications that ETV4 protein is more abundant in hypoxic and in PHD2 knockdown cells while ETV4 mRNA levels remain unaffected. ETV4 protein levels were also increased when cells were treated with a PHD inhibitor. That might hint to a hydroxylation-dependent regulation of ETV4 through PHDs that is inhibited when the O2-concentration is low or when PHDs are silenced. In conclusion, this work demonstrated that a synthetic transactivation screening can unravel so far unrecognized transcriptional pathway interactions that also have implications on clinical data of different cancer specimen.


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Item Type:Dissertation
Referees:Wenger R H, Stiehl D P, Hottiger M O, Frei C
Communities & Collections:04 Faculty of Medicine > Institute of Physiology
07 Faculty of Science > Institute of Physiology

04 Faculty of Medicine > Center for Integrative Human Physiology
Dewey Decimal Classification:570 Life sciences; biology
610 Medicine & health
Deposited On:22 Jan 2012 19:18
Last Modified:17 Oct 2012 10:04
Number of Pages:165

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