Prolylhydroxylase domain proteins (PHD) are cellular oxygen-sensing molecules that regulate the stability of the ␣-subunit of the transcription factor hypoxia inducible factor (HIF)-1. HIF-1 affects cardiac development as well as adaptation of the heart toward increased pressure overload or myocardial infarction. We have disrupted PHD2 in cardiomyocytes (cPhd ؊/؊ ) When oxygen availability is impaired, the resulting hypoxia activates homeostatic mechanisms at the systemic and cellular level (1). Hypoxia-inducible factors (HIFs) 2 are essential players in these responses because they regulate the transcription of a large number of genes that affect a myriad of cellular processes, including angiogenesis, metabolism, cell survival, and oxygen delivery (2). HIF is a heterodimeric protein comprising the oxygen-sensitive ␣-subunit HIF-1␣ or the more cell type-specifically expressed HIF-2␣ or HIF-3␣ and the oxygen-insensitive ␤-subunit (3). In the presence of oxygen, HIF␣ becomes hydroxylated at two critical proline residues by prolylhydroxylase domain (PHD) enzymes (4, 5). The PHD protein family responsible for HIF␣ regulation consists of three members called prolylhydroxylase domain (PHD)1, PHD2, and PHD3 (6, 7). Following prolyl-4-hydroxylation of the critical prolyl residues under normoxic conditions, the ubiquitin ligase von Hippel-Lindau tumor suppressor protein recognizes ⌯⌱F-1␣ subunits and targets them for rapid ubiquitination and proteasomal degradation (8 -10).Based on the ubiquitous expression pattern and its dominant effect in normoxia, it had to be assumed that PHD2 is the most critical HIF-1␣-regulating PHD isoform in most tissues (11)(12)(13). This notion, learned from in vitro studies, was confirmed by the up to now available genetically modified Phd2 mouse models (14). Phd2 knock-out embryos die between embryonic day (E) 12.5 and E14.5 (15). This time point coincides with the increased levels of PHD2 in wild-type (wt) mice starting from E9.0. A major role of PHD2 in regulating the HIF system is further underscored by mouse models with a somatic Phd2 Ϫ/Ϫ knock out, which enable to analyze the in vivo function of PHD2 in the adult mice. Two independent inducible Phd2 Ϫ/Ϫ mouse models were developed by Takeda et al. (16) and Minamishima et al. (17). The phenotype of these mice most obviously resembles the consequences of HIF␣ overexpression with increased angiogenesis, erythropoiesis, and extramedullar hematopoiesis (17,18). Most interestingly, these mice also develop a cardiac phenotype with symptoms of dilated cardiomyopathy. In the heart, HIF-1␣ and thereby also the PHDs are known to influence key components of heart development, morphogenesis, and function (19,20). Long term activation of HIF-1␣ in the heart seems to activate detrimental pathways resulting in the development of heart failure (21). Thus, it is tempting to speculate that loss of PHD2 in the heart is responsible for the dilated cardiomyopathy as observed in the inducible Phd2 Ϫ/Ϫ mice. However, because these mice also develop an inc...

Cells are responding to hypoxia via prolyl-4-hydroxylase domain (PHD) enzymes, which are responsible for oxygen-dependent hydroxylation of the hypoxia-inducible factor (HIF)-1␣ subunit. To gain further insight into PHD function, we generated knockdown cell models for the PHD2 isoform, which is the main isoform regulating HIF-1␣ hydroxylation and thus stability in normoxia. Induction of a PHD2 knockdown in tetracycline-inducible HeLa PHD2 knockdown cells resulted in increased F-actin formation as detected by phalloidin staining. A similar effect could be observed in the stably transfected PHD2 knockdown cell clones 1B6 and 3B7. F-actin is at least in part responsible for shaping cell morphology as well as regulating cell migration. Cell migration was impaired significantly as a consequence of PHD2 knockdown in a scratch assay. Mechanistically, PHD2 knockdown resulted in activation of the RhoA (Ras homolog gene family member A)/Rho-associated kinase pathway with subsequent phosphorylation of cofilin. Because cofilin phosphorylation impairs its actin-severing function, this may explain the F-actin phenotype, thereby providing a functional link between PHD2-dependent signaling and cell motility.

The prolyl-4-hydroxylase domain 1-3 (PHD1-3) enzymes are regulating the protein stability of the a-subunit of the hypoxiainducible factor-1 (HIF-1), which mediates oxygen-dependent gene expression. PHD2 is the main isoform regulating HIF-1a hydroxylation and thus stability in normoxia. In human cancers, HIF-1a is overexpressed as a result of intratumoral hypoxia which in turn promotes tumor progression. The role of PHD2 for tumor progression is in contrast far from being thoroughly understood. Therefore, we established PHD2 knockdown clones of MDA-MB-231 breast cancer cells and analyzed their tumorforming potential in a SCID mouse model. Tumor progression was significantly impaired in the PHD2 knockdown MDA-MB-231 cells, which could be partially rescued by re-establishing PHD2 expression. In a RNA profile screen, we identified the secreted phosphoprotein 1 (SPP1) as one target, which is differentially regulated as a consequence of the PHD2 knockdown. Knockdown of PHD2 drastically reduced the SPP1 expression in MDA-MB-231 cells. A correlation of SPP1 and PHD2 expression was additionally verified in 294 invasive breast cancer biopsies. In subsequent analyses, we identified that PHD2 alters the processing of transforming growth factor (TGF)-b1, which is highly involved in SPP1 expression. The altered processing capacity was associated with a dislocation of the pro-protein convertase furin. Thus, our data demonstrate that in MDA-MB-231 cells PHD2 might affect tumor-relevant TGF-b1 target gene expression by altering the TGF-b1 processing capacity.

Membrane trafficking is highly dynamic upon hypoxia. This phenotype is quickly reversible upon reoxygenation, which suggests that this mechanism participates in the cellular adaptation to hypoxia.