Purpose: Hypoxia in tumors is associated with poor prognosis and resistance to treatment.

The outcome of hypoxia is largely regulated by the hypoxia-inducible factors (HIF-1a and

HIF-2a). HIFs in turn are negatively regulated by a family of prolyl hydroxylases (PHD1-3).

The PHD2 isoform is the main down-regulator of HIFs in normoxia and mild hypoxia. This

study was designed to analyze the correlation of the expression and subcellular localization

of PHD2 with the pathologic features of human carcinomas and HIF-1a expression.

Experimental Design: The expression of PHD2 was studied from paraffin-embedded

normal tissue (n = 21) and head and neck squamous cell carcinoma (HNSCC; n = 44)

by immunohistochemistry. Further studies included PHD2 mRNA detection and HIF-1a

immunohistochemistry from HNSCC specimens as well as PHD2 immunocytochemistry

from HNSCC-derived cell lines.

Results: In noncancerous tissue, PHD2 is robustly expressed by endothelial cells. In

epithelium, the basal proliferating layer also shows strong expression, whereas the more

differentiated epithelium shows little or no PHD2 expression. In HNSCC, PHD2 shows strongly

elevated expression both at the mRNA and protein level. Moreover, PHD2 expression increases

in less differentiated phenotypes and partially relocalizes from the cytoplasm into the nucleus.

Endogenously high nuclear PHD2 is seen in a subset of HNSCC-derived cell lines. Finally,

although most of the tumor regions with high PHD2 expression show down-regulated

HIF-1a, regions with simultaneous HIF-1a and PHD2 expression could be detected.

Conclusions: Our results show that increased levels and nuclear translocation of the cellular

oxygen sensor, PHD2, are associated with less differentiated and strongly proliferating tumors.

Furthermore, they imply that even the elevated PHD2 levels are not sufficient to down-regulate

HIF-1a in some tumors