Hypoxia-inducible factor 1α is essential for hypoxic p27 induction in endometrioid endometrial carcinoma

Hypoxia-inducible factor 1α (HIF-1α) plays an essential role in the adaptive response of cells to hypoxia. The cyclin-dependent kinase inhibitor p27(Kip1) is highly expressed in the normal endometrium but is lost during endometrial carcinogenesis. However, in high-grade cancers, p27 re-expression is observed. We analysed the role of HIF-1α in hypoxia-induced expression of p27 in vitro and in vivo in endometrial cancer. Paraffin-embedded specimens from endometrioid endometrial carcinoma (n = 39) were stained immunohistochemically for HIF-1α, p27, and Ki67. HEC1B, an endometrial carcinoma cell line, was cultured under normoxic or hypoxic conditions in the presence or absence of transiently expressed short hairpin RNAs targeting HIF-1α. Protein expression of p27 and HIF-1α was assessed by western blotting. Immunohistochemical staining revealed perinecrotic HIF-1α expression in 67% of the cases and p27 staining centrally in the tumour islands, mostly around necrosis, in 46% of the cases. In 50% of the tumours with perinecrotic HIF-1α expression, p27 and HIF-1α perinecrotic/central co-localization was observed. In these tumour sections, hypoxia-associated p27 expression showed less proliferation around necrosis. Analysis of cultured endometrial carcinoma cells demonstrated that p27 protein expression is induced by hypoxia. This induction was abrogated by transient knockdown of HIF-1α using RNAi. Furthermore, hypoxia induced cell cycle arrest in HEC1B cells. We conclude that, in endometrioid endometrial carcinoma, p27 re-expression by hypoxia is HIF-1α-dependent and leads to cell cycle arrest. This may contribute to the survival of cancer cells in hypoxic parts of the tumour.