Notch Downregulation and Extramedullary Erythrocytosis in Hypoxia-Inducible Factor Prolyl 4-Hydroxylase 2-Deficient Mice

ABSTRACT

Erythrocytosis is driven mainly by erythropoietin, which is regulated by hypoxia-inducible factor (HIF). Mutations in HIF prolyl 4-hydroxylase 2 (HIF-P4H-2) (PHD2/EGLN1), the major downregulator of HIFα subunits, are found in familiar erythrocytosis, and large-spectrum conditional inactivation of HIF-P4H-2 in mice leads to severe erythrocytosis. Although bone marrow is the primary site for erythropoiesis, spleen remains capable of extramedullary erythropoiesis. We studied HIF-P4H-2-deficient (Hif-p4h-2^gt/gt) mice, which show slightly induced erythropoiesis upon aging despite nonincreased erythropoietin levels, and identified spleen as the site of extramedullary erythropoiesis. Splenic hematopoietic stem cells (HSCs) of these mice exhibited increased erythroid burst-forming unit (BFU-E) growth, and the mice were protected against anemia. HIF-1α and HIF-2α were stabilized in the spleens, while the Notch ligand genes Jag1, Jag2, and Dll1 and target Hes1 became downregulated upon aging HIF-2α dependently. Inhibition of Notch signaling in wild-type spleen HSCs phenocopied the increased BFU-E growth. HIFα stabilization can thus mediate non-erythropoietin-driven splenic erythropoiesis via altered Notch signaling.

KEYWORDS:

• erythrocytosis
• hypoxia
• hypoxia-inducible factor prolyl-4-hydroxylase 2
• spleen

ACKNOWLEDGMENTS

We thank K. I. Kivirikko for valuable comments on the manuscript; T. Aatsinki, E. Lehtimäki, R. Salmu, and M. Siurua for excellent technical assistance; S. Kauppila for evaluating the Leder-stained bone marrow samples; and the Biocenter Oulu core facilities for electron microscopy and transgenic animals.

Elitsa Y. Dimova, Peppi Koivunen, and Raisa Serpi designed the experiments; Mikko N. M. Myllymäki, Jenni Määttä, Elitsa Y. Dimova, Valerio Izzi, Timo Väisänen, Peppi Koivunen, and Raisa Serpi performed experiments and analyzed the data; Johanna Myllyharju contributed to generating the Hif-p4h-2^gt/gt mouse line; Peppi Koivunen supervised the work; and Peppi Koivunen and Raisa Serpi wrote the manuscript.

Johanna Myllyharju owns equity in FibroGen Inc., which develops HIF-P4H inhibitors as potential therapeutics, and the company currently supports research headed by her. We have no additional financial interests.

This study was supported by Academy of Finland grants 218129 (Peppi Koivunen) and Center of Excellence 2012-2017 grant 251314 (Johanna Myllyharju) and by the S. Jusélius Foundation (Peppi Koivunen and Johanna Myllyharju), the Emil Aaltonen Foundation (Peppi Koivunen), the Jane and Aatos Erkko Foundation (Peppi Koivunen and Johanna Myllyharju), and FibroGen Inc. (Johanna Myllyharju).