HIF-1a iS IMPLICATED IN EMBRYONIC STEM (ES) CELL GROWTH AND TUMORIGENESIS

G. Höpfl1, R. H. Wenger1, T. Stallmach², M. Gassmann1, and I. Desbaillets^1*
Institute of Physiology¹, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, and Pathology Department ², University Hospital of Zürich, 8091 Zürich

Rapid tumor growth is accompanied by the appearance of hypoxic/anoxic regions due to the lack of oxygen supply to the tumor cells. The hypoxia-inducible factor (HIF)-1a is known to induce a number of oxygen-dependent genes, including the angiogenic factor VEGF (vascular endothelial growth factor) which induces new blood vessel formation and therefore allows tumor oxygenation. To determine the role of HIF-1a in cell growth and tumorigenesis, we have generated HIF-1a-deficient (HIF-1a^-/-) ES cells by homologous recombination. We found that HIF-1a^+/+ ES cells grow slower than HIF-1a^-/- ES cells, when differentiated as monolayers in normoxia (21% O₂), the opposite occuring in hypoxia (1% O₂). Similar results are obtained by in vitro differentiation of these cells into embryoid bodies, suggesting that HIF-1a deficiency might cause cell growth arrest under low oxygen concentrations. To confirm these in vitro data, we injected the HIF-1a^+/+ and HIF-1a^-/- ES cells subcutaneously into nude mice allowing teratocarcinoma formation. Tumors originating from HIF-1a^+/+ ES cells (n=10) grew faster than the ones from HIF-1a^-/- cells. Interestingly, when HIF-1a^+/+ ES cells were added to HIF-1a^-/- ES cells at a proportion of only 1%, tumors developed as fast as the 100% HIF-1a^+/+ tumors. The same vessel density was observed in all tumors and HIF-1a^+/+ cells did not overgrow the HIF-1a^-/- during mixed tumor formation. Therefore HIF-1a^+/+ cells secrete factors that support the growth of the HIF-1a^-/-cells.