HIF-1 or HIF-2 induction is sufficient to achieve cell cycle arrest in NIH3T3 mouse fibroblasts independent from hypoxia

Abstract

Hypoxia is a severe stress which induces physiological and molecular adaptations, where the latter is dominated by the Hypoxia-inducible transcription Factor (HIF). A well described response on cellular level upon exposure to hypoxia is a reversible cell cycle arrest, which probably renders the cells more resistant to the difficult environment. The individual roles of hypoxia itself and of the isoforms HIF-1α and HIF-2α in cell cycle regulation are poorly understood and discussed controversially. In order to characterize the isolated effect of both HIFα isoforms on the cell cycle we generated tetracycline inducible, HIF-1α and -2α expressing NIH3T3 cells. The cDNAs for HIFα were mutated to generate stable and active HIF under normoxia. Upon activation of both HIFα subunits, the total number of living cells was reduced and long-term stimulation of HIF led to complete loss of transgene expression, implicating a strong negative selection pressure. Equally, colony forming activity was reduced by activation of both HIFα subunits. Cell cycle analyses showed that HIF activation resulted in a prominent cell cycle arrest in G1-phase, similarly to the hypoxic effect. Both, HIF-1α and HIF-2α were able to induce the expression of the cyclin-dependent kinase inhibitor p27 on reporter gene and protein level. Our study shows that HIF-1 and HIF-2 can individually arrest the cell cycle independent from hypoxia. These findings have implications for the resistance of tumor cells to the environment and treatment, but also for physiological cells. Importantly, recent approaches to stabilize HIFα in normoxia could have deleterious effects on proliferating tissues.