Hypoxia, the state of low oxygen conditions, occurs frequently in solid tumours and is a poor prognostic factor for patient outcome. The expression of Hif-1α, the main mediator of the hypoxic response pathway, and the up-regulation of its target genes have been implicated in tumour growth, invasion, angiogenesis, therapy resistance and metastasis. There are two forms of hypoxia present in a growing tumour: chronic hypoxia (caused by limitations in oxygen diffusion from abnormal tumour vasculature) and cycling hypoxia (caused by the aberrant, repeated and temporary closing and re-opening of blood vessels, leading to fluctuations in oxygen supply). The effects of these two different forms of hypoxia on tumour cells metastatic potential have not yet been comprehensively studied.
By exposing murine breast cancer cells to 9 days of 20% O2 (normoxia), 1% O2 (chronic hypoxia) or alternating 24 hour cycles in 20% O2 and 1% O2 (cyclic hypoxia), we observe that cyclic hypoxia generates tumour cells with greater metastatic potential. These cells have the ability for low-anchorage dependent growth, altered proliferation behavior, a modified secretory profile and undergo senescence. Cyclic hypoxia-treated cells express increased levels hypoxia response genes, but upon re-exposure to 1% O2, their hypoxic response is dampened. These cells also undergo a glycolytic switch, as evidenced by the up-regulation of several glucose metabolism genes. Using RNAseq data, we find that cyclic hypoxia causes the expression of a lung metastasis gene signature. Although these treatments produce no difference in primary tumour growth when orthotopically injected into the mammary fat pad, cyclic hypoxia-treated cells give rise to a greater number of lung metastases. This data suggests that cyclic hypoxia endows tumour cells with greater metastatic potential.