Progress Report

Rosalie Richards. Primary supervisor: Dr Violaine Sée.

Background

Tumour growth is dependent on the formation of new blood vessels however the vasculature within tumours is poorly organised and often shows severe structural abnormalities. This creates an inadequate supply of oxygen known as hypoxia. Hypoxia is associated with the increased malignancy, invasiveness and treatment resistance of tumours. Although hypoxia is associated with increased tumour aggressiveness under certain conditions it has also been shown to arrest cells in the first stage of the cell cycle. This stops the cell from dividing into more cells, reducing tumour growth. A greater understanding of the effect of hypoxia on cell proliferation and cell fate will help resolve this apparent contradiction and better tackle uncontrolled tumour growth.

Aim of the project

The aim of this research is to investigate the effects of different oxygen levels on Glioblastoma Multiforme (GBM) cell proliferation, cell fate and drug resistance.

Progress to date

1.      Characterisation of the effect of different oxygen levels on GBM cell cycle and cell survival. Exposure to 24, 48 or 72 hours of 1% or 0.5% oxygen does not cause cell cycle arrest in any of the GBM cell lines tested. It also does not affect cell survival since no significant cell loss could be detected at low oxygen levels. This suggests that cells can divide and survive at oxygen levels ranging from 21% (atmospheric) to 0.5% (intratumoural).

2.      Investigation into the effect of different oxygen levels on GBM cell proliferation. Results to date suggest that cells grow faster in 8% oxygen than in 20% or 1% oxygen. This suggests that the physiological conditions are the most optimum for cellular growth. However cells exposed to 1% oxygen grow slower than those exposed to 20% oxygen. Interestingly this happens in the absence of any detectable cell cycle arrest (see above). It suggests that exposure to 1% oxygen may cause the cell cycle as a whole to slow down, rather than causing arrest in a certain phase of the cell cycle. Live cell imaging experiments are currently being conducted to confirm this hypothesis. These imaging experiments will allow us to measure the length of the cell cycle in cells exposed to different oxygen conditions. If this hypothesis is correct, our aim will be to elucidate at the molecular level, the mechanisms of the changes in proliferation kinetics in order to better understand tumour cell growth.

3.      Primary cultures of GBM tumour samples. Samples of GBM tumour tissue have been successfully grown in culture. Our aim is to replicate the experiments detailed above in these cells in order to increase the physiological relevance of our findings. Preliminary results support the conclusions of experiments conducted with laboratory cell lines.

Future directions

1.      Investigation into the role of constant and cycling hypoxia in GBM drug resistance. Research has shown that there are fluctuations in oxygen levels within tumours so that cells are exposed to repeated cycles of hypoxia and re-oxygenation. The majority of research has been conducted using constant hypoxia; however it is becoming clear that there are important differences between constant and cycling hypoxia. Research indicates that cycling hypoxia may increase treatment resistance, although the mechanisms underlying this effect are poorly understood.  A greater understanding of this phenomenon may lead to an enhanced understanding of treatment resistance, an important factor in determining patient prognosis.

2.      Investigation into the effect of a physiologically relevant gradient of oxygen on GBM cellular characteristics. Previous research has shown that a hypoxic gradient exists within tumours. This gradient consists of a peripheral oxygenated layer, an intermediate mildly hypoxic layer and a highly hypoxic core. The most treatment resistance cells are found in the hypoxic core of the tumour. We would like investigation the effects of a gradient of oxygen on GBM cellular characteristics.