Relationship between radiobiological hypoxia in a C3H mouse mammary carcinoma and osteopontin levels in mouse serum

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Abstract
Purpose: To investigate the possible relationship between radiobiological hypoxia in a C3H mouse mammary carcinoma and osteopontin (OPN) levels measured in mouse serum. Material and methods: Experiments were performed in CDF1 mice that were either non-tumour bearing or with different sized tumours implanted in the right rear foot. Osteopontin levels in extracted mouse blood serum and tissue from the transplanted tumours were measured using an ELISA assay. The tumour oxygenation status was estimated using the Eppendorf Histograph and the fraction of oxygen partial pressure (pO2) values ≤5 mm Hg (HF5) was calculated. Necrosis was measured in haematoxylin and eosin-stained sections. Tumour hypoxia was increased by placing animals in a low-oxygen (10%) environment. Single radiation doses (240 kV x-rays) were given locally to tumours under ambient or clamped conditions and response assessed using a tumour control assay. Results: Serum OPN levels increased linearly with increasing tumour volume and this increase correlated with tumour OPN. HF5 and necrosis also increased with increasing tumour volume, but this increase was non-linear. Converting the HF5 results into equivalent tumour volume gave results that were directly correlated to OPN serum levels. Placing mice in a 10% oxygen environment for 12 hours significantly increased HF5. However, serum OPN only increased if reoxygenation occurred before measurement. Radiobiological hypoxic fraction in this tumour model did not change with increasing tumour size, but the total number of hypoxic cells did increase. Conclusions: These findings suggest that serum OPN measurement may predict the proportion of hypoxic cells in this tumour model, although increased serum OPN levels simply resulting from an increased tumour burden can not be ruled out.

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