September 2005

MRS measures elevated metabolite levels indicative of hypoxia in prostate tumors

By: Charles Bankhead

CONTEXT: Although increased total choline (tCho) and phosphocholine (PC) are hallmarks of tumor cells, heterogeneous distribution of tCho is often detected in solid tumors. Kristine Glunde, Ph.D., Zaver Bhujwalla, Ph.D., and their colleagues at Johns Hopkins University Medical Center conducted studies involving combined MR spectroscopy and optical imaging to evaluate tumors derived from human prostate cancer cells stably transfected with green fluorescent protein (GFP) and expressing GFP under hypoxic conditions. The studies routinely revealed coarse colocalization between tCho maps obtained with MRS and hypoxic fluorescing regions detected with optical imaging. Subsequent studies produced evidence suggesting that hypoxia increases tCho, which might contribute to the heterogeneous distribution of tCho observed in vivo.

RESULTS: Human prostate cancer cells were stably transfected with the hypoxia response elements of human VEGF-A ligated to enhanced GFP. The cells were exposed to hypoxic conditions for 24 hours. Water-soluble cell extract fractions and protein lysates were obtained from hypoxic and control cells. Fluorescence microscopy was performed before extraction. Fully relaxed proton MRS was performed on the water-soluble fractions. Choline kinase expression was determined by gel electrophoresis and Western blotting. The studies showed that the cells exhibited significantly increased tCho and PC levels after 24 hours of hypoxia. Choline kinase was significantly overexpressed in hypoxic cells compared with normoxic cells.

IMAGE: A: Triplanar view of a tCho map, displayed in red, fused with a colocalization spin density map obtained from a human prostate tumor xenograft expressing GFP under the control of a hypoxia response element. Heterogeneous distribution of tCho is apparent. B: Corresponding GFP distribution reflects hypoxia inducible factor activity and hypoxia in a fresh tissue slice matching the MRS slice, overlaid in a white light image. Comparison of the two figures reveals a coarse colocalization between GFP distribution and tCho distribution.

IMPLICATIONS: The results indicate that choline kinase expression in prostate cancer cells can be driven by hypoxia, leading to elevated tCho and PC levels in hypoxic tumor regions. The observation might have therapeutic implications related to targeting of the choline cycle.