Effects of choline kinase activity on phospholipid metabolism and malignant phenotype of prostate cancer cells

Abstract

High choline uptake and increased choline kinase activity have been reported in many cancers. This has motivated the use of choline as a biomarker for tumor imaging. Tumors in general are heterogeneous in nature with respect to oxygen tension. There are regions of hypoxia and normoxia that are expected to have different metabolism but regulation of choline metabolism under hypoxia is poorly understood. It is important to clarify the status of choline metabolism in hypoxic microenvironment as it will have an impact on potential of choline as a cancer biomarker. The primary goal was to determine the status of choline phosphorylation in hypoxic cancer cells and its effect on uptake of choline. This was examined by tracer studies in cancer cells exposed to hypoxia. It was observed that hypoxia universally inhibits choline uptake /phosphorylation in cancer cells. Decreased choline phosphorylation resulted in transient uptake of choline radiotracers in cultured cancer cells and 9L tumors suggesting potential problem in using choline as a biomarker for cancers in hypoxic microenvironment. To investigate the mechanism behind decrease in choline phosphorylation, steady state levels of choline metabolites were measured and choline kinase catalyzed choline phosphorylation step was found to be rate-limiting in PC-3 cells. This suggested that modulation in choline kinase levels can alter choline metabolism in hypoxic cancer cells. Expression and activity assays for choline kinase revealed that choline kinase expression is down-regulated in hypoxia. This regulation involved transcriptional level mediation by HIF1 at the conserved HRE7 site in choline kinase promoter. To further understand the importance of down-regulation of choline kinase in hypoxia, stable prostate cancer cell lines over-expressing choline kinase were generated. Effect of over-expression of choline kinase in hypoxia was evaluated in terms of malignant phenotypes like proliferation rate, anchorage independent growth and invasion potential. Both over-expression of choline kinase and hypoxia had a pronounced effect on malignant phenotypes of prostate cancer cells. Further study showed that increased choline kinase activity and hypoxic tumor microenvironment are important for progression of early-stage, androgen-dependent LNCaP prostate cancer cells but confer little survival advantage in undifferentiated, androgen-independent PC-3 prostate cancer cells.