The uptake and metabolism of methyl-choline by tumour cells and its relationship with proliferation : an 'in vitro' simulation of a [Methyl-Â¹Â¹C]-choline-PET scan
[Methyl-11C]-choline is proving to be a useful positron emission tomography imaging agent for several tumour types including prostate cancer but the identity of metabolites giving rise to the 11C signal in tumour tissue is currently unknown. Evidence suggests that choline metabolism is related to cellular proliferation. Methods: Tumour cells were incubated with [Methyl-11H]-choline for 10 minutes then in a cold medium to simulate the rapid blood clearance of [Methyl-11C]-choline. Cell were then extracted with organic and aqueous solvents to determine intracellular location of tracer. Aqueous extracts were subject to thin layer chromatography, ion exchange chromatography, and a choline extraction procedure to identify 3H-containing metabolites. Procedures were carried out on fast and slow growing populations of MCF-7 cells to determine the relationship of choline incorporation with proliferation. Results: Only about 5% of [Methyl-11H]-choline was present as phospholipid. [Methyl-11H]-choline incorporation was found to be related to S-phase fraction. In another experiment [Methyl-14C]-choline incorporation was found to correlate with [Methyl-3H]-thymidine incorporation. Km for choline transport in exponentially growing cells was significantly lower (26 mM) than in growth-inhibited cells (68 mM), whilst the Vmax for choline transport for exponentially growing cells (15.20 nmol/mg protein/10 min) was higher than for growth-inhibited cells (3.97 nmol/mg protein/10 min). Choline kinase activity was significantly higher in the faster growing cell populations. Conclusion: Choline incorporation into tumour cells under conditions that simulate rapid blood clearance of [Methyl-11C]-choline correlates with proliferation. Most of the activity (about 95%) was in the non-lipid fraction of the cell.