The mechanism of magnetization transfer (MT) between water and components of the proton spectrum was studied ex vivo in a perfused cell system and in vivo in the rat brain (n ‫؍‬ 5). Water was selectively labeled and spectral buildup consequential to transfer of longitudinal magnetization was followed as a function of time. At short mixing time (T m ), nitrogen-bound solventexchangeable protons were observed, predominantly assigned to amide groups of proteins and peptides. At longer T m , intramolecular nuclear Overhauser enhancement (NOE) was observed in the aliphatic proton region, leading to a mobile-macromolecule-weighted spectrum that resembles typical protein spectra described in the literature. This effect on the proton spectrum is distinct from that of classical off-resonance MT, which has been shown to be due to the immobile solid-like proton pool. When studying a solution of major brain metabolites under physiological concentrations and conditions (pH), no transfer effects were observed, in line with expectations based on reduced NOE effects in rapidly tumbling molecules and the fast proton exchange rates of amino, amine, SH, and OH groups. In vivo magnetization transfer (MT) effects from irradiated protons associated with immobile macromolecules and membranes can occur to water (1-3) and to metabolites (4 -13). The mechanism of MT during off-resonance irradiation (i.e., not on water) has been well described for both instances. The semisolid lattice protons are efficiently saturated via intramolecular dipolar transfer over the complete spin system (spin diffusion), which is followed by intermolecular transfer to interacting smaller molecules.This intermolecular saturation transfer can in principle occur both via proton exchange and dipolar cross relaxation to short-lived bound molecules (Fig. 1), the descriptions of which are theoretically equivalent. However, the sequence of events after transfer of saturation to metabolites is not yet well understood. For instance, for creatine in excised rat muscle, the off-resonance saturation effect was found to be pH-independent in H 2 O and to remain unchanged in D 2 O, interpreted as excluding a contribution of exchangeable NH protons to the transfer process (4,13). However, on-resonance water saturation was found to increase the saturation transfer effect to metabolites (4,12,13). In addition, selective inversion of the water magnetization caused a transient reduction in the creatine resonance in vivo, but not in vitro, the mechanism of which is also unknown.In the present article we investigate the effect of water perturbation on ex vivo and in vivo metabolite spectra using a selective water-exchange-filter experiment (WEXfilter) (14,15) in which the path of MT of RF-labeled water protons can be followed as a function of time. Using a detection scheme that does not suppress exchangeable protons, it is possible to follow the interaction between water, exchangeable protons, and other molecular sites on a time scale of a few milliseconds to a second. The present...

Choline kinase is overexpressed in breast cancer cells and activated by oncogenes and mitogenic signals, making it a potential target for cancer therapy. Here, we have examined, for the first time, the effects of RNA interference (RNAi)-mediated down-regulation of choline kinase in nonmalignant and malignant human breast epithelial cell lines using magnetic resonance spectroscopy (MRS) as well as molecular analyses of proliferation and differentiation markers. RNAi knockdown of choline kinase reduced proliferation, as detected by proliferating cell nuclear antigen and Ki-67 expression, and promoted differentiation, as detected by cytosolic lipid droplet formation and expression of galectin-3. The functional importance of RNAi-mediated choline kinase down-regulation on choline phospholipid metabolism was confirmed by the significant reduction of phosphocholine detected by MRS. These results strongly support the targeting of choline kinase in breast cancer cells with RNAi and show the potential ability of noninvasive MRS to detect and evaluate future treatments incorporating such strategies. (Cancer Res 2005; 65(23): 11034-43)

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