Active transport, tumors

It has been found that a similar type of self-regulatory active transport also operates in mammalian cells, in this case in the Ehrlich ascites tumor cell (41). These cells show an active concentrative ability for glucose at low ambient glucose concentrations. This enables the cell to... 

Schafer, J.A. Heinz, E. (1971). The effect of reversal of Na+- and K+-electrochemical potential gradients on the active transport of amino acids in Ehrlich ascites tumor cells. Biochim. Biophys. Acta 249,15-33. 

Energy-dependent, carrier-mediated uptake of thyroid hormone has been demonstrated in a variety of other cell types, including fibroblasts [109,110], pituitary tumor cells [111] and muscle cells [112]. In addition to the uptake processes in the plasma membrane, indirect evidence has been reported recently for active transport of T3 between subcellular compartments, i.e., from the cytoplasm to the nu-... 

Like their pyrimidine counterparts, these 3-halogenated adenosine-based nucleosides undergo conversion to the active triphosphate nucleotides (Fig. 42.25) after active transport into tumor cells. All are initially phosphorylated by... 

The characterization of a mitoxantrone-resistant breast cancer cell line that displayed an efflux-based MDR phenotype without expression of Pgp or MRPl led to the discovery of BCRP. This transporter, also known as mitoxantrone resistance protein (MXR) or placenta-specific ABC transporter (ABCP), is a half-transporter acting as a homo- or heterodimer to form an active transporter [22]. The substrate specificity of BCRP overlaps somewhat with that of Pgp and MRPl, suggesting a similar role in the pharmacokinetic of chemotherapeutic drugs. In fact, the overexpression of BCRP in tumor cells confers resistance to mitoxantrone, topotecan, SN38, flavopiridol, doxorubicin, bisantrene, etoposide, and methotrexate [16]. 

It is generally assumed that the inward movement of potassium is passive and is in some way coupled to the expulsion of sodium ions. However, active potassium transport has been observed in the red cell, and tumor cells appear to lose their permeability to potassium and require an active transport mechanism to maintain the intracellular potassium concentration. 

Choline, an amino alcohol, is the precursor of phospholipids, which are ubiquitous components of cell membranes. Choline can be labeled with "C (50) or F (51,52) and is taken up by tumor cells via an active transport mechanism. It is subsequently trapped through phosphorylation by choline kinase to... 

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