Pgp and cell volume control (Idriss, 2000). In order to prevent lysis, most cells have a mechanism for regulatory volume decrease (RVD). This often involves activation or inactivation of K or Cl channels. Pgp has been connected to regulation of cell volume, functioning like the Cl- channel which is defective in cystic fibrosis, the CFTR. As Pgp is structurally related to CFTR and Pgp expression is upregulated in cells which lack CFTR, it appears as though Pgp may be involved with cellular volume control. A model was proposed for this aspect of the role of Pgp in which the ATP pump which usually functions to efflux drugs can be modified so that it serves as a Cl channel under hyposmotic stress (Figure 28). This was supported by the fact that cell swelling inhibits Pgp drug pump activity, while the presence of intracellular cytotoxic compounds prevents Cl transport.

Figure 28

Pgp Ion Channel Activation (Idriss, 2000). If Pgp does have a dual role as an ion channel, there must be a means by which the protein alternates between being a membrane pump and a membrane channel. Like many other cellular activities, phosphorylation is a likely factor for such a functional shift. However, conclusive proof for the dual function of Pgp has yet to be substantiated. In some experiments phosphorylation appeared to have no effect upon drug efflux, while in different studies a direct link between kinase concentrations and Pgp drug efflux activity was determined. To further complicate matters, kinase inhibitors appear to play a role in inhibiting Cl channels. Available experimental evidence is frustratingly circular and incomplete, as kinase activity simultaneously does and does not effect both Pgp drug efflux and Cl channel activity. As a result, pending further experimental evidence, it is generally regarded that Pgp acts as a regulator of other ion channels rather than serving as an ion channel itself.

Alternative phosphorylation model (Idriss, 2000). An interesting hypothesis has been proposed which combines the conflicting results related to Pgp phosphorylation and Pgp ion channel function. Phosphorylation of Pgp on specific tyrosine or serine/threonine residues is known to play a role in modulating Pgp function. PKC, as previously discussed, appears to be the most prominent protein kinase involved with Pgp regulation. However, a variety of isozymes of PKC including protein kinase A (PKA) and casein kinase II (CKII) are also capable of phosphorylating available Thr, Ser, or Tyr residues of Pgp. Many of these enzyme isozymes are specific for particular amino acid residues, and while only a few of these sites appear to be critical for ATP-dependent drug efflux, the presence of multiple phosphorylation sites supports a multifunctional role of Pgp. In such a model, the site of phosphorylation may deterermine the eventual function of Pgp from apoptotic signalling, regulation of osmotic pressure or drug efflux. This would explain the apparently contradictory experimental evidence. Phosphorylation in a MDR cell line at amino acid residues which control drug efflux would not have an effect upon experimentally observed drug efflux because the MDR phenotype of the cells means the drug efflux role is already active. However, in contrast, phosphorylation at a residue which controls ion transport would have an observable effect, and kinase inhibitors for this particular phosphorylation would prevent the shift between drug efflux and ion transport. The phosphorylation cascade involving Pgp is much more complicated than previously thought, and apparently conflicting results may be explained by the specificity of enzyme isozymes. Additional research is necessary to adequately support this theory, but it remains an attractive possibility since it successfully integrates available information.

Applications of the alternative theory. Further information regarding the interaction between protein kinases and Pgp is of immense interest to the cancer community. One of the chief problems with treating cancer is the fact that tumor cells are autogenous - they arise from normal, "self" tissue. This makes it very difficult to destroy the tumor cell without destroying normal cells in the process. Both chemotherapy and radiation subject the patient to a barrage of highly toxic compounds in hopes that the tumor will die before the patient does. A more specific means of treating cancer cells is highly desirous as it would eliminate the caustic side effects of standard chemotherapy and radiation protocols. While inhibiting all protein kinases would have lethal effects upon an organism, the ability to identify and inhibit only the particular isozyme which is conferring MDR upon a tumor offers a potentially specific, sensitive and side-effect-free means of treating cancer.