Mutant receptors

The measurement of ER has become a standard assay in the clinical management of breast cancer. The presence of ERa identifies those breast cancer patients with a lower risk of relapse and better clinical outcome. Receptor status also provides a guideline for those tumors that may be responsive to hormonal intervention. But only about half of ER-positive patients respond to hormonal therapies. Of those who respond initially, most will eventually develop an estrogen unresponsive disease following a period of treatment even though ERa is often still present. Mutant receptors and constitutively active r eceptors as well as hormone-independent activation of the ERa are discussed. The involvement of ER 3 isoforms is under investigation. 

Mutagenesis studies have shown that morphine and sufentanil bind differently to the jj, receptor [83, 85]. Mutation of an aspartic acid at residue 114 of the // receptor to an asparagine resulted in a mutant that did not bind morphine and morphine was ineffective in inhibiting adenylyl cyclase via that receptor. In contrast, sufentanil bound to the mutant and wild-type receptors equally well and it effectively inhibited cAMP accumulation via the mutant receptor. These findings demonstrate that morphine and sufentanil have different requirements for binding to the // receptor. By binding differentially, these two agonists may induce the ft receptor to interact with different G proteins to induce distinct cellular effects. 

After cysteine substitution, if the mutant receptor is expressed and has close to wild-type functional properties, then its three-dimensional structure will be close to wild type, and the substituted cysteine acts as a reporter for the environment surrounding the wild-type side chain (Karlin and Akabas, 1998 Javitch et al., 2002). 

Milligan, G., Stevens, P. A., Ramsay, D., and Mclean, A. J. (2002) Ligand rescue of constitu-tively active mutant receptors. Neurosignals. 11, 29-33. 

Mutant receptors that become blocked during processing in the endoplasmic reticulum or Golgi apparatus and thus never reach the plasma membrane. 

Mutant receptors that bind LDL at the cell surface but are blocked in endocytosis and thus do not internalize LDL. 

Several cancer cell types are characterized by expressing a truncated EGF receptor. The related viral oncogene, V-er B, also encodes a truncated receptor which lacks most of the extracellular domain (the EGF receptor is also known as C-erbB). Mutant receptors that display inappropriate constitutive activity can lead to cellular transformation, due to the continuous generation of mitogenic signal. 

A central and as yet poorly understood concept in the arena of gefitinib and erlotinib pharmacology is the cellular and biochemical basis for sensitivity to these inhibitors. Early functional analyses of the mutant receptors in cell-based kinase assays had suggested they exhibit an approximately ten-fold increased sensitivity to inhibition by gefitinib relative to wild-type EGFR (45,55). 

An extended (seven-sided) ternary complex model (Fig. 2B) was proposed by Samama et al. (1993) to accommodate mutant receptors that exhibited constitutive activity and to link receptor affinity with efficacy. This model includes the isomerization of the receptor between two conformational states, inactive (R) and active (R ), and only allows for the active R conformational state to interact with the G protein. Conceptually, the model allows for the receptor to toggle between on and off states where ligand or G protein manipulates the population size of these two conformational states, rather than affecting the activation strength of a particular conformational state. The different types of ligands influence... 

Expression levels of the mutant thrombin receptors were determined in transiently transfected COS cells by assaying the level of surface expressed thrombin receptors by FACS analysis, usually for mutant receptors that showed a significant diminution of function. COS cells were transfected by calcium phosphate-DNA co-precipitation30 with 10 ng of DNA. The transfected cells were analyzed by FACS 48 h after transfection. A monoclonal antibody specific for the amino terminal region was used as the primary antibody to detect thrombin receptors present on the surface of the cells.31 A FITC conjugated goat anti-mouse IgG was used as a secondary antibody. Expression levels of the mutant receptors were determined relative to expression levels of wild-type controls. Expression-level data (% of control) for some of the mutant receptors are listed in Table 2 (wild-type thrombin... 

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