Both observations indicate that at low stoichiometric concentrations of ERBB3 and ERBB2, heterodimerization is inefficient and most ERBB3 remains in clusters of ERBB3, despite the high thermodynamic stability of ligand-bound heterodimers of ERBB2 and ERBB3
Both observations indicate that at low stoichiometric concentrations of ERBB3 and ERBB2, heterodimerization is inefficient and most ERBB3 remains in clusters of ERBB3, despite the high thermodynamic stability of ligand-bound heterodimers of ERBB2 and ERBB3. Previous studies had suggested that the heterodimerization of ERBB3 Araloside X and ERBB2 occurs under conditions Araloside X at which direct ERBB3 interactions are destabilized by ligand binding, yet our current study indicates that A30 cross-linking is detecting the proximity of ERBB3 receptors beyond direct ECD interactions and in the presence of ligand. signaling incompetent probe of its immediate receptor environment. This approach detects receptor clustering of endogenous ERBB3 in the breast cancer cell line Rabbit polyclonal to PDGF C MCF7 at levels as low as 25000 receptors per cell and at aptamer concentrations as low as 20 nM. Our analysis also indicates that ERBB3 receptors are apparently segregated from ERBB2 receptors in their resting state, and both ligand-activated ERBB3 and ERBB2 do not share the same microenvironment as inactive ERBB3. In recent years, the selection of nucleic acid aptamers by SELEX has emerged as a powerful route to macromolecules that exhibit high affinities and high specificity comparable to those of antibodies (1,2). Aptamers are on average one-tenth the size of antibodies and are in their final format usually within range of fully synthetic production. This provides a broad range of options for site-specific chemical modification. Significant efforts, in terms of both time and cost, have to be applied to create aptamers that are chemically stable and sufficiently Araloside X resistant to nucleases for use as therapeutics or related applications in whole organisms. However, to be used as a diagnostic tool in vitro, both in solution and in cell culture, RNA aptamers can be readily stabilized by the simple addition of RNase inhibitors to experiments. For example, RNA Araloside X aptamers have been selected for distinguishing cell lines on the basis of the presence or absence of clinically relevant biomarkers (35). We wanted to use an existing and inhibitory aptamer against a known cell surface target, the ERBB31receptor, to create a probe that can evaluate the microenvironment of ERBB3 in a live cell setting. The probe, which ultimately consists of both the targeted and inhibited receptor and the attached photo-cross-linkable aptamer, should preferably remain neutral during signaling to distinguish changes in its microenvironment from changes that occur to the status of the probe itself. In other words, we wanted to convert an ERBB3 receptor into a passive bystander species, capable of reporting on the status of its immediate surrounding beyond the well-documented stabilization of receptor heterodimers by ligand. ERBB receptors are cell surface receptor tyrosine kinases, and the human ERBB family consists of the EGF receptor (EGFR or ERBB1) and its homologues, ERBB2, ERBB3, and ERBB4 (also termed HER2/neu, HER3, and HER4, respectively). The protein interactions in the ERBB system in the presence and absence of ligand are very complex, and many aspects of it are still poorly understood. Additional questions arise when data derived in vitro from soluble segments of the receptors are translated to a live cell membrane setting. One question is the presence and role of higher-order receptor associations of either activated or resting receptors in signaling. We specifically wanted to evaluate whether the previously observed self-association of soluble ERBB3 extracellular domains (ECDs) and overexpressed recombinant cellular ERBB3 can be confirmed for endogenous and nonoverexpressed ERBB3 receptors, whether such cellular preassociation or clustering in the absence of ligand involves coclustered ERBB2, and whether the activation of ERBB3 by ligand results in the recruitment of ERBB2 into such clusters of inactive ERBB3. Alternatively, activated ERBB2 and ERBB3 receptors may be spatially segregated from clusters of inactive ERBB3. We have previously reported the selection of SELEX-derived RNA aptamers that bind the extracellular domains of ERBB3 with high specificity. The most potent aptamer (A30) binds in the low nanomolar range and interferes with ligand-induced signaling, but without competition for the ligand binding site of ERBB3 (6). A30 represented a good candidate aptamer to be targeted to ERBB3 and thereby convert the aptamer-tagged and inactive receptor into a passive probe of its microenvironment. While little is known about the extent and function of higher-order association states of ERBB receptors, a considerable body of work exists on the control and structure of receptor dimers, which are at the core of activation of ERBB receptors. Within the ligand-activated dimer, tyrosine phosphorylation on the cytoplasmic side of the receptors is critical to the transmission of signal (7). For EGFR, the respective ligands are EGF and EGF-like ligands such as TGF. ERBB4 and ERBB3 bind isoforms of neuregulin, a large family of EGF-related ligands. Isoforms of neuregulin 1 are also called heregulins. All ERBB3 receptors are capable of forming heterodimers with other ERBB family members, and indeed, ERBB3, the only kinase-deficient member in this family (8), relies exclusively on heterodimerization with ERBB1, ERBB2, or ERBB4 for signaling (9,10). The preferred heterodimerization partner for ERBB3 is ERBB2, which in turn stands out by being the only member of the Araloside X family lacking any identified.