The epidermal growth factor receptor HER2 is overexpressed in 20% of breast cancer
cases. HER2 is an orphan receptor that is activated ligand-independently by homodimerization.
In addition, HER2 is able to heterodimerize with EGFR, HER3, and HER4. Heterodimerization has
been proposed as a mechanism of resistance to therapy for HER2 overexpressing breast cancer. Here,
a method is presented for the simultaneous detection of individual EGFR and HER2 receptors in the
plasma membrane of breast cancer cells via specific labeling with quantum dot nanoparticles (QDs).
Correlative fluorescence microscopy and liquid phase electron microscopy were used to analyze the
plasma membrane expression levels of both receptors in individual intact cells. Fluorescent single-cell
analysis of SKBR3 breast cancer cells dual-labeled for EGFR and HER2 revealed a heterogeneous
expression for receptors within both the cell population as well as within individual cells. Subsequent
electron microscopy of individual cells allowed the determination of individual receptors label
distributions. QD-labeled EGFR was observed with a surface density of (0.5–5) × 101 QDs/µm2
,
whereas labeled HER2 expression was higher ranging from (2–10) × 102 QDs/µm2
. Although most
SKBR3 cells expressed low levels of EGFR, an enrichment was observed at large plasma membrane
protrusions, and amongst a newly discovered cellular subpopulation termed EGFR-enriched cells