Growth factor receptors in cancer cells
Greater understanding of the spatial distribution of transmembrane receptor tyrosine kinases may provide new insights into signaling mechanisms controlling cell growth. Within our research program, we study the role of these receptors in cancer cells using the unique characterization possibilities provided by liquid-phase electron microscopy (LP-EM), which is capable of studying proteins within intact cells in their native liquid environment with nanometer resolution as needed to resolve the individual constituents of protein complexes. Our recent research involves members of the epidermal growth factor receptor (EGFR) family. HER2, one of the EGFR family members, is overexpressed in certain types of breast cancer. Although HER2 is considered an orphan receptor because it has no ligand, it can form homodimers, thereby contributing significantly to a dysregulation of intracellular signaling and of cell growth. We have studied the intra- and intercellular variation of HER2 in SKBR3 breast cancer cells. The unique experimental capabilities of LP-EM allowed the quantifying of the stoichiometry of HER2 complexes, distinguishing between monomers, dimers, and higher order clusters, while mapping their locations in the cell. Compared to biochemical methods providing information about the average of pooled cellular material, our methodology provides unique information at the molecular level in single cells.
Figure at the left: LP-EM image of HER2 labeled with quantum dots (QDs) in SKBR3 breast cancer cells. A schematic drawing reflecting both a labeled monomer an a labaled HER2 homodimer is overlaid. From: Sci. Adv. 1, e1500165 (2015)
Drug resistance development in HER2 overexpressing of breast cancer
One of our key interests is the identification of differences in protein function between individual cancer cells (cancer cell heterogeneity) and between distinct functional membrane regions within the same cell. With our methodological approach using LP-EM, it is now possible to study the effect of cancer drugs on small sub-populations of cells, aiming at identifying predictive markers for increasing the effectiveness of HER2 targeting drugs in personalized medical therapies. Small sub-populations of breast cancer cells were found to be irresponsive to the prescription drug trastuzumab than bulk cancer cells.
This research is funded by the Else Kröner-Fresenius-Stiftung in the project entitled “Investigation of the Influence of Breast Cancer Drugs on HER2 Dimerization at the Molecular Level in Individual Cells Aiming to Find Clues for Causes of Drug Resistance: HERe” in conjunction with the project partners Prof. Stefan Wiemann of the Deutsches Krebsforschungszentrum in Heidelberg, Dr. Diana Peckys, Biophysics Department, Saarland University, and Dr. Gilda Schmidt and Prof. Erich-Franz Solomayer of the Universitätsklinikum des Saarlandes.
Figure at the left: Light (A, B) – and electron microscopy (C, D) images of whole SKBR3 cells in liquid. LP-EM reveals the locations of individual QD-labeled HER2. From: Mol. Biol. Cell 28, 3193 (2017)
Role of HER2 in drug resistance development of gastric cancer
We also examine the role of HER2 in drug resistance development in gastric cancer in a project funded by the Deutsche Krebshilfe: MetGaP – Long-term response in Trastuzumab-treated metastatic gastric- or gastroesophageal junction cancer patients via molecular HER2 surface and pathway analyses” with the project partner Prof. Timo Gaiser, Institute for Pathology, Medical Center Mannheim. We analyze biopsy samples of gastric cancer patients for cell heterogeneity in the tumor tissue.
Figure at the left: LP-EM of cells obtained from patient biopsy samples. QD labeled HER2 proteins are visible as yellow spots. From: Mol. Med. 25, 42 (2019)