Spatial Correlation of Constitutive HER2 Activities with Cell Membrane Deformation

Abstract

Approximately 20% of breast cancers (BC) are characterized by gene amplification and overexpression of the HER2 receptor tyrosine kinase. These tumors are clinically classified as HER2 “positive”, and they grow faster and more aggressively than tumors with normal HER2 levels. The poor prognosis for this BC subset is related to the constitutive activation of HER2 that results from its overexpression. However, how this activation occurs is not yet clear. In contrast to other closely related receptors, HER2 does not require ligand binding or conformational changes for activation. To gain a better understanding of this process, we developed a single molecule optical tracking method that can separate the diffusion states of individual HER2s that relate to their activation on living cells. We found that slow diffusion of HER2 correlates with the receptor activation. Using this information we constructed a map of the locations of “active” HER2s on the cell surface. We observed that at any given time, a greater fraction of HER2s were in “active” states in high HER2 expressing cells compared with cells that express lower levels. Interestingly, a large fraction of active HER2s were associated with the deformed membrane structures that are prevalent on cells that overexpress HER2s. This suggests that the membrane deformations caused by HER2 overexpression create localized conditions that enable constitutive receptor activation to take place.