The cytokine TGF-ß1 plays an important role in epithelial-mesenchymal transdifferentiation (EMT). In proximal tubular epithelial cells it induces formation of extracellular matrix proteins which can promote renal fibrosis. A hallmark for EMT and loss of epithelial function is the formation of stress fibers. An augmentation of cell stiffness according to stress fiber formation would conceivably contribute to transdifferentiation. The aim of the current study was to examine the mechanical properties of proximal tubular epithelial cells in response to TGF-ß1. The atomic force microscope (AFM) is a versatile tool for nano-imaging and mechanical analysis of cells. In rat kidney cells (NRK 52E) stimulated for 6 days with TGF-ß1, AFM was able to detect an increase in stress fiber formation which is typical for EMT. Moreover, anchoring sites were observed as nodular protrusions where bundles of stress fibers cross cell contacts. Concomitant to stress fiber formation the cells stiffened by 79% 25.6%. Furthermore, we developed a method to quantify the morphological alterations. A significant increase of cell junctional mass by a factor of 14 after TGF-ß1 stimulation could be detected. The rearrangement of cell junctions was associated with an altered distribution of N-cadherin as shown by immunofluorescence.

In summary, TGF-ß1-induced force generation might lead to formation of compensatory anchoring sites in cell junctions.