Process characterization of epithelial–mesenchymal transition in alveolar epithelial type II cells using surface-enhanced Raman scattering spectroscopy

Epithelial–mesenchymal transition (EMT), a process by which epithelial cells undergo a phenotypic conversion that gives rise to fibroblasts and myofibroblasts, plays an important role in tissue repair and fibrosis, chronic inflammation, cancer invasion and metastasis. Therefore, increasing attention has focused on the study of EMT. However, it is still a challenge to distinguish the undifferentiated and differentiated epithelial cells, which are closely related and morphologically similar. In this study, we employed the transactivator of transcription (TAT)-functionalized AuNSs as intracellular surface-enhanced Raman scattering (SERS) probes for characterizing the EMT process in alveolar epithelial type II (ATII) cells, induced by bleomycin (BLM). CCK8 assay and reliable aggregation tests indicated a low cytotoxicity and high stability of the intracellular SERS probes. The internalization of the TAT-functionalized AuNSs acting on the ATII cells was verified by transmission electron microscopy (TEM), fluorescence images, and energy-dispersed spectroscopy (EDS). SERS spectra were analyzed by principal component analysis (PCA), which was able to distinguish between ATII cells at different stages of EMT and monitor the cellular biochemical composition changes in this process. In addition, the expression of epithelial and fibroblastic markers on ATII cells suggested that incubation with BLM could induce ATII cells to differentiate into fibroblasts. The SERS detection of the internalized TAT-functionalized AuNSs provides a continuous, non-invasive, and label-free characterization for the EMT process.