Sonodynamic effects of hematoporphyrin monomethyl ether on CNE-2 cells detected by atomic force microscopy

Hematoporphyrin monomethyl ether (HMME) has been effectively used to treat solid tumors of some types. However, its application in nasopharyngeal carcinoma has not been studied yet. In this paper, the detailed sonodynamic effects of HMME-SDT (sonodynamic therapy) on CNE-2 cells including cell growth inhibition, apoptosis induction, and membrane toxicity were investigated. It was found that HMME alone had less cytotoxicity whereas HMME-SDT could suppress the cell proliferation in a dose-dependent manner as detected by MTT assay. The annexin V-based flow cytometric data indicated that upon SDT, different concentrations of HMME induce distinct types of cell death, apoptosis by low concentration (60 µg/ml) of HMME and necrosis by higher concentration (120 µg/ml). The immunofluorescence of cytoskeleton and nuclei morphology showed that upon HMME-SDT, the cells became rounding and the cytoskeletal network disappeared, and, the nuclei represented a total fragmented morphology of nuclear bodies. These alternations showed the apoptosis induction by HMME-SDT. Further AFM study showed that the cell membrane structure and cytoskeleton networks were destroyed, and, the Young's modulus, tip-cell-surface adhesion force decreased to 0.22 ± 0.11 Mpa, 35.4 ± 12.8 pN of cells with 120 µg/ml HMME-SDT from 0.48 ± 0.21 Mpa, 69.6 ± 22.3 pN of native cells, respectively. These membrane changes caused the collapse of mitochondrial transmembrane potential and disturbance of intracellular calcium homeostasis, which was consistent with the results detected by flow cytometry. Therefore, membrane toxicity and cytoskeleton disrupture induced by HMME-SDT maybe important factors to induce cell apoptosis, and, the disturbance of mitochondrial transmembrane potential and calcium channels might be the apoptosis mechanisms.