Cell biomechanics and metastatic spreading: a study on human breast cancer cells
Despite the intensive research of the past decades in oncology, cancer invasion and metastasis still represent the most important problem for treatment and the most common cause of death in cancer patients. Metastasis refers to the spread of malignant cells from a primary tumour to distant sites of the body and the adaptation of these cancer cells to a new and different tissue microenvironment. Usually, millions of cells can be released by a tumour into the circulation every day, but only a tiny minority of these cells are able to reach and colonize a distant organs: the utter inefficiency of the metastatic process implies that cells might strongly need biomechanical alterations that allow them to invade and colonize different tissues. The hypothesis that cellular biomechanics may play a significant role in tumour genesis and cancer invasion, gains every day more and more support: therefore characterizing these properties in connection with the membrane and cytoskeleton organization could be very important for understanding better the migration mechanisms and to develop new diagnostics and therapeutics tools. The goal of our study was the mechanical characterization of cell lines chosen as model of cancer progression using different biophysical techniques and the correlation of the mechanical properties with possible alterations of the cytoskeleton structure and plasma membrane composition. We used a custom built Optical Tweezers to extract the local viscoelastic properties of the cell plasma membrane, an Atomic Force Microscopy (AFM) to locally measure cell elasticity of cells, and a Microfluidic Optical Stretcher to measure the deformability of cells as whole bodies. We investigated then the actin organization of the cytoskeleton by STimulated Depletion and Emission (STED) and confocal microscopy. The lipid composition of cells was analysed by MALDI-mass spectroscopy in order to correlate the mechanical alterations of cells with alteration at the cytoskeleton and plasma membrane level. The cell lines analyzed derive from breast tissue and represent a model of human epithelial cells towards malignancy. In particular, two cell lines -MDA-MB-231 and MCF-7- provided by American Type Culture Collection (ATCC) were originally derived from breast cancers patients with different level of cancer aggressiveness. Cells were chosen according to the nowadays accepted classification of breast cancer based on gene expression pattern and proteomic expression, which divide breast cancers in subtypes that differ in terms of risk factor, distribution, prognosis, therapeutic treatment responsiveness, clinical outcomes and survival. The third cell line, HBL-100, is an immortalized but non-neoplastic cell line derived from the milk of a nursing mother with no evidence of breast lesions, representing a earlier stage of the cell transformation. A pulling membrane tether approach by means of Optical Tweezers has been chosen since it allows an accurate quantitative characterization of local viscoelastic properties of plasma membranes. Bovine Serum Albumine (BSA) coated silica beads were used to bind the plasma membrane and grab membrane tethers of several microns measuring the force exerted on the bead. By fitting with the Kelvin body model our force-elongation curves obtained by experimental data we extracted the parameters of interest: tether stiffness, membrane bending rigidity, and tether viscosity. We observed that lower values of tether stiffness and membrane bending rigidity corresponded to cells associated to a higher aggressive behaviour, while viscosity showed an inverse tendency. We also probed elasticity of the cells using by indentation experiments with AFM. We used a bead probe attached to the cantilever and measured the Young Modulus. The results obtained could not clearly discriminate the three cell types in terms of elasticity. Cell deformability was further investigated by means of Microfluidic Optical Stretcher. Cells in suspension were trapped by two counter propagating laser beams of low intensity from two optical fibers. Adjusting the intensity of the laser light, the forces acting on the cell surface increased, leading to a measurable elongation of the cell body along the laser beam axis. With MOS we were able to discriminate between cancer and control cells lines, while differences between the two cancer cell lines were not significant. However a trend could be observed: lower aggressive tumour cells were more resistant to deformation compared to the higher aggressive tumour cells. We investigated the cells cytoskeleton structure by STED and confocal microscopy confirming that malignancy involves cytoskeleton structure alterations. Differences in the organization. of the actin filaments and in the presence of actin drifts were observed. We peformed also a preliminary analysis of the cell lipid composition by MALDI MASS spectroscopy. We could observe that highly aggressive cells with softer membranes presented alterations at the level of Phosphatidylethanolamines (PEs) and Phosphatidylinositoles (PIs). The work of this thesis is partially published in the article “Custom Built Optical Tweezers for locally probing the viscoelastic properties of cancer cells” in the International Journal of Optomechatronics (June 2011). A second article including the comparative results of the biomechanical analysis on the breast cell lines is in preparation.