Poorly invasive MCF7 breast cancer cells become more invasive after a knockdown of heterochromatin protein 1a (HP1α) expression, and in many solid tumors, the loss of HP1α correlates with the onset of tumorcell invasion. In this work, the mechanical properties of nuclei isolated from MCF7 cells with a constitutive knockdown of HP1α expression are studied by using three mechanical measurement techniques that deform the nuclei in very different ways. They probe the outer regions of the nuclei (~100 nm) with forces on the order of ~10 pN (optical tweezers, OT) or ~1 nN (atomic force microscopy, AFM), or the bulk properties with whole-of-nuclei deformations (micropipette).
The cover image for the July 6 issue of Biophysical Journal is an artistic rendering of a microbead probing an extracted nucleus, created by Raoul Solomon, whose MSc work focuses on how the loss of HP1α alters nuclear integrity. The image reflects our exploration of the mechanical properties of nuclei isolated from malignant MCF7 breast cancer cells with reduced HP1α. The nuclear periphery with chromatin, lamina, and membrane is shown, while the mechanical studies are represented by the bead pushing into the nuclear membrane.
The work depicted was inspired by a collaboration between two groups within the School of Fundamental Sciences at Massey University, New Zealand: the Chromatin Research Group, whose focus is on HP1α dysregulation during tumorigenesis; and the Biophysics and Soft Matter Group, whose interests include developing measurement capability by using OT. Further collaborations with Dr. Gleb Yakubov and Prof. Geoff Willmott’s Dynamic Microfluidics Research group at the University of Auckland, facilitated by the MacDiarmid Institute, added AFM and micropipette-based techniques to our arsenal.
From a physical perspective, by using AFM or OT to locally indent nuclei, those extracted from MCF7 HP1α knockdown cells were found to have apparent Young’s moduli that were significantly lower than nuclei from MCF7 control cells, consistent with previous studies that assert that heterochromatin plays a major role in governing the mechanical response in such experiments. From the cell biologist perspective, we are interested in understanding how these changes in nuclear integrity contribute to tumor progression and metastasis. It is our hope that the changes in the nuclear periphery that result in altered nuclear elasticity might represent new targets for anti-cancer therapies that may render invasive cells more vulnerable.
- Susav Pradhan, Raoul Solomon, Ankita Gangotra, Gleb Yakubov, Geoff Willmott, Catherine Whitby, Tracy Hale, and Martin Williams