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Background: Multidrug resistance in cancer (MDR) occurs when tumours become crossresistant to a range of different anticancer agents. One mechanism by which MDR can be acquired is through cell to cell communication pathways. Membrane-derived microparticles (MPs) are emerging as important signaling molecules in this process. MPs are released from most eukaryotic cells and transfer functional proteins and nucleic acids to recipient cells conferring deleterious traits within the cancer cell population including MDR, metastasis, and angiogenesis. MP formation is known to be dependent on calpain, an intracellular cysteine protease which acts to cleave the cytoskeleton underlying the plasma membrane, resulting in cellular surface blebbing Objective: To establish the role of calpain in vesiculation in malignant and non-malignant cells by 1) comparing membrane vesiculation at rest and following the release of intracellular calcium, and 2) comparing vesiculation in the presence and absence of calpain inhibitor II (ALLM)., Method: This study examines the differences in vesiculation between malignant and non-malignant cells using high-resolution Atomic Force Microscopy (AFM). HBEC, MBE-F, MCF-7, and MCF- 7/Dx cells were analysed at rest and following treatment with calcium ionophore A23187 for 18 hours. Vesiculation of calcium activated and resting malignant and non-malignant cells was also assessed after 18 hour treatment of calpain inhibitor II (ALLM)., Results: We demonstrate that malignant MCF-7 and MCF-7/Dx cells have an intrinsically higher degree of vesiculation at rest when compared to non-malignant human brain endothelial cells (HBEC) and human mammary epithelial cells (MBE-F). Cellular activation with the calcium ionophore A23187 resulted in an increase in vesiculation in all cell types. We show that calpain-mediated MP biogenesis is the dominant pathway at rest in malignant cells as vesiculation was shown to be inhibited with calpain inhibitor II (ALLM)., Conclusion: These results suggest that differences in the biogenic pathways exist in malignant and non-malignant cells and have important implications in defining novel strategies to selectively target malignant cells for the circumvention of deleterious traits acquired through intercellular exchange of extracellular vesicles., (Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.org.)

Microparticles (MPs) are released from most eukaryotic cells after the vesiculation of the plasma membrane and serve as vectors of long and short-range signaling. MPs derived from multidrug resistant (MDR) cancer cells carry molecular components of the donor cell such as nucleic acids and proteins, and can alter the activity of drug-sensitive recipient cells through the transfer of their cargo. Given the substantial role of MPs in the acquisition and dissemination of MDR, we propose that the inhibition of MP release provides a novel therapeutic approach. This study characterises the effect of a panel of molecules known to act on MP-biosynthetic pathways. We demonstrate a differential effect by these molecules on MP inhibition that appear dependent on the release of intracellular calcium stores following activation with the calcium ionophore A23187. Calpain inhibitor, PD-150606; a selective inhibitor of Rho-associated, coiled-coil containing protein kinase (ROCK), Y-27632; and the vitamin B5 derivative pantethine, inhibited MP release only upon prior activation with A23187. Calpain inhibitor II showed significant inhibition in the absence of cell activation, whereas the vitamin B5 derivatives cystamine dihydrochloride and cysteamine hydrochloride showed no effect on MP inhibition under either condition. In contrast the classical pharmacological inhibitor of MDR, the calcium channel blocker Verapamil, showed an increase in MP formation on resting cells. These results suggest a potential role for calcium in the mechanism of action for PD-150606, Y-27632 and pantethine. These molecules, together with calpain inhibitor II have shown promise as modulators of MP release and warrant consideration as potential candidates for the development of an alternative therapeutic strategy for the prevention of MP-mediated MDR in cancer.