Targeting microparticle biogenesis: a novel approach to the circumvention of cancer multidrug resistance.

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.