Effects of charged lipids on the physicochemical and biological properties of lipid-styrene maleic acid copolymer discoidal particles.

Styrene maleic acid copolymers (SMA) form discoidal lipid nanoparticles (lipid nanodisks) that mimic plasma high-density lipoproteins. We have previously prepared and characterized lipid nanodisks composed of SMA and the neutral phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). In the present study, we tested whether the surface charges can alter the physicochemical and biological properties of lipid-SMA discoidal particles. Unlike the case of DMPC alone, addition of saline to the buffer was necessary to induce the formation of lipid-SMA complexes containing either 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) or 1,2-dimyristoyl-3-trimethylammonium-propane (DMTAP), with formation efficiency being dependent on the concentration of charged lipids. After purification, DMPG- or DMTAP-containing discoidal particles with an approximate size of 10 nm were obtained in a manner similar to DMPC alone. Although DMPG and DMTAP appeared to be similarly incorporated into the lipid nanodisks, the zeta potentials of both particles were comparable. That is, no significant differences were observed in the physicochemical properties between the lipid-SMA nanodisks. Compared to DMPC-SMA nanodisks, the uptake of DMPG or DMTAP-containing discoidal particles by RAW264 cells was increased for both particle types, whereas in MDA-MB-231 cells, only DMTAP-containing discoidal particle uptake was increased. In addition, fluorescence microscopy revealed that lipid-SMA nanodisks are localized adjacent to the plasma membrane of RAW264 cells but in MDA-MB-231 cells they accumulated in the center of the cell. Furthermore, these particles caused cytotoxicity in a cell-type dependent manner, with high toxicity in MDA-MB-231. These results raised the possibility that compositional alterations in lipid-SMA discoidal particles may modulate biological reactions in vivo.