Effect of chain length on cytotoxicity and endocytosis of cationic polymers

We investigated the effect of chain length on the cytotoxicity and endocytosis of rhodamine B end-labeled cationic linear poly(2-(N,N- dimethylamino)ethyl methacrylate) (RhB-PDMAEMA) polymers [M_w = (1.1-4.8) × 10⁴ g/mol and M_w/M_n ∼ 1.2], which were synthesized via atom transfer radical polymerization by using rhodamine B-based initiator. The 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide (MTT), lactate dehydrogenase (LDH), and caspase-3/7 assays revealed that the cytotoxicity and cellular membrane disruption of HepG2 cells induced by PDMAEMA depend on both the polymer concentration and chain length, and the apoptosis depends on chain length at the concentration of 37.6 μg/mL. In the concentration range 10-110 μg/mL, PDMAEMA chains with different lengths are cytotoxic to HepG2 cells by different mechanisms. Namely, (1) for short PDMAEMA chains [M_w = (1.1-1.7) × 10⁴ g/mol], the cytotoxicity, membrane disruption, and apoptosis are very low, independent of the chain length; (2) in the medium range (1.7 × 10 ⁴ < M_w < 3.9 × 10⁴ g/mol), the cytotoxicity increases with the chain length and polymer concentration, mainly due to the cooperative effect of membrane disruption and apoptosis; and (3) for long chains [M_w = (3.9-4.8) × 10⁴ g/mol], they are very disruptive to the cellular membrane, pro-apoptotic, and able to enter the cytoplasm and nucleus faster than short chains, as revealed by the real-time confocal laser scanning microscopy images, and their much higher cytotoxicity is independent of the PDMAEMA chain length.