Stimulation of albumin endocytosis by cationized ferritin in cultured aortic smooth muscle cells

Anionic microdomains within the aortic smooth muscle cell (SMC) surface glycocalyx represent a potential barrier to the endocytosis of anionic plasma proteins. Cultured SMCs exposed briefly to cationized ferritin (CF) exhibit ultrastructural aggregations of surface anionic sites resulting in intervening areas essentially devoid of anionic charge. Preincubation of cultured aortic medial SMCs with 0.2 mg/ml CF for 1 minute at 37 C resulted in a 4-fold increase in binding and a 13-fold increase in internalization of ¹²⁵I-human serum albumin (¹²⁵I-HSA) relative to cells pretreated with native ferritin. When both the CF preincubation and the endocytosis were performed at 4 C, the influence of CF was abolished. Studies at 4 C indicated that CF pretreatment of SMC at 37 C induced high affinity (K(d) = 1.5 nM) saturable ¹²⁵I-HSA binding, in addition to low-affinity nonsaturable binding. These results were further confirmed by binding competition studies using increasing concentrations of unlabeled HSA. In contrast, low-density lipoprotein, a large anionic molecule, failed to compete with ¹²⁵I-HSA for binding sites on CF-pretreated SMCs at either 4 or 37 C. Pulse-chase studies at 37 C indicated that 20-30% of internalized ¹²⁵I-HSA was degraded, and 40-50% exocytosed within 24 hours in CF-treated cells. CF pretreatment of the SMCs did not significantly enhance the uptake of ¹⁴C-sucrose as a measure of fluid-phase endocytosis at 30 and 60 minutes. The results of these studies emphasize the potentially important regulatory roles of cell-surface anionic charge distribution and cationic molecules in cellular endocytosis.