*2.1. Materials*

Mouse anti-megalin monoclonal antibody was purchased from Abcam (Cambridge, MA, USA). Goat anti-cubilin polyclonal antibody was purchased from Santa Cruz Biotechnology (Dallas, TX, USA). Rabbit anti-transferrin receptor polyclonal antibody was purchased from Abnova (Taipei, Taiwan). Rabbit anti-β-actin polyclonal antibody, rabbit anti-Early Endosome Antigen 1 (EEA1) antibody, anti-rabbit IgG HRP-linked antibody, anti-mouse IgG HRP-linked antibody, and anti-goat IgG HRP-linked antibody were purchased from Cell Signaling Technology (Danvers, MA, USA). Alexa 555 anti-rabbit IgG antibody was purchased from Invitrogen (Carlsbad, CA, USA). Albumin-fluorescein isothiocyanate conjugate was purchased from Sigma-Aldrich (St. Louis, MO, USA), and Alexa Fluor® 488-conjugated ChromPure Mouse Transferrin was purchased from Jackson ImmunoResearch Laboratories (West Grove, PA, USA). Immortalized human renal proximal tubular epithelial cells (hRPTECs: CRL-4031) was obtained from ATCC (American Type Culture Collection, Manassas, VA, USA).
