A challenge in advanced
drug delivery is selectively traversing the plasma membrane, a barrier that prohibits the intracellular delivery of most
peptide and
nucleic acid-based
therapeutics. A variety of short amino acid sequences termed
protein transduction domains (PTDs) first identified in
viral proteins have been utilized for over 20 years to deliver
proteins nondestructively into cells, however, the mechanisms by which this occurs are varied and cell-specific. Here we describe the results of live cell imaging experiments with
AZX100, a cell-permeable anti-fibrotic
peptide bearing an "enhanced" PTD (PTD4). We monitored fluorescently labeled
AZX100 upon cell surface binding and subsequent intracellular trafficking in the presence of cellular process inhibitors and various well-defined fluorescently labeled cargos. We conclude that
AZX100 enters cells via caveolae rapidly, in a manner that is independent of
glycoconjugates, actin/microtubule polymerization,
dynamins, multiple
GTPases, and
clathrin, but is associated with
lipid rafts as revealed by methyl-beta-cylodextrin.
AZX100 treatment increases the expression of phospho-
caveolin (Y14), a critical effector of focal adhesion dynamics, suggesting a mechanistic link between
caveolin-1 phosphorylation and actin cytoskeleton dynamics. Our results reveal novel and interesting properties of PTD4 and offer new insight into the cellular mechanisms facilitating an advanced
drug delivery tool.