The use of membrane-permeable peptides as carrier vectors for the intracellular delivery of various proteins and macromolecules for modifying cellular function is well documented. Arginine-rich peptides, including those derived from human immunodeficiency virus 1 Tat protein, are among the representative classes of these vectors. The internalization mechanism of these vector peptides and their protein conjugates was previously regarded as separate from endocytosis, but more recent reevaluations have concluded that endocytosis is involved in their internalization. In this report, we show that the uptake of octa-arginine (R8) peptide by HeLa cells was significantly suppressed by the macropinocytosis inhibitor ethylisopropylamiloride (EIPA) and the F-actin polymerization inhibitor cytochalasin D, suggesting a role for macropinocytosis in the uptake of the peptide. In agreement with this we observed that treatment of the cells with R8 peptide induced significant rearrangement of the actin cytoskeleton. The internalization efficiency and contribution of macropinocytosis were also observed to have a dependency on the chain length of the oligoarginine peptides. Uptake of penetratin, another representative peptide carrier, was less sensitive to EIPA and penetratin did not have such distinct effects on actin localization. The above observations suggest that penetratin and R8 peptides have distinct internalization mechanisms.

Nonviral gene complexes can enter mammalian cells through different endocytic pathways. For efficient optimization of the gene carrier it is important to profile its cellular uptake, because this largely determines its intracellular processing and subsequent transfection efficiency. Most of the current information on uptake of these gene-delivery vehicles is based on data following the use of chemical inhibitors of endocytic pathways. Here, we have performed a detailed characterization of four commonly used endocytosis inhibitors [chlorpromazine, genistein, methyl-beta-cyclodextrin (MbetaCD), and potassium depletion] on cell viability and endocytosis in five well-described cell lines. We found that chlorpromazine and to a lesser extent MbetaCD significantly decreased cell viability of some cell lines even after short incubation periods and at concentrations that are routinely used to inhibit endocytosis. Through analyzing the uptake and subcellular distribution of two fluorescent endocytic probes transferrin and lactosylceramide (LacCer) that are reported to enter cells via clathrin-dependent (CDE) and clathrin-independent (CIE) mechanisms, respectively, we showed poor specificity of these agents for inhibiting distinct endocytic pathways. Finally, we demonstrate that any inhibitory effects are highly cell line dependent. Overall, the data question the significance of performing endocytosis studies with these agents in the absence of very stringent controls.

Activation of myofibroblast rich stroma is a rate-limiting step essential for cancer progression. The responsible factors are not fully understood, but TGFβ1 is probably critical. A proportion of TGFβ1 is associated with extracellular nano-vesicles termed exosomes, secreted by carcinoma cells, and the relative importance of soluble and vesicular TGFβ in stromal activation is presented. Prostate cancer exosomes triggered TGFβ1-dependent fibroblast differentiation, to a distinctive myofibroblast phenotype resembling stromal cells isolated from cancerous prostate tissue; supporting angiogenesis in vitro and accelerating tumour growth in vivo. Myofibroblasts generated using soluble TGFβ1 were not pro-angiogenic or tumour-promoting. Cleaving heparan sulphate side chains from the exosome surface had no impact on TGFβ levels yet attenuated SMAD-dependent signalling and myofibroblastic differentiation. Eliminating exosomes from the cancer cell secretome, targeting Rab27a, abolished differentiation and lead to failure in stroma-assisted tumour growth in vivo. Exosomal TGFβ1 is therefore required for the formation of tumour-promoting stroma.

The exact mechanisms by which arginine-rich cell-penetrating peptides enter cells are still the subject of debate. Here, we have analyzed in detail the effects of serum and extracellular concentration on the internalization of oligoarginines (R n; n = 4, 8, 12, 16). The presence of serum in the incubation medium had a major influence on the uptake of R12 and R16 peptides but did not affect the uptake of R4 and R8 significantly. Incubation of cells at 37 degrees C with R12 and R16 peptides in serum-containing medium showed that the majority of labeling was confined to punctate endocytic structures. Performing the same experiments in serum-free media led to a dramatic increase in cytosolic labeling, and similarly diffuse R12 and R16 labeling was observed in cells treated with peptides at 4 degrees C. This suggests, in both cases, that the peptides were entering via a nonendocytic mechanism. Further studies on R12 peptide suggest that the initiation of nonendocytic uptake and cytosolic labeling is also dependent on serum concentration and extracellular peptide concentration. At relatively low concentrations, the peptide labels endocytic structures, but upon raising the peptide concentration, the fraction labeling the cytosol increases dramatically and this accompanies a nonlinear increase in total cellular fluorescence. Membrane-associated proteoglycans also contribute to increasing the peptide concentration at the cell surface by enhancing their recruitment via electrostatic interactions. These results demonstrate that uptake mechanisms of these compounds are highly dependent on both the presence of serum and the effective extracellular peptide concentration.