Cell-permeable peptides induce dose- and length-dependent cytotoxic effects

We have explored the threshold of tolerance of three unrelated cell types to treatments with potential cytoprotective peptides bound to Tat4s-57 and AntP(43-58) cell-permeable peptide carriers. Both Tat(48-57) and AntP(43-58) are well known for their good efficacy at crossing membranes of different cell types, their overall low toxicity, and their absence of leakage once intemalised. Here, we show that concentrations of up to 100 mu M of Tat(48-57) were essentially harmless in all cells tested, whereas AntP(43-58) was significantly more toxic. Moreover, all peptides bound to Tat(48-57) and AmP43-58 triggered significant and length-dependent cytotoxicity when used at concentrations above 10 mu M in all but one cell types (208F rat fibroblasts), irrespective of the sequence of the cargo. Absence of cytotoxicity in 208F fibroblasts correlated with poor intracellular peptide uptake, as monitored by confocal laser scanning fluorescence microscopy. Our data further suggest that the onset of cytotoxicity correlates with the activation of two intracellular stress signalling pathways, namely those involving JNK, and to a lesser extent p38 mitogen-activated protein kinases. These responses are of particular concern for cells that are especially sensitive to the activation of stress kinases. Collectively, these results indicate that in order to avoid unwanted and unspecific cytotoxicity, effector molecules bound to Tat(48-57) should be designed with the shortest possible sequence and the highest possible affinity for their binding partners or targets, so that concentrations below 10 mu M can be successfully applied to cells without hann. Considering that cytotoxicity associated to Tat(48-57)(-) and AntP(43-58) bound peptide conjugates was not restricted to a particular type of cells, our data provide a general framework for the design of cell-penetrating peptides that may apply to broader uses of intracellular peptide and drug delivery. (C) 2007 Elsevier B.V. All rights reserved.