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256 Cell-Penetrating Peptides: Processes and Applications
activity of the respective proteins. Modulation of G-protein activity by transportan
can be considered a drawback for a peptide-type delivery vector. However, the
concentration of transportan used in cell delivery experiments is an order of magnitude
lower than what is necessary to influence G-protein activity. 42
The predecessor of transportan, galparan ( 13 Pro-transportan) possesses activity
in different experimental systems. Galparan activates Na + ,K + -ATPase, 43 and induces
acetylcholine release in the frontal cortex 44 and release of insulin from rat pancreatic
β-cells. 45 Transportan can probably also have some analogous activity in these assays
because of very similar structure to that of galparan. The above-mentioned activities
of transportan can hardly have adverse effects when using it in vitro, since most of
its side effects are exerted at higher concentrations than those used in cell delivery
experiments. Furthermore, very few cell lines express galanin receptors and receptormediated
effects of transportan can be avoided in vitro by choosing a cell line without
galanin receptors.
11.2.3.2 Tat
Transcription transactivating factor of HIV, Tat, exerts a multitude of activities in
addition to activation of viral gene expression and replication. The secreted factor
Tat is implicated in angiogenesis, 46 and monocyte chemotaxis, 47 induces apoptosis
in endothelial, 48 and neuronal cells, 49 potentiates glutamate excitotoxicity, 50 and
decreases cAMP synthesis, to mention a few.
So far, it has not been established whether the respective activities of full-length
Tat can be confined to certain regions in the protein or if the effects can be mimicked
by short peptide sequences. However, a recent study by Jia and collaborators demonstrates
that some effects of Tat whole protein can be exerted by the 12 amino
acids-long peptide corresponding to basic region (known to penetrate into cells), or
a peptide of 20 amino acids corresponding to the cysteine-rich domain. 51 The Tat
(46–57) peptide inhibits binding of vascular endothelial growth factor ( 125 I-VEGF 165)
to KDR and neuropilin receptors in human umbilical vein endothelial cells (HUVEC
IC 50 of 42 µM). This peptide was able to inhibit VEGF induced ERK activation and
mitogenesis in endothelial cells and inhibited angiogenesis in vitro. Tat (46–57) also
inhibited ERK activation, mitogenesis, and angiogenesis induced by basic fibroblast
growth factor. 51
11.3 CPP TOXICITY IN VIVO
The limited number of reports describing in vivo application of cell-penetrating
peptides does not afford generalizations about the toxicity of CPPs. Supposedly, no
formal study about the general and acute toxicity of CPPs in vivo has been performed.
However, in experiments that applied CPP-mediated delivery in vivo, toxic effects
of cell-penetrating constructs and, sometimes, also the transport peptide have been
estimated. In most cases, no toxicity or unwanted side effects have been detected.
The in vivo studies with application of CPP-mediated transport will here be briefly
summarized, concentrating mainly on observations that can be related to possible
toxic effects.