Mechanism of cellular uptake of HIV-TAT peptide & effects of TAT-SOD against ultraviolet induced skin damage
TAT peptide is one of the best-characterised cell penetrating peptides (CPPs) derived from the transactivator of transcription protein from the human immunodeficiency virus 1 (HIV-1). TAT peptide is able to cross the cell membrane and deliver various biomolecules into cells with low immunogenicity and no toxicity. However, the exact mechanism of internalization still remains a subject of controversy. Lamellar neutron scattering was used to determine the location of TAT peptide in the negativelycharged phospholipids bilayers. The results reveal two locations, one in the peripheral aqueous phase between the adjacent bilayers and the second one below the glycerol backbone region of the lipid bilayer. A concentrationindependent membrane thinning above a peptide concentration threshold (1mol%) and a contiguous transbilayer water channel at the largest peptide concentration (10mol%) were also found. This evidence led to the suggestion that the toroidal pore model might be involved in the transmembrane mechanism at high peptide concentration. Another set of neutron diffraction experiments examined the interaction between the TAT peptide and neutral phospholipids showed that TAT peptide preferentially intercalated into the hydrophobic core and the glycerol backbone region of the neutral lipid bilayer at the lowest peptide concentration investigated (0.1mol%), indicating that the insertion did not require negatively-charged phospholipids. There was also clear evidence for the concentration-dependent reorientation of TAT peptide. A plasmid containing the human copper-zinc SOD gene linked with the coding sequence for a 11-aa HIV-TAT peptide (pGEX-TAT-SOD, 513bp) was constructed and used to express a recombinant fusion protein in Escherichia coli strain BL21 (DE3). High-level expression of TAT-SOD soluble protein with a GST tag (44-kDa) was achieved under optimal expression conditions and a small-scale glutathione affinity column or large-scale ion-exchange chromatography used for its purification. The potential protective effect of TAT-SOD against UV-induced cell damage was studied on UVC-irradiated MDCK epithelial cells. Before any further clinical study, the UV full-length absorption of TAT-SOD protein was measured. The results showed the potential UV protective effect of TAT-SOD was not due to the physical absorption of UV irradiation. In a preclinical study with five healthy volunteers, the penetration of TAT-SOD through human stratum corneum on the inner upper arm was identified by the tape stripping and specific SOD activity analysis. Significant increases on SOD activity were found on the outer layers of stratum corneum in TAT-SOD treated group, compared to placebo treated control, indicating that the TAT peptide assisted SOD to penetrate into the human stratum corneum . In a clinical study with ten healthy volunteers, eight showed a significant increase of minimal erythema dose (MED) with TAT-SOD pre-treatment. The median blood flow value of ten subjects at the UVB-irradiated site decreased with TAT-SOD pretreatment. Taken together, this evidence showed that TATvi SOD did have a marked protective effect against UVB induced skin damage. In a second clinical study, five healthy volunteers were challenged with a series of UVB doses. Skin punch biopsies were taken from four test sites on the lower back for H&E and immunohistochemical staining analysis. UVB-induced apoptotic sunburn cell (SBC) formation, p53 up-regulation and thymine dimer formation in epidermis were not attenuated by pretreatment with TAT-SOD. These data suggest that transdermal superoxide scavenger TAT-SOD reduced the UVB-induced inflammation, but did not abrogate the direct DNA damage of UVB irradiation on the skin. However, the hope of TAT-SOD could reduce UVA indirect DNA damage remains.