Grb7 has potential importance in the progression of
cancer. We have previously identified a novel
peptide that binds to the SH2 domain of Grb7 and inhibits its association with several different
receptor tyrosine kinases. We have synthesised the Grb7
peptide,
G7-18NATE, with two different
cell penetrating peptides,
Penetratin and Tat. In this study, we have shown that both
Penetratin- and Tat-conjugated
G7-18NATE peptides are able to inhibit the proliferation of SK-BR-3, ZR-75-30, MDA-MB-361 and MDA-MB-231
breast cancer cells. There was no significant effects on
breast cancer MCF-7cells, non-malignant MCF 10A or 3T3 cells. In addition, there was no significant inhibition of proliferation by
Penetratin or Tat alone or by their conjugates with arbitrary
peptide sequence in any of the cell lines tested. We determined the EC50 of G7-18NATE-P
peptide for SK-BR-3 cell proliferation to be 7.663 x 10(-6) M. Co-treatment of G7-18NATE-P
peptide plus
Doxorubicin in SK-BR-3
breast cancer cells resulted in an additional inhibition of proliferation, resulting in 56 and 84% decreases in the
Doxorubicin EC50 value in the presence of 5 x 10(-6) and 1.0 x 10(-5) M G7-18NATE-P
peptide, respectively. Importantly, the co-treatment with
Doxorubicin and the delivery
peptide did not change the
Doxorubicin EC50. Since Grb7 associates with ErbB2, we assessed whether the
peptide inhibitor would have a combined effect with a molecule that targets ErbB2,
Herceptin. Co-treatment of
Herceptin plus 1.0 x 10(-5) M G7-18NATE-P
peptide in SK-BR-3 cells resulted in a 46% decrease in the
Herceptin EC50 value and no decrease following the co-treatment with
Herceptin and
penetratin alone. This Grb7
peptide has potential to be developed as a therapeutic agent alone, in combination with traditional
chemotherapy, or in combination with other targeting molecules.