Bifunctional recombinant proteins in cancer therapy: cell penetrating peptide aptamers as inhibitors of growth factor signaling.

The major aim of molecular cancer research is the development of new therapeutic strategies and compounds that target directly the genetic and biochemical causes of malignant transformation. Therapeutic genes, antibodies and their derivatives, but also small molecular weight compounds, have been used for this purpose. Small peptides might be able to complement these agents because of their ability to recognize specific protein domains and thus to interfere with enzymatic functions or protein-protein interactions. A variation of the yeast-two-hybrid procedure allows to select specifically binding peptides, so called peptide aptamers, from a peptide library of high complexity. This selection procedure can be adapted to any protein or protein fragment as a bait construct and selects for the intracellular interaction between the bait of choice and the peptide aptamer prey. Peptide aptamers thus selected can be cloned, provided with a protein transduction domain, expressed in bacteria and introduced into cancer cells. There they might disrupt protein-protein interactions crucial for cancer cell growth or survival. We introduce an example in which the Stat3 arm of EGF receptor signaling is selectively inhibited by a peptide aptamer and causes the growth arrest of EGF receptor-dependent tumor cells. The aptamer constructs can be supplemented with additional functional domains to enhance their inhibitory effects.