INHIBITION OF CANCER CELL-GROWTH AND C-MYC TRANSCRIPTIONAL ACTIVITY BY A C-MYC HELIX L-TYPE PEPTIDE FUSED TO AN INTERNALIZATION SEQUENCE

c-Myc is a nuclear protein with important roles in cell transformation , cell proliferation, and gene transcription, It has been previously s hown that a 14-amino acid (aa) modified peptide (H1-S6A,F8A) derived f rom the helix 1 (H1) carboxylic region of c-Myc can interfere in vitro with specific c-Myc DNA binding, sere, ae have linked the above Myc-d erived 14-aa peptide to a 16-aa sequence from the third helix of Anten napedia (Int), It has been repeatedly reported that this 16-aa Antenna pedia peptide is able to cross mammalian cell membranes and to work as a vector for short peptides. Using fluorescent (dansylated or rhodami nated) peptides, we have shown that the fusion peptide with the Antenn apedia fragment (Int-H1-S6A,F8A) but not the c-Myc derived fragment al one (H1-S6A,F8A) mas capable of internalization inside MCF-7 human bre ast cancer cells. Int-H1-S6A,F8A and H1-S6A,F8A were the only two pept ides capable of inhibiting coimmunoprecipitation of the c-Myc/Max hete rodimer in vitro. We have treated (continuously for 10-11 days) MCF-7 cells with four different peptides: Int, H1-S6A,F8A, Int-H1-S6A,F8A, a nd Int-H1wt [a peptide differing from Int-H1-S6A,F8A by 2 aa (S6 and F 8) in the H1 region]. In intact MCF-7 cells, Int-H1-S6A,F8A was the on ly active peptide capable of inducing the following biological effects : (a) inhibition of cloning efficiency on plates; (b) inhibition of ce ll growth and induction of apoptosis in subconfluent/confluent cells; and (c) inhibition of transcription of two c-Myc-regulated genes (ODC and p53). Int-H1-S6A,F8A was active in the 1-10 mu M range. Int-H1-S6A ,F8A may represent a lead molecule for peptidomimetic compounds that h ave a similar three-dimensional structure but are more resistant to pe ptidases and, therefore, suitable for in vivo treatment of experimenta lly induced tumors.