Alpha-fetoprotein derived growth inhibitory peptide (GIP) inhibits expression of cyclin E1

4918 Alpha-fetoprotein (AFP) promotes both ontogenic growth and oncogenic progression of some tumor types, and has been shown to enhance cell proliferation via protein kinase A activity. In contrast, the alpha-fetoprotein derived Growth Inhibitory Peptide (GIP), a 34-amino acid fragment of the AFP molecule synthetically produced, suppresses growth of an array of tumor types in vitro and in vivo. The GIP site lies buried in a molecular cleft of naturally folded human alpha-fetoprotein, but it is partially exposed following introduction of fetal stress. It was previously demonstrated that treatment with low levels of GIP for eight days inhibited proliferation of MCF-7 cell cultures in what was noted as a cytostatic as opposed to a cytotoxic effect. The present study was initiated to determine whether cell cycle constituents required for G1-to-S phase transition were altered by treatment with GIP. MCF-7 breast cancer cultures were treated with various concentrations of GIP (10-5 to 10-14 M) for eight days after which cells were harvested and RNA or protein isolated. Analysis of gene expression with real time RT-PCR revealed that levels of Cyclin E1 were decreased as much as 5-fold after exposure to concentrations of GIP as low as 10-12 M. Concomitantly, mRNA levels of Rb1 were enhanced from 1.7 to 3.4 fold following GIP treatment. However, GIP had no effect on the mRNA expression of cdk2, cdk4, cdk6, Cyclin D1 or Cyclin B1. A decrease in protein levels of Cyclin E1 was detected with Western blot analysis after treatment with 10-9 M GIP as was a decrease in phosphorylated p27 suggesting that treatment of MCF-7 cells with physiologic levels of GIP may result in G1/S phase arrest.