PREVENTION OF MAMMARY PRENEOPLASTIC TRANSFORMATION BY NATURALLY-OCCURRING TUMOR INHIBITORS

Aberrant hyperproliferation (AH) is a late occurring post-initiational event that precedes mammary tumorigenesis in vivo. Experiments on the spontaneously immortalized, non-tumorigenic murine mammary epithelial C57/MG and MMEC cells were designed to validate AH as an in vitro cel lular marker for preneoplastic transformation. Colony forming efficien cy (% CFE) in anchorage-independent conditions of growth represented t he quantitative parameter for AH. C57/MG and MMEC cells, upon treatmen t with chemical carcinogens or transfection with oncogenes, exhibited at least a 60-300-fold increase in AH relative to that seen in appropr iate untreated controls. Transplantation of mammary epithelial cells i nitiated either by chemical carcinogens or by oncogenes into mammary f at pads of syngeneic mice produced rapidly growing tumors at the trans plant site within 4-6 weeks. The tumor-derived T-1/Pr-1 and myc(3)/Pr- 1 cell lines (positive controls) exhibited at least an 800-900-fold in crease in AH. Treatment of initiated cells with naturally occurring tu mor inhibitors eicosapentaenoic acid (EPA), indole-3-carbinol (I3C), ( -)epigallocatechin gallate (EGCG), squalene (SQE), and perillyl alcoho l (PA) at non-toxic doses, resulted in a 70-99% inhibition of AH, depe nding on the initiator and the chemopreventive test compound. Upregula tion of AH in initiated mammary epithelial cells in vitro prior to tum origenesis in vivo, and persistent inhibition of AH by diverse natural ly occurring tumor inhibitors, provides evidence for AH as a cellular surrogate endpoint for induction and modulation of mammary neoplastic transformation. (C) 1997 Elsevier Science Ireland Ltd.