Binding sites and mechanistic pathways for a novel human anti-breast cancer peptide

About two-thirds of the 211,240 women diagnosed each year with breast cancer have tumors which have estrogen receptors (ER+) and grow in response to estrogen (E2). We have synthesized a peptide, cAFPep, that stops the growth of ER+ human breast cancer growing as xenografts in immune-deficient mice. This peptide is derived from alpha-fetoprotein (AFP), a safe, naturally-occurring protein produced during pregnancy which itself has anti-breast cancer activity. Although the anti-oncotic activity of this peptide and its precursors is well documented, the mechanism of its action has not been elucidated.; In order to study the mechanism of action of this peptide, a reliable cell culture assay was required. Standard culture of human breast cancer cells on tissue culture grade plastic had been shown to be insufficient to demonstrate the anti-breast cancer effects of the peptide. However, when defined components of the extracellular matrix (fibronectin and collagen) were added to this system, significant growth inhibition by peptide was demonstrable. This defined extracellular matrix (ECM) system was used to examine the effect of cAFPep on expression and phosphorylation of ER. Treatment with cAFPep reduced total cellular levels of ER, reduced the E2-stimulated phosphorylation of serine 118 of the ER, and reduced transcriptional activity of the ER. FAK, Shc, p38, and SAPK/JNK were also found to be modulated in a way that was consistent with the anti-proliferative effects of cAFPep.; Ligand affinity chromatography showed that cAFPep interacted with members of the 70 kDa heat shock protein (HSP) family which are known to be required for stabilization and activation of the ER.; Taken together, these data suggest a mechanism by which cAFPep uses ECM and HSPs to downregulate the ER and block E2-stimulated growth of breast cancer. The action of this peptide interferes with ER in a way that is unlike that of any other drug currently used to treat ER+ breast cancer. Further understanding of the nature of the interactions with ECM and HSPs to alter the metabolism of the ER will shed light on a novel pharmacological interference with estrogen particularly as it relates to prevention of estrogen-stimulated breast cancer growth.