Importance of the central region of lamprey gonadotropin-releasing hormone III in the inhibition of breast cancer cell growth

Naturally occurring isoforms of the decapeptide gonadotropin-releasing hormone (GnRH) share residues 1-4 and 9-10. lGnRH-III, the third isoform isolated in the sea lamprey has no endocrine effect in mammals but shows a direct antiproliferative effect on human breast, prostate and endometrial cancer cell lines. To investigate these features, residues 5-8 of lGnRH-III were systematically replaced with Ala. The ability of the synthetic analogs to interact with receptors on MDA-MB 231 human breast cancer cells and their effect on the growth of the same cell line were investigated. [Ala⁶]lGnRH- III and [Ala⁷]lGnRH-III have neither receptor binding nor antiproliferative activity. Replacement of His⁵ with Ala resulted in an analog that binds to the receptor but does not have antiproliferative activity. The results are in agreement with previous reports that modifications of Lys at position 8 are well tolerated.