Further characterization and cloning of the growth inhibitor mammastatin.

Abstract

Current breast cancer statistics indicate that new treatments are needed for breast cancer. Studies of the growth control mechanisms of human mammary cells may suggest methods to prevent and control abnormal cancer cell growth. Studies of factors influencing normal human mammary cell growth have identified the growth inhibitor mammastatin. Mammastatin is a growth inhibitory protein identified in normal human mammary cells. The protein inhibits the growth of mammary carcinoma cells in a tissue specific manner. This growth inhibitory activity has not been identified in media conditioned by the growth of mammary carcinoma cells. Lack of growth inhibitory activity in carcinoma cell conditioned media suggests a lack of mammastatin expression in human breast carcinoma cells. We have studied mammastatin protein expression in normal and transformed human mammary cells to determine whether carcinoma cells produce the protein. We describe here the identification of 44, 49 and 53 kD species of mammastatin in normal human mammary cells. Mammastatin is identified in carcinoma cells and tumor lysates only as the 44 kD protein species. Phosphorylation of mammastatin in normal cells may account for the identification of different species of the mammastatin protein as well as differential inhibitory activity in normal and tumor cells. Mammastatin phosphorylation modulates its growth inhibitory activity and may effect its interaction with the Ras signal transduction pathway. We have identified a candidate cDNA clone for mammastatin that shares sequence homology with the p53, anti-oncogene. The mammastatin cDNA clone will allow synthesis of recombinant protein which will aid studies to determine mammastatin's role in growth control.