Delicate balance between cellular plasticity and differentiation is critically maintained during mammary development. Disruption of this balance leads to breast cancer initiation and metastatic progression. Recent findings from our lab have revealed that N-Myc and STAT Interactor (NMI) protein is decreased in 70% of primary patient specimens with metastatic breast cancer. Mammary specific Nmi knock out mouse model revealed that conditional Nmi loss disrupts luminal differentiation in the mammary gland affecting alveologenesis and prompted the progression of tumors with aggressive metastatic characteristics.

Our studies showed that Nmi is induced at the onset of pregnancy and its expression remains throughout lactation. Furthermore, prolactin stimulation and differentiation of HC11 murine mammary epithelial cells is accompanied by up-regulation of Nmi. STAT5a is one of the downstream effector of prolactin and is essential for differentiation of secretory alveolar epithelium. However, functional relationship of NMI and STAT5a was not known.

Here we present our finding that NMI and STAT5a expression has a direct relationship in normal mammary development as well as in breast primary and metastatic tumor specimens. We demonstrate that loss of Nmi in vivo caused a decrease in STAT5a activity and a subsequent transcriptomic shift in mammary epithelial and breast cancer cells. Detailed examination of STAT5a mammary specific controlled genetic program in the context of transcription profiles of NMI knockout and overexpressing cell lines as well as mammary tumors revealed ISG20, interferon stimulated exonuclease gene 20, as a unique negatively regulated transcript. Here we show that expression of ISG20 is kept in check by NMI by miR17-92 cluster and that ISG20 has a positive influence on tumor progression and metastasis.

Citation Format: Rajeev Sharad Samant, Heba A Alsheikh, Brandon J Metge, Dongquan Chen, Shi Wei, Lalita A Shevde. N-Myc and STAT interactor regulates STAT5a in mammary development and metastasis [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-01-14.