Prolactin, epidermal growth factor or transforming growth factor-α activate a mammary cell-specific enhancer in mouse mammary tumor virus-long terminal repeat

Abstract

Mammary specific expression of elevated levels of mouse mammary tumor virus (MMTV) contributes to mammary carcinogenesis. Mechanisms which regulate provirus expression have not been completely defined. Using a MMTV-long repeat terminal (MMTV-LRT) directed chloramphenicol-acetyltransferase (CAT) reporter gene system and a human breast cancer cell line T47D, we demonstrate that prolactin (PRL), epidermal growth factor (EGF), or transforming growth factor-α (TGF-α) act on a mammary cell-specific enhancer at the extreme 5' end of the MMTV-LTR involving sequences - 1094 through - 858. PRL and either EGF or TGF-α exert concerted roles in this activation of these sequences. In contrast, using a plasmid construct lacking this mammary cell-specific enhancer, EGF or TGF-α, but not PRL, act synergistically with progesterone to induce CAT activity, indicating that the action of PRL on regulatory elements of the MMTV-LTR is restricted to this mammary cell-specific enhancer involving sequences - 1094 through - 858. A mobility shift assay was used to demonstrate that PRL, EGF or TGF-α induce nuclear factors (MP4, MAF, and MGF) which bind directly to this mammary cell-specific enhancer element.