Targeted expression of stromelysin-1 in mammary gland provides evidence for a role of proteinases in branching morphogenesis and the requirement for an intact basement membrane for tissue-specific gene expression [published erratum appears in J Cell Biol 1996 Feb;132(4):following 752]

Abstract

Abstract. The extracellular matrix (ECM) is an important regulator of the differentiated phenotype of mammary epithelial cells in culture. Despite the fact that ECM-degrading enzymes have been implicated in morphogenesis and tissue remodeling, there is little evidence for a direct role for such regulation in vivo. We generated transgenic mice that express autoactivated isoforms of the matrix metalloproteinase stromelysin-1, under the control of the whey acidic protein gene promoter, to examine the effect of inappropriate expression of this enzyme. Stromelysin-1 is implicated as the primary player in the loss of basement membrane and loss of function in the mammary gland during involution. The transgene was expressed at low levels in maramary glands of virgin female mice, leading to an unexpected phenotype: The primary ducts had supernumerary branches and showed precocious development of alveoli that expressed /3-casein at levels similar to that of an early-to midpregnant gland. Lactating glands showed high levels of transgene expression, with accumulation at the basement membrane, and a decrease in laminin and collagen IV, resulting in a loss of basement membrane integrity; this was accompanied by a dramatic alteration of alveolar morphology, with decreased size and shrunken lumina containing little/3-casein. During pregnancy, expression of endogenous whey acidic protein and r-casein was reduced in transgenic glands, confirming the observed dependence of milk protein transcription on ECM in mammary epithelial cells in culture. These data provide direct evidence that stromelysin-1 activity can be morphogenic for mammary epithelial cells, inducing hyperproliferation and differentiation in virgin animals, and that its lytic activity can, indeed, disrupt membrane integrity and reduce mammary-specific function. We conclude that the balance of ECM-degrading enzymes with their inhibitors, and the associated regulation of ECM structure, is crucial for tissue-specific gene expression and morphogenesis in vivo.T HE differentiated state is plastic, requiring continuous and active control for both its acquisition and its maintenance (Bisseli, 1981;Blau, 1992). In vertebrate cells in culture, there is an increasing body of evidence that the extracenular matrix (ECM) t plays a seminal role both in in-

Putting tumours in context

“…In transgenic mice that express SL-1 in mammary luminal epithelial cells, the mammary glands show morphogenesis defects and contain pre-neoplastic lesions 44,53,54 that eventually lead to full malignancies 50,54 . Here, the causative mechanism seems to be that SL-1 -expressed ectopically at low levels in the epithelial cells -is subsequently produced at much higher levels by the stromal fibroblasts 44 Macrophage migration inhibitory factor (MIF) is another immunomodulator that is associated with tumour progression.…”

“…Cellular context determines the response of mammary epithelial cells to SL-1 treatment: when grown in basement-membrane gels, mammary epithelial cells undergo growth arrest and become functionally differentiated; subsequent treatment of these cells with SL-1 causes apoptosis 52 . However, when cultured on two-dimensional matrices and allowed to continuously proliferate, mammary epithelial cells react to treatment with SL-1 by undergoing an epithelial-mesenchymal transition and becoming tumorigenic 50 .In transgenic mice that express SL-1 in mammary luminal epithelial cells, the mammary glands show morphogenesis defects and contain pre-neoplastic lesions 44,53,54 that eventually lead to full malignancies 50,54 . Here, the causative mechanism seems to be that SL-1 -expressed ectopically at low levels in the epithelial cells -is subsequently produced at much higher levels by the stromal fibroblasts 44 Macrophage migration inhibitory factor (MIF) is another immunomodulator that is associated with tumour progression.…”

The matrix metalloproteinase stromelysin-1 acts as a natural mammary tumor promoter

“…Either way, Str1 could act as an apoptotic stimulus. Indeed, Str1 does induce apoptosis in di erentiated mammary alveolar epithelial cells in culture and in vivo, however it also promotes the proliferation and branching of ductal epithelium Boudreau et al, 1995;Sympson et al, 1994;Thomasset et al, 1998;Witty et al, 1995). These seemingly contradictory e ects can be reconciled by noting that ductal epithelial cells normally divide during branching morphogenesis and persist throughout involution, whereas alveolar epithelial cells do not.…”

“…Thus the di erentiation status of the target cell may determine its response to Str1. These e ects were ®rst observed in transgenic mice with an autoactivating rat Str1 transgene ± (the autoactivating rat Str1 cDNA contained a Val 92 -to-Gly 92 transition within its propeptide domain, thus destabilizing thè cysteine switch' that otherwise maintains enzyme latency (Sanchez-Lopez et al, 1988)) ± targeted to mammary epithelium by the whey acidic protein (WAP) gene promoter (Sympson et al, 1994) and mouse mammary tumor virus (MMTV) enhancer/ promoter (Witty et al, 1995). In these mice, Str1 transgene expression resulted in increased ductal branching and precocious lobulo-alveolar development during puberty, basement membrane disruption and unscheduled involution during pregnancy, and alveolar collapse and low milk-protein production during lactation.…”

Expression of Matrix Metalloprotease-2-Cleaved Laminin-5 in Breast Remodeling Stimulated by Sex Steroids

“…17 These enzymes are ubiquitously located in the body, are capable of degrading several proteins, including BM components, 10 and are considered to have a key role in tissue morphogenesis and in cancer metastasis as well. 10,18 In a transgenic mouse model, stromelysin-1 has been shown to induce branching morphogenesis 16,19 and also to trigger a malignant phenotype in epithelial mammary cells after extended contact. 20 While sex steroids regulate branching morphogenesis of the mammary gland, as is proteolytic ECM remodeling, no mechanistic link between these two processes has yet been established.…”