Expression of Pulmonary Metallothionein Genes in Late Gestational Lambs

Abstract

ABSTRACT. Metallothioneins (MT) are low molecular weight proteins that are important in providing protection against heavy metals such as cadmium. Other precise physiologic roles for this family of proteins are less clear, but fetal hepatic cell proliferation and differentiation may be regulated through changes in MT levels and attendant MT-mediated regulation of zinc levels. The role of MT in other developing tissue, most notably lung, is far less clear. Although expression of MT has been reported to be extremely low in early postnatal and mature lung, we hypothesized that MT has a more ubiquitous role in organ development and that pulmonary MT levels may be elevated during periods of rapid lung growth. Thus, we studied expression of MT in late-gestation fetal lambs. Sheep are particularly useful because alveolarization of lung parenchyma occurs before birth (by d 120 of a normal 1474 gestation). Immunoreactive MT was localized to bronchial epithelium of fetal, newborn, and mature sheep. The intensity of staining was greatest in the 1304 gestational age (saccular) lung, where positive reaction product was noted in the cytoplasm and nucleus of alveolar epithelial and interstitial cells. We next evaluated MT expression in developing lung tissue using Northern blot analysis and 32P-cDNA probes against the 3'-untranslated regions of mRNA encoding each of four known functional sheep MT (sMT) isoforms. Expression of sMT-11, sMT-Ia, and sMT-Ib was restricted to the saccular stage (120-132 d gestational age), and sMT-Ic mRNA was not detected in pulmonary samples from any stage of development. In contrast, all mRNA isoforms were expressed in fetal and newborn liver and at levels considerably greater than lung. These data show that pulmonary MT gene expression is apparent in the saccular stage of lung development when considerable remodeling of lung parenchyma occurs. In addition, an organ-and age-specific expression of MT mRNA isoforms in developing sheep lung and liver was noted for the first time, suggesting that MT isoforms may play specific roles in organ development. MT are small (approximately 7 kD), cysteine-rich (approximately 30 mol %) metal-binding proteins (1, 2). All vertebrates have two isoforms of MT, MT-I and MT-11, based on elution profiles from diethylaminoethyl columns. In humans (3) and sheep (4), several subisoforms have been identified by chromatography and genes encoding these isoforms have been sequenced. Although precise physiologic roles for members of this multigene family remain unknown, it has been suggested that one major function of MT is to affect intracellular metal ion homeostasis (1, 2, 5,6). MT are the major zinc-binding proteins of cells, and thus indirect regulation of numerous biologic processes that involve zinc metalloenzymes (i.e. differentiation, proliferation, energy metabolism, transcription, and antioxidant systems) is highly likely. These possibilities underscore investigations of the regulation of MT expression in developing liver (7-10) in which MT has been hypothesized to...

Mechanical Ventilation Injury and Repair in Extremely and Very Preterm Lungs

“…MT is a strong antioxidant [18], [19] induced by metals, glucocorticoids, oxidative stress and inflammatory mediators [18] and its expression has previously been reported in lungs of humans [20], lambs [21] and mice [22]. There is evidence that MT is involved in lung repair and the prevention of lung injury.…”

“…Knockout mice lacking MT are more sensitive to acute lung injury caused by lipopolysaccharide [23], ozone [24] and allergic airway inflammation [25]. MT expression is greatest in bronchiolar epithelium, alveolar macrophages and endothelial cells [20], [21].…”