SIBLING expression patterns in duct epithelia reflect the degree of metabolic activity

The SIBLING (Small Integrin-Binding LIgand, N-linked Glycoprotein) family of secreted glycophosphoproteins includes bone sialoprotein (BSP), dentin matrix protein-1 (DMP1), dentin sialophosphoprotein (DSPP), osteopontin (OPN), and matrix extracellular phosphoglycoprotein (MEPE). For many years, they were thought in normal adults to essentially be limited to metabolically active mesenchymal cells that assembled the mineralized matrices of bones and teeth. Over the last decade they have also been upregulated in a variety of tumors. Three of these proteins (BSP, OPN, and DMP1) have been shown to interact with three matrix metalloproteinases (MMP-2, MMP-3, and MMP-9, respectively). Recently, all five SIBLINGs and their MMP partners when known were observed in specific elements of normal ductal epithelia in salivary gland and kidney. We have hypothesized that the SIBLINGs and their MMP partners may be expressed in ductal cells with high metabolic activity. In this paper, we show that all the SIBLINGs (except MEPE) and their MMP partners are expressed in the metabolically active epithelia of human eccrine sweat gland duct but not in the more passive ductal cells of the macaque (monkey) lacrimal gland. It is hypothesized that MEPE expression may be limited to cells involved in active phosphate transport. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.