Expression, location, and interactions of ErbB2 and its intramembrane ligand Muc4 (Sialomucin complex) in rat mammary gland during pregnancy

Muc4 (also called Sialomucin complex) is a heterodimeric glycoprotein complex consisting of a peripheral O-glycosylated subunit ASGP-1 (ascites sialoglycoprotein-1) tightly but non-covalently bound to an N-glycosylated transmembrane subunit ASGP-2. Muc4/SMC can act as an intramernbrane ligand for Erb132 via an EGF-like domain present in the transmembrane subunit. The complex is developmentally regulated in normal rat mammary gland and overexpressed in a number of mammary tumors. Overexpression of Muc4/SMC has been shown to block cell-cell and cell-matrix interactions, protect tumor cells from immune surveillance, promote metastasis, and protect from apoptosis. We have investigated whether Muc4/SMC and Erb132 are co-expressed and co-localized in normal rat mammary gland and whether Muc4/SMC-ErbB2 complex formation is developmentally regulated in this tissue. Muc4/SMC and Erb132 have different expression patterns and regulatory mechanisms in the developing rat mammary gland, but both are maximally expressed during late pregnancy and lactation. The two proteins form a complex in lactating mammary gland which is not detected in the virgin gland. Moreover, this complex does not contain Erb133. Erb132 is colocalized with Muc4/SMC at the apical surfaces of ductal and alveolar cells in lactating gland; however, another form of Erb132, recognized by a different antibody, localizes to the basolateral surfaces of these cells. Erb132 phosphorylated on Tyr 1248 colocalized with Muc4/SMC at the apical surface but not at the basolateral surfaces of these cells. To investigate the function of Muc4 in the mammary gland, transgenic mice were derived using an MMTV-Muc4 construct. Interestingly, mammary gland development in the transgenic mice was aberrant, exhibiting a bifurcated pattern, including invasion down the blood vessel, similar to that exhibited by transgenic mice inappropriately expressing activated Erb132 in the mammary gland. These data provide further evidence of the ability of Muc4/SMC to interact with ErbB2 and influence its behavior in normal epithelia. (C) 2004 Wiley-Liss, Inc.