TGF-beta regulation of Muc4/SMC in the mammary epithelial cell

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Biochemistry and Molecular Biology

First Committee Member

Kermit L. Carraway - Committee Chair


Muc4/Sialomucin complex (SMC) is a heterodimeric glycoprotein complex derived from a single gene that is post-translationally processed into mucin (ASGP-1) and transmembrane (ASGP-2) subunits. The mucin subunit is a high molecular weight sialomucin that is highly O-glycosylated. The transmembrane subunit (ASGP-2) is N-glycosylated. Muc4/SMC is tightly regulated in the rat mammary gland, low in the virgin, increased during pregnancy and lactation, and overexpressed in some aggressive mammary tumors. Investigations of primary rat mammary epithelial cells (MEC) have shown that Muc4/SMC expression is post-translationally regulated through inhibition of Muc4/SMC precursor processing by transforming growth factor-beta (TGF-beta). Mechanistic studies suggest that TGF-beta inhibition of Muc4/SMC expression is mediated through Smad2, a TGF-beta effector that, when activated, functions as a transcription factor. Smad2 antisense oligonucleotide blocks the inhibition of Muc4/SMC expression by TGF-beta. The TGF-beta effect on Muc4/SMC expression is repressed by interferon-gamma (IFN-gamma). IFN-gamma treatment of MEC activates and relocalizes signal transducer and activator of transcription-1 (STAT-1) to induce an inhibitor Smad, Smad7. Smad7 antisense oligonucleotide prevents IFN-gamma from blocking the TGF-beta inhibition of Muc4/SMC expression. Immunohistochemical studies of mammary tissues derived from transgenic mice that express a truncated, kinase-defective, dominant-negative type II TGF-beta receptor regulated by the MMTV promoter/enhancer (MMTV-DNIIR) show significantly higher expression levels of Muc4/SMC as compared to its wild-type counterpart. Cleavage of the precursor form of Muc4/SMC to ASGP-1/ASGP-2 is an essential step in Muc4/SMC processing. In the presence of Pefabloc SC, a serine protease inhibitor, Muc4/SMC processing is substantially inhibited. Increasing the concentration of inhibitor increases the levels of Muc4/SMC precursor with a concomitant decrease in ASGP-2 expression. These results suggest that TGF-beta regulates Muc4/SMC expression via the Smad pathway by a transcriptional effect on a protein, possibly a protease or protease inhibitor, in the Muc4/SMC processing step.


Biology, Molecular; Chemistry, Biochemistry

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