MUC1: a multifaceted oncoprotein with a key role in cancer progression

Abstract

The transmembrane glycoprotein Mucin 1 (MUC1) is aberrantly glycosylated and overexpressed in a variety of epithelial cancers, and plays a crucial role in progression of the disease. Tumor-associated MUC1 differs from the MUC1 expressed in normal cells with regard to its biochemical features, cellular distribution, and function. In cancer cells, MUC1 participates in intracellular signal transduction pathways and regulates the expression of its target genes at both the transcriptional and post-transcriptional levels. This review highlights the structural and functional differences that exist between normal and tumor-associated MUC1. We also discuss the recent advances made in the use of MUC1 as a biomarker and therapeutic target for cancer.

Keywords: ABC pumps; EMT; Mucin 1 (MUC1); VNTR; cancer; cancer stem cell.

Figure 1

Schematic representation of the structure of MUC1, cleavage by proteases, and potential binding motifs on MUC1 cytoplasmic tail (CT). (A) The N-terminal subunit (MUC1-N) and C-terminal subunit (MUC1-C) of MUC1 associate around the SEA domain, forming a stable heterodimeric complex. MUC1-N contains the signal peptide, variable number tandem repeat (VNTR) region, and SEA domain. The VNTR region of MUC1-N is composed of 20 amino acids that are extensively O-glycosylated (red) at the serine and threonine residues. MUC1-N and MUC1-C are sparingly N-glycosylated (violet) at asparagine residues. The C-terminal domain (MUC1-C) consists of the extracellular domain (ECD), transmembrane domain (TMD), and CT. (B) The ECD of MUC1-C contains one N-glycosylation site (asparagine residue). Based on the extent of N-glycosylation, the size of MUC1-C can range between 23 and 25 kDa. MUC1-C lacking N-glycosylation has a molecular weight of 17 kDa. (C) Under normal growth conditions, MUC1 remains in a heterodimeric form (Stage 1). The ECD of MUC1-C is cleaved (Stage 2) by enzymes such as tumor necrosis factor (TNF)-α converting enzyme (TACE), also called disintegrin and metalloprotease domain containing protein-17 (ADAM17) or membrane type 1 matrix metalloprotease (MMP-MT1), to generate peptide fragments: MUC1* (16 kDa) or MUC1-CTF₁₅ (15 kDa) with shorter ECD. Functionally, MUC1* acts as a growth factor receptor for NM23-H1 and MUC1-CTF₁₅ acts as a substrate for γ-secretase. (D) The amino acid sequence of MUC1 CT, highlighting the potential sites of phosphorylation and protein-binding partners. The tyrosine (red), threonine (green), and serine (blue) residues of MUC1 CT are phosphorylated by growth factor receptors and intracellular kinases. The p85 subunit of PI3K, PKC-δ, GSK-3β, and c-Src phosphorylate the tyrosine residue of the YHPM motif, the threonine residue of the TDR motif, the serine residue of the SPY motif, and the tyrosine residue of the YEKV motif, respectively. β-Catenin directly binds to the serine-rich SAGNGGSSLS motif. These phosphorylated residues form potential binding sites for intracellular signaling molecules and thus MUC1 can integrate different signaling cascades or modulate their activation status. For example, the phosphotyrosine residue of the SANL motif acts as a docking site for the Grb-2 protein [109]. MUC1-C monomers dimerize around the CQC motif to form a functional homodimer. The putative nuclear localization motif RRK binds to importin-β, and allows translocation to the nucleus via Nup-62. Adapted from [18]. Abbreviations: MUC1, Mucin 1; SEA, sea urchin sperm protein, enterokinase, and agrin.

Figure 2

Schematic representation of the MUC1 gene and the different isoforms of MUC1. (A) The MUC1 gene consists of seven exons (E1 to E7, indicated by different colored boxes) and six introns (I to VI, blue lines). Exons 1–3 encode the MUC1 N-terminal and exons 4–7 encode the MUC1 C-terminal subunits. Exons encoding the corresponding domains are indicated by an arrow. Exon 1 (E1) encodes the signal peptide (SP), E2 encodes the N-terminal degenerate sequence (DS) and the VNTR, and E3 encodes the C-terminal DS. E4, E5, E6, and E7 together encode the extracellular domain (ECD), transmembrane domain (TMD), and cytoplasmic tail (CT). MUC1 is encoded as a single polypeptide chain that undergoes spontaneous cleavage at the GSVVV site (red) to generate the MUC1-N and MUC1-C subunits. (B) MUC1 pre-mRNA is spliced into four main variants of mature MUC1 mRNA – MUC1/A, MUC1/B, MUC1/C, and MUC1/D, all encoding ‘full-length’ MUC1. These isoforms are generated by alternative splicing between the fixed splice donor site near the 5′ end of intron I (red) and multiple splice acceptor sites near the 3′ and 5′ end of intron I and exon II, respectively (green, orange, magenta, and blue). In MUC1/A, a portion of the intron I (27 bp) is retained, coding an alternative signal peptide without causing a reading frameshift. In MUC1/B, intron I is completely removed. Portions of exon 2 are spliced out while retaining the reading frame in MUC1/C and MUC1/D, resulting in shorter VNTRs [31]. (C) MUC1 mRNA also contains a cryptic intron in exon 2. Several alternative splice acceptor sites around the 3′ end of exon 2 (brown, magenta, blue) can link with the fixed splice donor site at the 5′ end of exon 2 (red). Splicing between these sites results in the formation of the splice variants MUC1/X (or MUC1/Z), MUC1/Y, and MUC1/ZD, which completely lack the VNTR region. MUC1/SEC, the secreted isoform, is generated by a failure to excise intron 2, leading to premature abortion of transcription caused by the presence of a stop codon within intron 2 [112,113]. Abbreviations: MUC1, Mucin 1; E, exon; VNTR, variable number tandem repeat; MUC1-N, MUC1 N-terminal; MUC1-C, MUC1 C-terminal; S.D., splice donor; S.A., splice acceptor.

Figure 3

Differences between the glycosylation patterns in normal and tumor-associated MUC1 (TA-MUC1). (A) N-Acetylgalactosamine (GalNAc) is first added to the serine and threonine residues of the MUC1-N VNTR, catalyzed by αGalNAc transferases to generate the Tn antigen. Core 1 β1,3-galactose transferase (Core 1 β1,3-Gal T) catalyzes the addition of galactose to Tn antigen to generate Core 1 O-glycan (also called T antigen). GlcNAc (N-acetyl glucosamine) is then added to the Core 1 O-glycan catalyzed by Core 2 β1,6-N-acetyl glucosamine transferase (Core 2 β1,6-GlcNAc T). The sugar branches of Core 2 glycans undergo further chain elongation followed by termination upon addition of fucose, or sialic acid to the terminal sugar [114]. (B) MUC1 is hypoglycosylated in cancer cells. Tn and T antigens are sialylated to sialyl Tn and sialyl T, respectively, which are catalyzed by the enzyme α2, 6-sialyltransferase and α2, 3-sialyltransferase, respectively. Sialylation causes premature termination of chain elongation. Abbreviations: GalNAc T, N-acetylgalactosamine transferase; GlcNAc T, N-acetyl glucosamine transferase; Gal T, galactose transferase; Sialyl T, sialyl transferase; MUC1, Mucin 1; VNTR, variable number tandem repeat.

Figure 4

MUC1 overexpression and loss of polarity in cancer cells. MUC1 and growth factors are confined to the apical surface and basolateral surface of normal epithelial cells, respectively. However, tumor cells lose apicobasal polarity, and hypoglycosylated MUC1 is overexpressed all over the surface of the tumor cells, often in close proximity to the growth factors and their receptors. Abbreviation: MUC1, Mucin 1.