Dystroglycan in Skin and Cutaneous Cells: β-Subunit Is Shed from the Cell Surface

Abstract

In skin, hemidesmosomal protein complexes attach the epidermis to the dermis and are critical for stable connection of the basal epithelial cell cytoskeleton with the basement membrane (BM). In muscle, a similar supramolecular aggregate, the dystrophin glycoprotein complex links the inside of muscle cells with the BM. A component of the muscle complex, dystroglycan (DG), also occurs in epithelia. In this study, we characterized the expression and biochemical properties of authentic and recombinant DG in human skin and cutaneous cells in vitro. We show that DG is present at the epidermal BM zone, and it is produced by both keratinocytes and fibroblasts in vitro. The biosynthetic precursor is efficiently processed to the alpha- and beta-DG subunits; and, in addition, a distinct extracellular segment of the transmembranous beta-subunit is shed from the cell surface by metalloproteinases. Shedding of the beta-subunit releases the alpha-subunit from the DG complex on the cell surface into the extracellular space. The shedding is enhanced by IL-1beta and phorbol esters, and inhibited by metalloproteinase inhibitors. Deficiency of perlecan, a major ligand of alpha-DG, enhanced shedding suggesting that lack of a binding partner destabilizes the epithelial DG complex and makes it accessible to proteolytic processing.

Enzymatic processing of β‐dystroglycan recombinant ectodomain by MMP‐9: Identification of the main cleavage site

“…Therefore, the specific cleavage produced by MMP-9 at the amino acid positions 715-716, overlapping with the end of the a-DG binding epitope, removes the a-DG binding epitope impairing the a-DG/b-DG interface. This might explain the presence of the 30 kDa b-DG fragment, which con- tains the entire cytosolic domain, produced by MMP-9 in healthy tissues, such as peripheral nerve, bladder, lung, kidney (10) and in normal human keratinocytes (11). Indeed, the b-DG ectodomain shedding produced by MMP-9 removes the a-DG binding epitope interrupting the link between the extracellular matrix and the cytoskeleton.…”

“…It is well known from the literature that b-DG is a target for MMP-9, whose enzymatic activity produces a typical 30 kDa b-DG fragment detected both in pathological, as well as in physiological conditions (9)(10)(11). It was also demonstrated that the 30 kDa fragment lacks the ectodomain, as it can be still detected by a monoclonal antibody directed against the C-terminal cytosolic region of b-DG (9).…”

“…Further studies suggested that some metalloproteinases could be responsible for the production of the 30 kDa b-DG truncated form which was also detected, although in reduced amounts, in healthy tissues, such as peripheral nerve, bladder, lung, kidney (10) and in normal human keratinocytes (11). The presence of the 30 kDa b-DG fragment has been reported under different pathological conditions of the skeletal muscle, such as Duchenne muscular dystrophy, Fukuyama-type congenital dystrophy and sarcoglycanopathy (12).…”

Expression of dystroglycan correlates with tumor grade and predicts survival in renal cell carcinoma

“…However, evidence has grown for the importance of non-integrin receptors such as dystroglycan (DG), a ubiquitously expressed cell-surface receptor responsible for crucial interactions between ECM and the cytoplasmic compartment, whose expression has been shown to be reduced or lost in a variety of human cancer cell lines and primary tumors. 15,18,19,[21][22][23][24] DG is expressed in podocytes at the base of foot processes and DG-laminin interaction has been shown to be essential for epithelium development in kidney organ culture (Durbeej, 1995 #969). Moreover, it has been demonstrated that the DG complex undergoes changes in human glomerular diseases in which flattening of foot processes is directly associated with dissociation of laminin-DG complexes.…”

“…7,15,16 We and others recently demonstrated that DG expression, and mainly aDG, is reduced or lost in a variety of human cancer cell lines and primary tumors. 15,[17][18][19][20][21][22][23][24] A reduction in the expression levels of DG was shown to be most pronounced in high-grade diseases 17,21 and to have a prognostic significance in breast cancer patients. 21 We also recently demonstrated that expression of an exogenous DG cDNA inhibits proliferation of both normal and cancer breast epithelial cells and reduces the anchorage-independent growth and tumorigenicity of breast cancer cells, thus confirming an important role of this molecule in the maintenance of the transformed phenotype of these cells.…”