Collagen, estimated to account for 25 – 35% of all protein in the human body (Canty and Kadler, 2005), is responsible for maintaining the structure of fibrous tissues such as bone, tendons, ligaments and skin (DiLullo et al., 2002). However, it is increasingly clear that collagen also functions in molecular signalling networks.
Using phosphoproteomic analysis, a team from the Institute of Cancer Research in London has delineated part of the collagen signalling network, showing that the tyrosine kinase receptor, DDR2, is activated by collagen (Iwai et al., 2013). This is a particularly interesting finding as activating mutations of the DDR2 receptor can cause lung squamous cell carcinoma, indicating that collagen signalling is active in the lung.
In addition to its function in the lung and in maintaining skin elasticity, collagen is also an integral part of the glomeruli of nephron (Miner, 2012) – the part of the kidney responsible for blood filtration – meaning that collagen functions in all three organs affected by BHD syndrome. Indeed, mutations in the COL IV family of collagen genes causes Alport syndrome and Goodpasture syndrome, which are characterised by advanced kidney disease, and autoimmune lung and kidney disease respectively.
Furthermore, a number of signalling pathways and proteins that have previously been linked to the BHD gene, FLCN, also converge on collagen.
Lim kinase 1 and 2 (LIMK1/2) inhibition reduces collagen contraction, which in turn reduces the invasiveness of tumour cells (Scott et al., 2010). The LIMKs work antagonistically with the slingshot genes, to regulate phosphorylation of Cofilin, and earlier this year it was reported that the Slingshot 2 gene is a synthetic lethal target of FLCN, suggesting that, like collagen, FLCN intersects with the LIMK/SSH/Cofilin pathway. Additionally, the LIMKs are downstream targets of RhoA signalling (Scott and Olsen, 2007), which is also dysregulated in FLCN-null cells.
The studies that identified RhoA signalling is dysregulated in FLCN-null cells, also found that cell-cell adhesion and junction formation was perturbed in these cells, as shown by reduced desmosome formation and a mis-localisation of E-cadherin. The integrin and DDR collagen receptors have been shown to act antagonistically to regulate E-cadherin’s localisation to the plasma membrane (Yeh et al., 2012), indicating that collagen signalling may also control cell-junction formation.
The structure of FLCN’s DENN domain was published last year; in vitro experiments suggested that FLCN could be function as GEF towards Rab proteins and three separate microarray studies have shown that Rab27b expression is activated by FLCN. Interestingly, Rab27b is required for correct collagen secretion from developing osteoblasts (Nabavi et al., 2012).
Finally, MMP9 stimulates collagen synthesis, and is in turn inhibited by SMAD7 (Yu et al., 2013). Dysregulation of matrix metalloproteinase expression, including MMP9, has been implicated in BHD lung pathogenesis, and FLCN has been shown to activate the expression of SMAD7 through regulation of TGF-β signalling (Hong et al., 2010a).
While it is important to note that there is no evidence directly linking FLCN function to collagen function, taken together the above studies suggest that FLCN may regulate collagen function via RhoA signalling, TGF-β signalling, and/or its function in membrane trafficking, and that collagen may subsequently regulate E-cadherin localisation, cell junction formation and cell migration. Determining whether collagen does have a role in the pathogenesis of BHD syndrome will shed light on FLCN’s function and may suggest further avenues of research and drug development.
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Hong SB, Oh H, Valera VA, Stull J, Ngo DT, Baba M, Merino MJ, Linehan WM, & Schmidt LS (2010). Tumor suppressor FLCN inhibits tumorigenesis of a FLCN-null renal cancer cell line and regulates expression of key molecules in TGF-beta signaling. Molecular cancer, 9 PMID: 20573232
Iwai LK, Payne LS, Luczynski MT, Chang F, Xu H, Clinton RW, Paul A, Esposito EA, Gridley S, Leitinger B, Naegle KM, & Huang PH (2013). Phosphoproteomics of collagen receptor networks reveals SHP-2 phosphorylation downstream of wild-type DDR2 and its lung cancer mutants. The Biochemical journal, 454 (3), 501-13 PMID: 23822953
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Yu H, Huang J, Wang S, Zhao G, Jiao X, & Zhu L (2013). Overexpression of Smad7 suppressed ROS/MMP9-dependent collagen synthesis through regulation of heme oxygenase-1. Molecular biology reports, 40 (9), 5307-14 PMID: 23661026