Much less is known about the mechanism of DDR clustering
Much less is known about the mechanism of DDR clustering and its role in the receptor function. It is understood that DDR1 exists as an inactive dimer on the cell surface and undergoes further oligomerization upon ligand binding (Abdulhussein et al., 2008, Noordeen et al., 2006, Mihai et al., 2009). Thus oligomerization seems to be a prerequisite for receptor activation. At present we are unable to define the reasons behind the heterogeneity observed in DDR1 oligomer sizes. We speculate that like the EGFR receptor, the large-scale (>10nm) clusters of DDR1 may dampen receptor activation and only the tetramer–octamer population (4–5nm in height) may be responsible for signaling. It is interesting to note that the time taken (4h) for formation of maximum number of DDR1 ECD tetramer–octamer in-vitro, coincides with that for maximal receptor phosphorylation (Vogel et al., 1997). An increase in oligomer size with collagen stimulation time was also observed for full length DDR1b–YFP in our earlier studies (Mihai et al., 2009). Further studies using ras inhibitor against phophorylated DDR1 may help elucidate how oligomer size correlates with receptor phosphorylation.
Endocytosis of the oligomerized receptor may also contribute to receptor function. In our previous study (Mihai et al., 2009), DDR1b oligomers formed upon ligand binding underwent rapid internalization and incorporation into the early endosomal compartments, an important site of receptor-initiated signal transduction (von Zastrow and Sorkin, 2007). In addition to their role as active signaling kinases it has been suggested (Fu et al., 2013) that DDRs, may act as molecular scaffolds similar to other kinases such as integrin-linked kinase (ILK) and kinase suppressor of ras1 (KSR1) (Hannigan et al., 2011, Zhang et al., 2012). Integrin clustering was shown to modulate downstream signaling at focal adhesion sites where these molecular scaffolds develop and it is possible that DDR clustering could play a similar role (Shi and Boettiger, 2003, Ye et al., 2010). Further studies would be needed to fully characterize the role played by DDR oligomerization in receptor function and signaling.
In conclusion, DDR1 ECD was shown to be sufficient for collagen mediated DDR1 oligomerization, and the oligomerized form binds to collagen with increased affinity. In full length receptors expressed on live cells, DDR1 oligomerization occurred on the cell surface in agreement with the role of DDR1 ECD in mediating oligomer formation. Together with our previous observations using cells expressing DDR1b–YFP (Mihai et al., 2009), that the kinetics of DDR1 oligomerization is very fast (minutes), we propose that the receptor oligomerization precedes its phosphorylation upon collagen stimulation. These insights into receptor oligomerization may help design strategies to modulate DDR1-collagen interaction, receptor function, and downstream signaling.
Introduction Ranked as the seventh most common cancer in women, Epithelial ovarian cancer (EOC) generally has a poor prognosis due to its insidious onset that defy early detection. Most EOC patients are presented at advanced stages and the overall survival merely ranges from 30 to 50% (Ferlay et al., 2015). EOC consists of a heterogeneous entity with distinct histopathological subtypes. The more common subtypes are serous, mucinous, and endometrioid carcinomas; while the less common subtypes include clear cell, transitional, squamous, mixed, and undifferentiated subtype (Hennessy et al., 2009, Silverberg, 1989). Regardless of the different histological subtypes, the treatment for advanced EOC patients remains as the standard surgical debulking followed by taxane/platinum-based chemotherapy (Coleman et al., 2013). This points to the need for better patient stratification based on tumour phenotypes, as well as more targeted, phenotype-specific treatments. By large-scale gene expression profiling, a few studies have classified EOC into different molecular subtypes (Tothill et al., 2008, Cancer Genome Atlas Research Network, 2011, Tan et al., 2013). All three studies identified a mesenchymal subtype in EOC that is associated with an evolutionarily conserved developmental pathway—epithelial-mesenchymal transition (EMT).