HA/CD44 Interaction with Cytoskeletal Proteins, Ankyrin, and ERM

19 March, 2011 (23:45) | Diseases | By: Health news

Ankyrin is a membrane-associated cytoskeletal protein that directly binds CD44.10 This CD44-ankyrin interaction causes cytoskeleton activation. Hyaluronan binding to CD44 promotes colocalization of CD44 and ankyrin in cholesterol-containing lipid rafts, and this colocalization appears to be a key mechanism in regulating HA-mediated cytoskeleton function and tumor cell-specific behaviors (eg, cell survival, growth, and migration). Ankyrin- and CD44-containing lipid rafts have been documented in three different tumor cell lines (breast cancer, ovarian cancer, and HNSCC).10

Up-regulated expression of the ezrin-radixin-moesin (ERM) family of cytoskeletal proteins is seen in HNSCC and is associated with poor prognosis.45 ERM proteins are linkers between membrane molecules such as CD44 and the cytoskeleton. Ezrin is a key regulator of tumor metastasis. CD44 interacts with ERM proteins and with merlin, a related protein. Loss of merlin results in increased HA/CD44-mediated tumorigenesis, and overexpression of merlin diminishes tumor cell growth. In HNSCC, moesin appears to interact with CD44 to degrade the ECM at the invasive front of oral cancers. Both the level of expression and the subcellular localization (cytoplasmic) of ERM proteins are indicators of clinical outcome in HNSCC. Higher expression and cytoplasmic localization of ERM proteins are indicators of poorer survival in HNSCC. Osteopontin, a ligand for CD44, colocalizes with CD44 and ezrin in fibroblasts, metastatic breast cancer cells, and HNSCC.
RhoA-Regulated PLC, ROK, and Ca2+ Signaling and Cytoskeleton Activation

Similar mechanisms for CD44-mediated Ca2+ mobilization pathways to promote tumor migration appear to exist in HNSCC. In HNSCC, Bourguignon et al37 reported that CD44 physically associates in a multimolecular complex with LARG and EGFR. HA/CD44 interaction induces LARG-specific RhoA signaling. LARG molecules isolated from HNSCC cells were found to function as a GDP/GTP exchange factor for RhoGTPases, and the basal rate of bound GTP increased at least 2.4-fold with the addition of HA.

To establish a linkage between HA/CD44-mediated LARG-RhoA signaling and intracellular Ca2+ regulation, one member of the PLC family, PLCε, was isolated from HNSCC cells with GDP- or GTP-loaded forms of RhoA-GST-conjugated beads. PLCε-RhoA interaction was found to be GTP-dependent. LARG-activated RhoA was shown to stimulate both the PLCε-mediated IP3 production and the IP3 receptor-triggered intracellular Ca2+ mobilization in HNSCC cells. HNSCC cells incubated with Fura-2/AM were treated with HA with or without pretreatment with inhibitors of PLC and IP3 receptor; fluorescence spectrophotometry demonstrated a rise in intracellular Ca2+ with HA treatment, but not in the presence of pretreatment with various inhibitors.40 These findings suggest that Ca2+ signaling in HNSCC cells involves both HA/CD44-dependent and RhoA/PLC/IP3 receptor-regulated processes.