Many adult connective tissues undergo continuous remodeling to maintain matrix homeostasis. Physiological remodeling involves the degradation of collagen fibers by the intracellular cathepsin-dependent phagocytic pathway. We considered that a multidomain, small GTPase activating protein, IQGAP1, which is involved in the generation of cell extensions, is required for collagen phagocytosis, possibly arising from its interactions with cdc42 and the actin-binding protein Flightless I (FliI). We examined the role of IQGAP1 in collagen phagocytosis by human gingival fibroblasts (HGFs) and by IQGAP1+/+ and IQGAP1-/- mouse embryonic fibroblasts. IQGAP1 was strongly expressed by HGFs, localized to vinculin-stained cell adhesions and sites where cell extensions are initiated, and colocalized with FliI. Immunoprecipitation showed that IQGAP1 associated with FliI. HGFs showed 10-fold increases of collagen binding, 6-fold higher internalization, and 3-fold higher β1 integrin activation between 30 and 180 min after incubation with collagen. Compared with IQGAP1+/+ fibroblasts, deletion of IQGAP1 reduced collagen binding (1.4-fold), collagen internalization (3-fold), β1 integrin activation (2-fold), and collagen degradation (1.8-fold). We conclude that IQGAP1 affects collagen remodeling through its regulation of phagocytic degradation pathways, which may involve the interaction of IQGAP1 with FliI.
Keywords: cell adhesion; cell extensions; collagen; phagocytosis; small GTPases.
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