New Roles of Filamins in Cell Signaling, Transcription and Organ Development

Abstract

Filamins are large actin-binding proteins that stabilize delicate three-dimensional actin networks and link them to cellular membranes. They integrate cell architectural and signaling functions and are essential for cell locomotion and development. This thesis includes studies of two abundantly expressed filamin members, filamin A (FLNA) and B (FLNB). FLNA has recently been shown to bind to the proteins that are related to cell motility and are implicated in diseases. The number of known FLNA interacting proteins is increasing, thus a complete understanding of the role of FLNA in diseases still requires intensive study. We identified hypoxia-inducible factor-1α (HIF-1α), a transcription factor, as a novel interacting partner of FLNA and studied the influence of their interaction on HIF-1α signaling in FLNA-deficient and FLNA-expressing human tumor cells. At hypoxia, cleavage of FLNA by calpain was induced. The cleaved C-terminal fragment interacted with HIF-1α and facilitated nuclear translocation and transactivation activity of HIF-1α. As a consequence, FLNA-deficient tumor cells produced less VEGF-A and exhibited an impaired ability to induce proliferation and migration of endothelial cells. In addition, we discovered that the interaction between FLNA and another transcription factor SMAD2 partially regulates c-MET expression. FLNA-deficient tumor cells expressed less c-MET and displayed impairments in c-MET signaling and hepatocyte growth factor-induced cellular migration. These results suggest that FLNA is important for cellular motility and may influence tumor growth by regulating angiogenesis and tumor metastasis in response to chemoattractants. FLNB mutations in humans are associated with devastating congenital malformations. However, the causal role of FLNB in these genetic disorders is unknown. Using a gene-trapping technique, we generated a mouse model of Flnb-deficiency, which led to a high embryonic lethality. A few Flnb-deficient mice that reached term displayed severe skeletal malformations and disorganized microvasculature. Flnb-deficiency impaired the cell motility of embryonic fibroblasts, which may partly explain the observed developmental consequences. Generation of in vivo and in vitro models of Flnb-deficiency will advance our understanding of the biological importance of FLNB in organ development and disease progression.