Research
Research in our laboratory focuses on the molecular and cellular basis for the functional relationships between integrins and growth factors in mediating physiological and pathological responses, ranging from hemostasis/thrombosis, angiogenesis to tumor growth. Recent studies from our lab (Byzova et al, JCB 1998, Molecular Cell 2000) have suggested that b3 integrin, aVb3, can be classified as "activatable" receptor. Since aVb3 is expressed broadly on vascular cells and has been implicated in complex processes ranging from angiogenesis to apoptosis to restenosis, regulation of aVb3 functional activity has the potential to play many crucial roles in vascular biology. We found that VEGF165 is the most efficient in activating integrin aVb3. We began to determine intracellular signaling molecules involved in this process and found that Akt kinase plays a role in activation of aVb3 and this role can be most convincingly demonstrated by comparing the consequences of inactivation of the Akt gene on the function of aVb3 and aIIbb3. We are in the process of characterization of role of Akt in aVb3 activation in vivo in the process of VEGF-stimulated angiogenesis in normal and Akt-1 null animals and in aIIbb3 activation in platelet mediated responses in vivo.
Our second project focuses on the role of b3 phosphorylation in integrin activation and in mediating both angiogenesis and lymphangiogenesis. The overall goal is to determine the structural requirements for the VEGFRs to transmit an activating signal to integrin; and to determine the requirement for aVb3 to receive the activation signal from the VEGFRs. In particular, we will focus on the role of beta 3 integrin cytoplasmic domain and its phosphorylation in the regulation of aVb3 activity during the process of VEGF-induced angiogenesis. We are using mice in which the two tyrosines that are targets for phosphorylation have been mutated (knock-in mice). In our laboratory we have become proficient in the isolation and characterization of murine endothelial cells and have developed unique in vivo models to study both angiogenesis and lymphangiogenesis. Thus we will: a) Determine the molecular mechanisms for interaction between VEGFR-2 and VEGFR-3 and aVb3. b) Evaluate the role of b3 integrin phosphorylation in VEGF-induced angiogenesis and lymphangiogenesis o in different tissues using subcutaneous and muscle models developed in our previous studies (Byzova et al, Blood, 2002).
Integrin activation and recognition of extracellular matrix plays a crucial role in the tumor development and, in particular, in the process of metastasis. Bone is by far the primary site of prostate cancer metastasis and it is clear that bone-specific matrix proteins and their receptors on the surface of prostate cancer cells (integrins) play a pivotal role in the process. Thus, another project centers on the mechanisms of the functional communications between integrins, and a subset of its ligands, the bone matrix proteins in the context of prostate cancer. We focus on a component of bone matrix, SPARC (secreted protein, acidic and rich in cysteine). In our studies we have defined a new molecular pathway that may account for the extraordinarily high osteotropism of prostate cancer (De et al, JBC 2003). Using SPARC-deficient mice and recombinant SPARC, we demonstrated that SPARC selectively supports the migration of highly metastatic relative to less metastatic prostate cancer cell lines to bone. Increased migration to SPARC can be traced to activation of integrins aVb3 and aVb5 which is induced by an autocrine VEGF/VEGFR-2 loop on tumor cells. A particularly relevant pathophysiological consequence of SPARC recognition by aVb5 is the upregulation of VEGF production, which provides prostate cancer cells with significant growth advantage in bone tissue. At sites of metastasis characterized by abundant SPARC, high VEGF production and enhanced neovascularization, aVb3 and aVb5 integrin activation is substantially higher in comparison to the tumor localized in the prostate.