The Regulated Expression of Focal Adhesion Kinase (FAK) Family Kinases Modulates Mononuclear Phagocyte Differentiation, Migration, and Breast Tumor Progression

A hallmark of all unicellular and multicellular organisms is the maintenance of homeostasis, which is achieved through the interpretation of extracellular stimuli coordinated with the appropriate effector response to these cues. This quality is exquisitely evident in cells of the mononuclear phagocyte system (MPS). Sharing lineage with primitive phagocytic cells and amoebocytes that reside in invertebrates, the MPS has surfaced in more evolved organisms to populate every tissue. The MPS is tightly regulated, which is reflected in the heterogeneity of cell populations that comprise this system and the effector capabilities that these cells harbor. Evidence indicates that, during steady state and disease conditions, the essential characteristics of phenotypic plasticity and migration associated with these cells are utilized to maintain balance as well as potentiate pathologies.
Intracellular signaling cascades have evolved to link the receptors that sense the microenvironment with intrinsic cell properties that permit effector function. My thesis research has focused on the role of the related non-tyrosine kinases focal adhesion kinase (FAK) and proline rich tyrosine kinase 2 (Pyk2) in the regulation of MPS development and function. Data from these studies have revealed that expression of FAK and Pyk2 are regulated during MPS differentiation, and that Pyk2 is expressed prior to FAK as lineage-committed monocytic cells differentiate. This is evident during the generation of monocyte cell populations in response to differentiation factors, where FAK can be seen to enhance monocyte differentiation and/or survival while Pyk2 blocks differentiation and/or enhances survival under certain conditions. This work has implications for a wide array of biological processes, including hematopoiesis in the bone marrow, the maintenance of circulating Ly6Chigh and Ly6Clow monocyte populations, and the generation of MPS populations in peripheral tissues during steady state and disease.
Prior studies have shown that FAK regulates the migration of fully differentiated macrophages toward chemokines and growth factors in vitro. In addition, FAK is necessary to generate macrophage accumulation in the peritoneal cavity during inflammation. We demonstrate here that the regulated expression of FAK in monocytic cells has functional implications. Our work revealed that FAK is nearly undetectable in Ly6Chigh monocytes but that expression increases significantly as monocytes differentiate and downregulate Ly6C. Utilizing monocytes derived from wild type (WT) and conditional FAK knockout mice (FAKΔmyeloid), we demonstrated that Ly6Clow monocytes lacking FAK migrated less efficiently toward macrophage colony stimulating factor (M-CSF) than those from WT mice. Additionally, Ly6Chigh monocytes displayed reduced migration relative to Ly6Clow monocytes, and the Ly6Chigh populations derived from WT and FAKΔmyeloid migrated equivalently. This suggests that migratory properties associated with monocytes are enhanced in congruence with FAK expression, and that efficient migration of these cells is FAK-dependent.
Evidence from human cancer patients and murine tumor models indicate that mononuclear phagocytes localize to the tumor microenvironment and influence tumor progression. We assessed if FAK expression in monocyte populations may also regulate progression of the MMTV-PyVmT murine breast carcinoma model. The rate of early stage tumor outgrowth was enhanced in FAKΔmyeloid mice crossed onto this background relative to WT MMTV-PyVmT mice, indicating either that FAK in myeloid lineage cells is necessary for an efficient anti-tumor response and/or that it blocks pro-tumor functions of these cells. Our evidence indicates that this regulation is limited to early stage events, as tumor vasculature and metastatic tumor burden are similar in both cohorts. We are currently investigating whether the migratory or functional capacity of tumor associated Ly6Clow monocytes is involved in orchestrating MPS control of early tumor outgrowth.