Abstract —Apolipoprotein E (apoE) is a multifunctional protein synthesized by the liver and tissue macrophages. ApoE-deficient mice have severe hyperlipidemia and develop accelerated atherosclerosis on a chow diet. Both liver-derived and macrophage-derived apoEs have been shown to reduce plasma lipoprotein levels and slow the progression of atherosclerosis in apoE-deficient mice, but regression of atherosclerosis has not been demonstrated in this model. We utilized second-generation adenoviruses to achieve hepatic expression of human apoE in chow-fed, apoE-deficient mice with established atherosclerotic lesions of different stages. As expected, hepatic expression of human apoE3 significantly reduced plasma cholesterol levels. Liver-derived apoE also accumulated substantially within preexisting atherosclerotic lesions, indicating that plasma apoE gained access to the arterial intima. Hepatic expression of human apoE3 for 6 weeks resulted in significant quantitative regression of both early fatty streak lesions as well as advanced, complex lesions in both the aortic root and the aortic arch. In addition, hepatic expression of apoE induced substantial morphological changes in lesions, including decreased foam cells and increased smooth muscle cells and extracellular matrix content. In parallel, human apoE4 and apoE2 were also expressed in the liver by using recombinant adenoviruses. ApoE4 reduced cholesterol levels to the same extent as did apoE3 and also prevented progression but did not induce significant regression of preexisting lesions. ApoE2 reduced cholesterol levels to a lesser degree than did apoE3 and apoE4 and lesion progression was reduced, but regression was not induced. In summary, (1) regression of preexisting atherosclerotic lesions in apoE-deficient mice can be rapidly induced by hepatic expression of apoE, despite the absence of macrophage-derived apoE; (2) the morphological changes seen in this model of regression resemble those in other animal models, induced over longer periods of time; (3) liver-derived apoE gained access to and was retained by intimal atherosclerotic lesions; and (4) apoE4 was less effective in inducing regression, despite its effects on plasma lipoproteins that were similar to those of apoE3. The rapid regression of preexisiting atherosclerotic lesions induced by apoE gene transfer in apoE-deficient mice could provide a convenient murine model for investigation of the molecular events associated with atherosclerosis regression.
Great inroads have been made in defining the oncogenic pathways intrinsic to neoplastic cells and the mechanisms by which they are activated in tumors. Knowledge of these pathways provides numerous opportunities that are actively being pursued to develop targeted therapies for cancer. Complementary studies, focused on the non-transformed components of the tumor microenvironment (TME), have revealed that the extrinsic cues provided by the TME are also essential for tumor cells to manifest a fully transformed phenotype, angiogenesis and metastasis. Delineation of these cues and their underlying cellular and molecular pathways will thus lead to a new era of integrative cancer therapy based on combinatorial drug regiments that act synergistically to destroy the neoplastic cells by targeting both the intrinsic and extrinsic pro-oncogenic pathways. Tumor-associated fibroblasts (TAFs) and proteases are two of the key regulators of epithelial-derived tumors that represent potential targets of such integrative therapies. Herein, we consider the potential therapeutic benefit of inhibiting the function of fibroblast activation protein (FAP), a cell surface serine protease with dipeptidyl peptidase and endopeptidase activity that is expressed on TAFs and pericytes, in an integrative approach to treating cancer.
CD44, a receptor for the extracellular matrix glycosaminoglycan hyaluronan, has been implicated in many adhesion‐dependent cellular processes including tumor growth and metastasis. Soluble CD44 has been identified in the serum of normal individuals. Furthermore, tumor progression is often associated with marked increases in plasma levels of soluble CD44. Release of soluble CD44 by proteolytic cleavage (shedding) of membrane‐anchored CD44 is likely to alter cellular responses to the environment due to modification of the cell surface and the potential for soluble CD44 to influence CD44‐mediated hyaluronan binding to cell surfaces. Cellular activation is typically required to induce hyaluronan binding to cell surface CD44 but the affinity of endogenous soluble CD44 for hyaluronan remains unknown. In this study, we demonstrate that oncostatin M and transforming growth factor β1 (TGF‐β1) which stimulate hyaluronan binding to HTB58 lung epithelial‐derived tumor cells, also induce the release of soluble CD44. Interestingly, soluble CD44 released by oncostatin M‐treated cells retained the ligand‐binding properties of the membrane‐anchored receptor. In contrast, soluble CD44 released from TGF‐β1‐treated HTB58 cells differed in its hyaluronan‐binding capacity from cell surface CD44 expressed on TGF‐β1‐stimulated cells. These data indicate that the mechanisms that regulate the generation of soluble CD44 may also govern the binding of the released receptor to hyaluronan and therefore determine the impact on CD44‐dependent physiologic and pathologic processes.
