| Summary: | Angiotensin II (Ang II) is an important modulator for development of atherosclerosis from early stage foam cell formation to advanced stage plaque rupture. Recently, the importance of locally generated Ang II, especially in macrophages, has become more evident. Generation of Ang II by several enzymes other than ACE and renin has been shown mainly in vitro. Cathepsin G is one these enzymes which is expressed in neutrophils and macrophages. Macrophages are one of the primary and crucial cells in atherosclerotic lesions which become lipid-laden foam cells through lipoprotein uptake. We hypothesized that activation of nuclear factors in foam cells increases Ang II by modulation of the renin angiotensin system (RAS) genes and cathepsin G. We also hypothesized that cathepsin G, through its Ang II generating activity and its other catalytic functions, promotes atherosclerosis.
The present study assessed the Ang I and II levels and expression of the RAS genes in THP-1 cells, a human acute monocytic leukemia cell line, and in peritoneal and bone marrow-derived macrophages after exposure to acetylated LDL (ac-LDL). I also evaluated how RAS blockade would affect foam cell formation in THP-1 cells. In parallel, I assessed the role of cathepsin G in Ang II generation and in the progression of atherosclerosis in cathepsin G heterozygous knockout mice on an Apoe-/- background (Ctsg+/-Apoe-/- mice).
Ac-LDL treatment increased Ang I and Ang II levels in cell lysates and media from THP-1 cells but not in peritoneal or bone marrow-derived macrophages from wild type C57BL/6 mice. In ac-LDL-treated THP-1 cells, ACE and cathepsin G mRNA levels and activities were elevated. Angiotensinogen mRNA is increased but not the angiotensinogen protein concentration. Renin mRNA level and activity were not altered by ac-LDL treatment. Blocking RAS by an AT1 receptor blocker, ACE inhibitors or a renin inhibitor decreased cholesteryl ester content of THP-1 cells after exposure to ac-LDL. To confirm that the Ang II effect on foam cell formation was not unique to ac-LDL, we treated the THP-1 macrophages with a renin inhibitor or an AT1 receptor inhibitor after exposure to oxidized LDL (ox-LDL). RAS blockade in ox-LDL-treated cells also abolished cholesteryl ester formation. To see how Ang II plays a role in foam cell formation we assessed the effect of RAS inhibitors on SR-A, the principal receptor for mediating ac-LDL entry into the cells and on acyl-CoA:cholesterol acyl transferase (ACAT-1), the enzyme responsible for intracellular cholesterol esterification. RAS blockade in both ac-LDL- and ox-LDL-treated cells decreased SR-A and ACAT-1 protein levels.
Cathepsin G partial deficiency on an Apoe-/- background did not change Ang II levels in peritoneal or bone marrow-derived macrophage cell lysates or media. This deficiency also did not affect immunoreactive angiotensin peptide levels in atherosclerotic lesions. After 8 weeks on a high fat diet Ctsg+/-Apoe-/- mice were similar to Ctsg+/+Apoe-/- mice in terms of lesion size and serum cholesterol levels but the Ctsg+/+Apoe-/- mice had more advanced lesions with more collagen and smooth muscle cells and fewer macrophages. Moreover, Ctsg+/+Apoe-/- mice had more apoptotic cells than their Ctsg+/-Apoe-/- littermates.
Overall, our findings indicate that Ang II is increased in foam cells and this endogenous Ang II is involved in cholesteryl ester formation, possibly by regulating the levels of ACAT-1 and SR-A. We did not find any role for cathepsin G in generation of Ang II in mice but cathepsin G does, nevertheless, promote the progression of atherosclerotic lesions to a more advanced stage.
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