SUMMARY Macrophage apoptosis in advanced atheromata, a key process in plaque necrosis, involves the combination of ER stress with other pro-apoptotic stimuli. We show here that oxidized phospholipids, oxidized LDL, saturated fatty acids (SFAs), and lipoprotein(a) trigger apoptosis in ER-stressed macrophages through a mechanism requiring both CD36 and toll-like receptor 2 (TLR2). In vivo, macrophage apoptosis was induced in SFA-fed, ER-stressed wild-type but not Cd36−/− or Tlr2−/− mice. For atherosclerosis, we combined TLR2 deficiency with that of TLR4, which can also promote apoptosis in ER-stressed macrophages. Advanced lesions of fat-fed Ldlr−/− mice transplanted with Tlr4−/−Tlr2−/− bone marrow were markedly protected from macrophage apoptosis and plaque necrosis compared with WT → Ldlr−/− lesions. These findings provide insight into how atherogenic lipoproteins trigger macrophage apoptosis in the setting of ER stress and how TLR activation might promote macrophage apoptosis and plaque necrosis in advanced atherosclerosis.

Proteins are covalently modified with a variety of lipids, including fatty acids, isoprenoids, and cholesterol. Lipid modifications play important roles in the localization and function of proteins. The focus of this review is S‐palmitoylation, the reversible addition of palmitate and other long‐chain fatty acids to proteins at cysteine residues in a variety of sequence contexts. The functional consequences of palmitoylation are diverse. Palmitoylation facilitates the association of proteins with membranes, mediates protein trafficking, and more recently has been appreciated as a regulator of protein stability. Members of a family of integral membrane proteins that harbor a DHHC cysteine‐rich domain mediate most cellular palmitoylation events. Here we focus on DHHC proteins that modify Ras proteins in yeast and mammalian cells.

Pharmacologic approaches to studying palmitoylation are limited by the lack of specific inhibitors. Recently, screens have revealed five chemical classes of small molecules that inhibit cellular processes associated with palmitoylation (Ducker, C. E., L. K. Griffel, R. A. Smith Compounds that selectively inhibited palmitoylation of Nmyristoylated vs. farnesylated peptides were identified in assays of palmitoyltransferase activity using cell membranes. Palmitoylation is catalyzed by a family of enzymes that share a conserved DHHC (Asp-His-His-Cys) cysteine-rich domain. In this study, we evaluated the ability of these inhibitors to reduce DHHC-mediated palmitoylation using purified enzymes and protein substrates. Human DHHC2 and yeast Pfa3 were assayed with their respective N-myristoylated substrates, Lck and Vac8. Human DHHC9/GCP16 and yeast Erf2/Erf4 were tested using farnesylated Ras proteins. Surprisingly, all four enzymes showed a similar profile of inhibition. Only one of the novel compounds, 2-(2-hydroxy-5-nitro-benzylidene)-benzo [b]thiophen-3-one [Compound V (CV)], and 2-bromopalmitate (2BP) inhibited the palmitoyltransferase activity of all DHHC proteins tested. Hence, the reported potency and selectivity of these compounds were not recapitulated with purified enzymes and their cognate lipidated substrates. Further characterization revealed both compounds blocked DHHC enzyme autoacylation and displayed slow, time-dependent inhibition but differed with respect to reversibility. Inhibition of palmitoyltransferase activity by CV was reversible, whereas 2BP inhibition was irreversible.

Rationale Myeloid-derived C/EBP-homologous protein (CHOP), an effector of the endoplasmic reticulum (ER) stress-induced Unfolded Protein Response, promotes macrophage apoptosis in advanced atherosclerosis, but the role of CHOP in vascular smooth muscle cells (VSMCs) in atherosclerosis is not known. Objective To investigate the role of CHOP in SM22α+ VSMCs in atherosclerosis. Methods and Results Chopfl/fl mice were generated and crossed into the Apoe−/− and SM22α-CreKI+ backgrounds. SM22α-CreKI causes deletion of floxed genes in adult SMCs. After 12 wks of Western-type diet feeding, the content of α-actin-positive cells in aortic root lesions was decreased in Chopfl/flSM22α-CreKI+Apoe−/− vs. control Chopfl/flApoe−/− mice, and aortic explant-derived VSMCs from the VSMC-CHOP-deficient mice displayed reduced proliferation. Krüppel-like factor 4 (KLF4), a key suppressor of VSMC proliferation, was increased in lesions and aortic VSMCs from Chopfl/flSM22α-CreKI+Apoe−/− mice, and silencing Klf4 in CHOP-deficient VSMCs restored proliferation. CHOP deficiency in aortic VSMCs increased KLF4 through two mechanisms mediated by the ER stress effector ATF4: transcriptional induction of Klf4 mRNA and decreased proteasomal degradation of KLF4 protein. Conclusions These findings in SM22α-CHOP-deficient mice imply that CHOP expression in SM22α+ VSMCs promotes cell proliferation by down-regulating KLF4. The mechanisms involve newly discovered roles of CHOP in the transcriptional and post-translational regulation of KLF4.