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E R stress occurs in macrophage-rich areas of advanced atherosclerotic lesions and contributes to macrophage apoptosis and subsequent plaque necrosis. Therefore, signaling pathways that alter ER stress–induced apoptosis may affect advanced atherosclerosis. Here we placed Apoe^–/– mice deficient in macrophage p38α MAPK on a Western diet and found that they had a marked increase in macrophage apoptosis and plaque necrosis. The macrophage p38α–deficient lesions also exhibited a significant reduction in collagen content and a marked thinning of the fibrous cap, which suggests that plaque progression was advanced in these mice. Consistent with our in vivo data, we found that ER stress–induced apoptosis in cultured primary mouse macrophages was markedly accelerated under conditions of p38 inhibition. Pharmacological inhibition or genetic ablation of p38 suppressed activation of Akt in cultured macrophages and in atherosclerotic lesions. In addition, inhibition of Akt enhanced ER stress–induced macrophage apoptosis, and expression of a constitutively active myristoylated Akt blocked the enhancement of ER stress–induced apoptosis that occurred with p38 inhibition in cultured cells. Our results demonstrate that p38α MAPK may play a critical role in suppressing ER stress–induced macrophage apoptosis in vitro and advanced lesional macrophage apoptosis in vivo.