In vitro studies suggest a role for c-Jun N-terminal kinases (JNKs) in proatherogenic cellular processes. We show that atherosclerosis-prone ApoE ^–/– mice simultaneously lacking JNK2 ( ApoE ^–/– JNK2 ^–/– mice), but not ApoE ^–/– JNK1 ^–/– mice, developed less atherosclerosis than do ApoE ^–/– mice. Pharmacological inhibition of JNK activity efficiently reduced plaque formation. Macrophages lacking JNK2 displayed suppressed foam cell formation caused by defective uptake and degradation of modified lipoproteins and showed increased amounts of the modified lipoprotein-binding and -internalizing scavenger receptor A (SR-A), whose phosphorylation was markedly decreased. Macrophage-restricted deletion of JNK2 was sufficient to decrease atherogenesis. Thus, JNK2-dependent phosphorylation of SR-A promotes uptake of lipids in macrophages, thereby regulating foam cell formation, a critical step in atherogenesis.

Dysfunction and loss of insulin-producing pancreatic beta cells represent hallmarks of diabetes mellitus. Here, we show that mice lacking the mitogen-activated protein kinase (MAPK) p38delta display improved glucose tolerance due to enhanced insulin secretion from pancreatic beta cells. Deletion of p38delta results in pronounced activation of protein kinase D (PKD), the latter of which we have identified as a pivotal regulator of stimulated insulin exocytosis. p38delta catalyzes an inhibitory phosphorylation of PKD1, thereby attenuating stimulated insulin secretion. In addition, p38delta null mice are protected against high-fat-feeding-induced insulin resistance and oxidative stress-mediated beta cell failure. Inhibition of PKD1 reverses enhanced insulin secretion from p38delta-deficient islets and glucose tolerance in p38delta null mice as well as their susceptibility to oxidative stress. In conclusion, the p38delta-PKD pathway integrates regulation of the insulin secretory capacity and survival of pancreatic beta cells, pointing to a pivotal role for this pathway in the development of overt diabetes mellitus.

The AP-1 transcription factor c-Jun is a key regulator of hepatocyte proliferation. Mice lacking c-Jun in the liver (c-jun^Δli*) display impaired liver regeneration after partial hepatectomy (PH). This phenotype correlates with increased protein levels of the cdk-inhibitor p21 in the liver. We performed PH experiments in several double-knockout mouse models to genetically identify the signaling events regulated by c-Jun. Inactivation of p53 in c-jun^Δli* mice abrogated both hepatocyte cell cycle block and increased p21 protein expression. Consistently, liver regeneration was rescued in c-jun^Δli*p21^−/− double-mutant mice. This indicated that c-Jun controls hepatocyte proliferation by a p53/p21-dependent mechanism. Analyses of p21 mRNA and protein expression in livers of c-jun^Δli* mice after PH revealed that the accumulation of p21 protein is due to a post-transcriptional/post-translational mechanism. We have investigated several candidate pathways implicated in the regulation of p21 expression, and observed increased activity of the stress kinase p38 in regenerating livers of c-jun^Δli* mice. Importantly, conditional deletion of p38α in livers of c-jun^Δli* mice fully restored hepatocyte proliferation and attenuated increased p21 protein levels after PH. These data demonstrate that c-Jun/AP-1 regulates liver regeneration through a novel molecular pathway that involves p53, p21, and the stress kinase p38α.