Inhibition of insulin signaling by oxidized low density lipoprotein. Protective effect of the antioxidant Vitamin E.

Oxidative stress is involved in several pathological conditions, including diabetes. Reactive oxygen species (ROS) have been demonstrated to act as second messengers for several hormones and cytokines, including insulin (INS). The effect of Cu(2+)-oxidized LDL (CuLDL) on INS-induced generation of ROS and on INS signaling was investigated on cultured human fibroblasts. Intracellular ROS generation was observed either in CuLDL- or in INS-treated cells. Moreover, CuLDL and INS had an additive effect on ROS formation in human fibroblasts. CuLDL by itself increased the phosphorylation of ERK without affecting the PKB/Akt phosphorylation. CuLDL also stimulated the DNA binding activities of the transcription factors AP1 and NFkappaB. However, CuLDL dose-dependently prevented the INS-signaling pathway, by inhibiting the INS-induced phosphorylation of the signaling kinases ERK and PKB/Akt and the INS-induced activation of the transcription factors AP1 and NFkappaB. Finally, the lipophilic antioxidant Vitamin E (Vit E) partially restored all the studied signaling events initiated by INS and impaired after pretreatment with CuLDL. These studies demonstrate that the oxidative stress generated by CuLDL has a negative effect on the INS-signaling pathway, independently of the INS-induced generation of ROS. Thus, oxidized LDL might be involved not only in atherosclerosis, as it is commonly admitted, but also in the INS-resistance observed in type 2 diabetes mellitus.

Impairment of the EGF signaling pathway by the oxidative stress generated with UVA.

Ultraviolet A (UVA) is a component of sunlight reaching the surface of the earth and involved in photodegenerescence and photocarcinogenesis. The effect of UVA was investigated on the EGF-induced activation of the signaling kinase ERK and the transcription factors AP1, NFkappaB, and STAT1. UVA prevented the Epidermal Growth Factor (EGF)-induced stimulation of ERK in a dose-dependent manner within the range of 1.5-9 J/cm(2). Concomitantly, the DNA binding activity of AP1, NFkappaB, and STAT1 under EGF were markedly inhibited by UVA within the same dose range. UVA by itself induced an activation of ERK activity, and a stimulation of AP1, NFkappaB, and STAT1 binding activity. UVA decreased EGF binding in a dose-dependent manner. Furthermore, the highest dose of UVA (9 J/cm(2)) prevented the EGF-induced Tyr-phosphorylation of the EGF-receptor (EGF-R). The generation of reactive oxygen species (ROS), as assessed by the fluorescent probe dichloro-fluorescein, showed an additive effect of EGF and UVA, within the studied range of UVA doses. Finally, the antioxidant Vitamin E prevented the inhibitory effect of UVA on ERK, AP1, NFkappaB, and STAT1. These results demonstrate that an overproduction of ROS, initiated by two different and successive triggering agents such as UVA and EGF, leads to inactivation of the EGF signaling pathway. This inhibition of gene expression control by EGF might play a role in the photodegenerative processes observed after exposition of skin cells to solar radiation.

Oxidized low density lipoprotein induces the cyclin-dependent kinase inhibitor p21(waf1) and the tumor suppressor Rb.

Oxidized low density lipoprotein (OxLDL) is known to be cytotoxic towards different cell types of the arterial wall, leading to progression of an atherosclerotic plaque. We previously reported that OxLDL activates the tumor suppressor p53 in human fibroblasts [Biochem. Biophys. Res. Commun. 276 (2000) 718]. In the present work, we demonstrate that OxLDL increased intracellular levels of the kinase inhibitor p21(waf1) (p21) and of the tumor suppressor Rb. Concomitantly, level of the hypophosphorylated active form of Rb (HypoP-Rb) was also enhanced. Cycloheximide prevented the OxLDL-induced increase in p21, Rb, and HypoP-Rb, whereas okadaic acid had no effect. This increase was also prevented by the antioxidant vitamin E. In addition, the lipid extract of OxLDL, which includes the lipid peroxidation products, reproduced the action of the OxLDL particle itself. OxLDL and its lipid extract induced an oxidative stress, as assessed by the intracellular levels of reactive oxygen species and lipid peroxidation products. Finally, OxLDL induced a dose-dependent inhibition of DNA synthesis as assessed by thymidine incorporation. These results demonstrate that OxLDL or its lipid peroxidation products, by generation of an oxidative stress, enhances the expression of p21 and Rb genes, leading to an accumulation of the Hypo-P active form of the tumor suppressor Rb. This phenomenon is in accordance with the fact that p21 is a mediator of p53-dependent cell-cycle arrest in G1 and is most probably involved in the cytotoxicity of OxLDL.