Nicotine induces apoptosis in human osteoblasts via a novel mechanism driven by H2O2 and entailing Glyoxalase 1-dependent MG-H1 accumulation leading to TG2-mediated NF-kB desensitization: Implication for smokers-related osteoporosis

Nicotine contained in cigarette smoke contributes to the onset of several diseases, including osteoporosis, whose emerging pathogenic mechanism is associated with osteoblasts apoptosis. Scanty information is available on the molecular mechanisms of nicotine on osteoblasts apoptosis and, consequently, on an important aspect of the pathogenesis of smokers-related osteoporosis. Glyoxalase 1 (Glo1) is the detoxification enzyme of methylglyoxal (MG), a major precursor of advanced glycation end products (AGEs), potent pro-apoptotic agents. Hydroimidazolone (MG-H1) is the major AGE derived from the spontaneous MG adduction of arginine residues. The aim of this study was to investigate whether, and by means of which mechanism, the antiglycation defence Glo1 was involved in the apoptosis induced by 0.1 and 1µM nicotine in human primary osteoblasts chronically exposed for 11 and 21 days. By using gene overexpression/silencing and scavenging/inhibitory agents, we demonstrated that nicotine induces a significant intracellular accumulation of hydrogen peroxide (H2O2) that, by inhibiting Glo1, drives MG-H1 accumulation/release. MG-H1, in turn, triggers H2O2 overproduction via receptor for AGEs (RAGE) and, in parallel, an apoptotic mitochondrial pathway by inducing Transglutaminase 2 (TG2) downregulation-dependent NF-kB desensitization. Measurements of H2O2, Glo1 and MG-H1 circulating levels in smokers compared with non-smokers or in smokers with osteoporosis compared with those without this bone-related disease supported the results obtained in vitro. Our findings newly pose the antiglycation enzymatic defense Glo1 and MG-H1 among the molecular events involved in nicotine-induced reactive oxygen species-mediated osteoblasts apoptosis, a crucial event in smoker-related osteoporosis, and suggest novel exposure markers in health surveillance programmes related to smokers-associated osteoporosis.