p38 and Activating Transcription Factor-2 Involvement in Osteoblast Osmotic Response to Elevated Extracellular Glucose

Abstract

Poorly controlled or untreated type I diabetes mellitus is characterized by hyperglycemia and is associated with decreased bone mass and osteoporosis. We have demonstrated that osteoblasts are sensitive to hyperglycemia-associated osmotic stress and respond to elevated extracellular glucose or mannitol by increasing c-jun and collagen I expression. To determine whether MAPKs are involved in this response, MC3T3-E1 osteoblasts were treated with 16.5 mM glucose, mannitol, or contrast dye for 1 h. Immunoblotting of phosphorylated p38 demonstrated activation of p38 MAPK by hyperosmotic stress in vitro and in vivo. Activation peaked at 20 min, remained detectable after 24 h, and was protein kinase C-independent. Activating transcription factor-2 (ATF-2) activation followed the same pattern as phospho-p38. Transactivation of cAMP response element (CRE)-and c-jun promoter (containing a CRE-like element)-reporter constructs increased following hyperosmotic treatment. SB 203580 (a p38 MAPK inhibitor) blocked ATF-2 phosphorylation, CRE transactivation, and c-jun promoter activation. Hyperosmotic activation of collagen I promoter activity was also inhibited by SB 203580, consistent with the involvement of c-jun in collagen I up-regulation. Therefore, we propose that hyperglycemia-induced increases in p38 MAPK activity and ATF-2 phosphorylation contribute to CRE activation and modulation of c-jun and collagen I expression in osteoblasts.

A novel rat model for the study of deficits in bone formation in type-2 diabetes

“…However, several metabolic disturbances postulated to affect bone formation negatively are characteristic of 9-week-old male ZDF rats-including deficiency in leptin signaling, insulin resistance, and hyperglycemia (McCarthy et al 1999, Zayzafoon et al 2002, Tamasi et al 2003. Evidence has accumulated to suggest that in type-2 diabetes, bone formation and/or turnover are impaired due to glucose metabolic and hormonal disturbances that might modulate osteoblastic activities.…”

“…Several studies have demonstrated that type-2 diabetic osteoporosis is associated with depression of osteoblastic activities, resulting in a decrease in bone formation (Inaba et al 1999, Takizawa et al 2003. In in vitro studies, elevated extracellular glucose has been shown to change osteoblast phenotype by inhibiting osteocalcin expression (Inaba et al 1995, Terada et al 1998, Zayzafoon et al 2002. Irreversible advanced glycosylation end-products (AGEs) are generated by even sporadic elevations in blood glucose and appear to act through specific receptors (RAGEs) to increase the levels of inflammatory cytokines (tumor necrosis factor-α and interleukin-6), which have been shown to negatively affect the balance of bone formation/resorption.…”

The p38-Mediated Stress-Activated Checkpoint: A Rapid Response System for Delaying Progression through Antephase and Entry into Mitosis

“…32 In the case of osmotic stress, p38 and JNK phosphorylation is first detected 5 min after the insult is applied 30,33 and their activity reaches a maximum level 20-60 min later. 30,34 Does activating p38 during antephase delay entry into mitosis? The answer here is clearly yes: activating p38 in antephase cells with anisomycin, 22 or exogenous α and β, but not γ, isoforms of p38, 20 delays progression into mitosis (see also ref.…”

Cetuximab Attenuates Its Cytotoxic and Radiosensitizing Potential by Inducing Fibronectin Biosynthesis

“…Transcription of the FN gene is accomplished by p38 mitogen-activated protein kinase (MAPK) activation and cyclic AMP response element (CRE)/ CCAAT site-dependent phosphorylation of the activating transcription factor 2 (ATF2; refs. 28,29). Additional transcription factors involved in FN gene transcription are CRE-binding protein (CREB) and early growth response-1 (EGR-1; refs.…”