Taken together our results indicate that HSF and NRF

Taken together, our results indicate that HSF1 and NRF2 reciprocally regulate HO-1 expression. Although each of HSF1 and NRF2 controls a distinct cytoprotective pathway, mounting evidence points that they engage in crosstalk [15,22]. The protein-protein interaction between HSF1 and NRF2 may be interfered in either of the function. However, this possibility is unlikely because heat shock-induced HSP70 expression in Nrf2-konochout CVT-313 receptor was the same as that in the wild-type cells, indicating the presence of the same levels of HSF1 activation in both types of cells. Another possibility is that the binding of HSF1 to HSE may interrupt the binding of NRF2 to MARE, and vice versa. It was reported that heme triggers dynamic changes in the composition of enhancer-binding proteins and histone modification of HO-1 gene enhancers [7,23]. We previously reported that HSF1 partially opens the chromatin structure of interleukin-6 promoter for an activator or a repressor to bind [19]. There are three regulatory regions in the HO-1 gene; the promoter-proximal and distal enhancers (E1, E2) in human, mouse, and rat [11,24]. E1 and E2 regions contain MARE, and E1 and the promoter-proximal regions contain HSE. Both MAREs in E1 and E2 were shown to be involved in heme-induced HO-1 expression [7]. In human and rat cells, a unique HSE in the promoter-proximal region was shown to be responsible for HSF1-mediated transcriptional regulation [12,21]. It has not been elucidated yet that which HSE is involved in the transcriptional regulation of mouse HO-1 gene in vivo.
Conflict of interest
Acknowledgments We are grateful to Dr. Hozumi Motohashi (Tohoku University) and Dr. Akira Nakai (Yamaguchi University) for suggestion. This work was supported in part by Grants-in- Aid for Scientific Research from Ministry of Education, Science, and Culture of Japan and a grant from Yasuda Women’s University.
Introduction Glucocorticoids (GCs) are effective agents used in treating cancer, autoimmune diseases and inflammatory disorders [1], [2]. However, high-dose or long-term use of GCs is the primary cause of secondary osteoporosis because GCs exert direct effects on the skeletal system. GCs suppress bone formation and increase bone resorption, which finally lead to GC-induced osteoporosis (GIOP) [3], [4]. Osteoporotic fractures may occur in 30–50% of patients receiving long-term GC administration [4]. Thus, there is a great need to clarify the pathogenesis of GIOP and develop effective therapeutic strategies. Coordinated cycles of bone formation and resorption require an orchestrated interplay among osteoblasts, osteocytes and osteoclasts [5]. GCs can increase osteoclast generation and induce osteocyte apoptosis [6], [7], [8]. Osteoblasts are responsible for bone formation. The inhibition of bone formation is critical to the pathogenesis of GIOP. GCs significantly decrease the number of osteoblasts. On the one hand, GCs inhibit the production of new osteoblast precursors. On the other hand, GCs induce the apoptosis of mature osteoblasts [9], [10], [11]. Previous studies have reported that GCs can induce osteoblast apoptosis through triggering endoplasmic reticulum (ER) stress, inducing proapoptotic proteins or suppressing tissue inhibitor of metalloproteinase-1 [12], [13], [14]. However, the underlying mechanism of GC-induced osteoblast apoptosis has not been fully elucidated. Haem oxygenase (HO) is the rate-limiting enzyme that degrades haem to biliverdin, carbon monoxide and free iron [15]. Three isoforms of HO (HO-1, HO-2 and HO-3) have been identified. HO-1 is a protective protein possessing antiapoptotic, antiinflammatory and antioxidant effects [16], [17], [18]. HO-1 was also found to play a critical role in bone and fat metabolism [19], [20], [21]. Cobalt (III) protoporphyrin IX chloride (CoPP) is a potent HO-1 inducer. HO-1 induction by CoPP can protect HepG2 cells from cadmium-induced apoptosis in vitro [22]. The antiapoptotic effects of HO-1 induction may attribute to activating the extracellular signal–regulated kinase (ERK)/NF-E2–related factor 2 (Nrf2) signalling pathway, inhibiting ER stress and JNK activation [18]. Although the cytoprotective effect of HO-1 has been extensively studied, the mechanisms of HO-1 against GC-induced osteoblast apoptosis remain unclear.