HSP plays a crucial role

HSP90 plays a crucial role as a main molecular chaperone in the regulation of proteostasis such as protein refolding under stress conditions [30]. It has been established that HSP90 binds a client protein such as Raf-1 and forms more continuous and stable functional complex [29]. On the other hand, numerous co-chaperons are also known to bind to HSP90 and regulate its chaperon activities [30]. Thus, we further investigated the effect of onalespib on the canonical HSP90 clients Raf-1, HSP90 co-chaperones HSP70 or even HSP90 levels themselves, however, the expression levels of Raf-1, HSP70 and HSP90 were not affected by onalespib with or without PGD2 stimulation. These unexpected results may be due to the limitation of our experimental conditions. In addition, evidence is accumulating that HSP90 is involved in a variety of pathophysiological cell functions such as responses to steroid hormones and neurodegenerative diseases [30]. With regard to HSP90 on osteoblasts, it has been reported that the expression of HSP90 is upregulated by bisphosphonate, a widely using therapeutic agent for osteoporosis, in osteoblasts [16]. It has also been shown that low-intensity pulsed ultrasound stimulation enhances the HSP90 expression in osteoblasts, resulting in mineralized nodule formation [17]. We have previously reported that HSP27 at phosphorylated state has a suppressive role in the calcification of osteoblast-like MC3T3-E1 TASIN-1 whereas HSP27 at un-phosphorylated state upregulates the calcification [15]. The post translational modification of HSP27 such as phosphorylation is generally recognized to modulate functions of HSP27 [34]. Our present results strongly indicate that HSP90 negatively regulates the induction of HSP27 elicited by PGD2 in osteoblast-like MC3T3-E1 cells. These findings lead us to speculate that HSP90 might control the calcification of osteoblasts via negatively regulating the expression levels of HSP27. Thus, our present findings might provide a novel therapeutic strategy as an applied utility of HSP90 inhibitors for metabolic bone diseases such as osteoporosis and the distress of bone fracture healing. However, the exact roles of HSP90 and HSP27 in osteoblasts have not yet been precisely clarified. Regarding the concentration of onalespib, published Kd for HSP90 binding is 0.71nM, and the LD50 for a large number of human tumor cell lines is under 30nM [35]. Thus, the concentration required for HSP27 induction in osteoblasts may be higher than the doses for chemotherapeutic effects. In addition, the concentration lower than the present study (800 pM) have been reported to modulate a host of cell signaling pathways, a number of which could regulate the stability of HSP27 in multiple tumor cell lines [36]. Osteoblast-like MC3T3-E1 cells, which we used, are not derived from tumor cells but from neonatal mouse calvaria [19]. Therefore, the discrepancy is possibly due to the differences among their origins or species. Further investigations would be required to investigate the details underlying the effects of HSP90 and HSP27 on bone metabolism. In conclusion, our results strongly suggest that HSP90 acts as a negative regulator in the HSP27 induction stimulated by PGD2 in osteoblasts, and that the inhibitory effect of HSP90 is mediated through the suppression of SAPK/JNK and p38 MAP kinase.
Funding sources
Author contributions
Declaration of Interest
Acknowledgement
Introduction Melanogenesis is a complex process, which serves as the determinant of skin and hair color. In human, skin hyperpigmentation is physiologically stimulated by UV radiation in order to protect skin against harmful UV injury. However, excess melanin production or abnormal distribution can cause skin hyperpigmentation. In melanocytes and melanoma cells, melanin production is mainly regulated by melanogenic enzymes such as tyrosinase (TYR), tyrosinase-related protein 1 (TRP-1), and TRP-2. MITF, a basic helix–loop–helix leucine zipper transcription factor, is important for melanocyte differentiation, survival and proliferation [1]. The expression of critical melanogenic enzymes is mainly regulated by MITF in melanocytes.