Effect of titanium ions on the Hippo/YAP signaling pathway in regulating biological behaviors of MC3T3-E1 osteoblasts.

Titanium (Ti) and its corresponding alloys have been widely applied in dental and orthopedic implants. Owing to abrasion and corrosion of implants in the unfavorable electrolytic aqueous environment of the host body, Ti ions could be released from implants and accumulated in local tissues. Recent studies have found that excessive Ti ions were toxic to osteoblasts in adjacent bone tissues and subsequently influenced long-term effects on implant prostheses. However, the potential molecular mechanisms underlying the damage to osteoblasts induced by Ti ions remained unclear. Hippo signaling has been confirmed to be involved in organ size and tissue regeneration in many organs, while its roles in osteoblasts differentiation and bone repair remained elusive. Therefore, we hypothesize that YAP, a regulator of Hippo pathway, inhibited osteoblast growth, skeletal development and bone repair, as well as excessive Ti ions promoted the progression of YAP activation. This study aimed to explore the role of Hippo/YAP signaling pathway in the biotoxicity effect of Ti ions on osteoblast behaviors. Here, we confirmed that 10 ppm Ti ions, a minimum concentration gradient previously reported that was capable of suppressing osteoblasts growth, induced nuclear expression of YAP in osteoblasts in our study. Furthermore, 10 ppm Ti ion-induced YAP activation was found to downregulate osteogenic differentiation of MC3T3-E1 cells. Most importantly, the hypothesis we proposed that knockdown of YAP did reverse the inhibitory effect of 10 ppm Ti ions on osteogenesis has been verified. Taken together, our work provides insights into the mechanism of which YAP is involved in regulating osteoblast behaviors under the effect of Ti ions, which may help to develop therapeutic applications for Ti implant failures and peri-implantitis.