HSP70 — A Key Regulator in Chondrocyte Homeostasis Under Naturally Coupled Hydrostatic Pressure-Thermal Stimuli

Objective During physical activities, chondrocytes experience coupled stimulation of hydrostatic pressure (HP) and a transient increase in temperature (T), with the latter varying within a physiological range from 32.5 °C to 38.7 °C. Previous short-term in vitro studies have demonstrated that the combined HP-T stimuli more significantly enhance chondroinduction and chondroprotection of chondrocytes than isolated applications. Interestingly, this combined benefit is associated with a corresponding increase in HSP70 levels when HP and T are combined. The current study therefore explored the indispensable role of HSP70 in mediating the combined effects of HP-T stimuli on chondrocytes. Design In this mid-long-term study of in vitro engineered cartilage constructs, we assessed chondrocyte responses to HP-T stimuli using customized bioreactor in standard and HSP70-inhibited cultures. Results Surprisingly, under HSP70-inhibited conditions, the usually beneficial HP-T stimuli, especially its thermal component, exerted detrimental effects on chondrocyte homeostasis, showing a distinct and unfavorable shift in gene and protein expression patterns compared to non-HSP70-inhibited settings. Such effects were corroborated through mechanical testing and confirmed using a secondary cell source. A proteomic-based mechanistic analysis revealed a disruption in the balance between biosynthesis and fundamental cellular structural components in HSP70-inhibited conditions under HP-T stimuli. Conclusions Our results highlight the critical role of sufficient HSP70 induction in mediating the beneficial effects of coupled HP-T stimulation on chondrocytes. These findings help pave the way for new therapeutic approaches to enhance physiotherapy outcomes and potentially shed light on the elusive mechanisms underlying the onset of cartilage degeneration, a long-standing enigma in orthopedics.