Chaperonic proteins, including inducible HSP70 (HSP70i) and constitutive HSP70 (HSC70), have been implicated as essential players in the cellular adaptive protection. Ensuing studies demonstrated that overexpression of either protein individually protects against thermal and oxidative challenges. The present study aimed to determine whether a concurrent overexpression of both HSC70 and HSP70i confers a better metabolic protection than the expression of each protein alone. Using a rat heart-derived H9c2 cardiac myoblast cell line, we found that HSP70i was rapidly induced within 2-8 h following a mild thermal preconditioning (43 degrees C for 20 min) in both parental cells and an established H9/70c clonal sub-line overexpressing HSC70. The level of HSP70i protein in heat pretreated H9170c clonal cells reached only 50% of that in heat pretreated H9c2 parental cells. Nevertheless, protection against lethal hyperthermia, menadione (an oxidant) and hydrogen peroxide (H2O2) exposure in the pretreated H9/70c clonal cells was significantly higher than the sum of protection afforded by the early induction of HSP70i in the pretreated parental cells and protection afforded by the pre-existing HSC70 in the H9/70c cells without preconditioning. Using dosimetric analysis, we also found that menadione resistance in the pretreated parental cells increased linearly with cellular HSP70i level (10-300 ng/mg total protein). However, the resistance in the pretreated H9170c cells showed a biphasic relationship with cellular HSP70i level; when HSP70i concentration reached >250 ng/mg protein, survivability after menadione exposure was markedly enhanced. Similar results were observed in H9c2 cells genetically manipulated to overexpress both HSC70 and HSP70i. The survival benefit against lethal hyperthermia, oxidant treatment, and hypoxia/reoxygenation conferred by a concerted HSC70 and HSP70i overexpression was greater than the sum of benefits contributed by individual protein overexpression. Together, these findings suggest that HSC70 and HSP70i may complement each other in a synergistic manner to preserve cellular integrity during metabolic challenges. (C) 2012 Elsevier Inc. All rights reserved.