Hsp90 regulates protein synthesis by activating the heme-regulated eukaryotic initiation factor 2α (eIF-2α) kinase in rabbit reticulocyte lysates

Inhibition of Hsp90 in the spinal cord enhances the antinociceptive effects of morphine by activating an ERK-RSK pathway

Morphine and other opioids are commonly used to treat pain despite their numerous adverse side effects. Modulating μ-opioid receptor (MOR) signaling is one way to potentially improve opioid therapy. In mice, the chaperone protein Hsp90 mediates MOR signaling within the brain. Here, we found that inhibiting Hsp90 specifically in the spinal cord enhanced the antinociceptive effects of morphine in mice. Intrathecal, but not systemic, administration of the Hsp90 inhibitors 17-AAG or KU-32 amplified the effects of morphine in suppressing sensitivity to both thermal and mechanical stimuli in mice. Hsp90 inhibition enabled opioid-induced phosphorylation of the kinase ERK and increased abundance of the kinase RSK in the dorsal horns of the spinal cord, which are heavily populated with primary afferent sensory neurons. The additive effects of Hsp90 inhibition were abolished upon intrathecal inhibition of ERK, RSK, or protein synthesis. This mechanism downstream of MOR, localized to the spinal cord and repressed by Hsp90, may potentially be used to enhance the efficacy and presumably decrease the side effects of opioid therapy.

Activation of HRI is mediated by Hsp90 during stress through modulation of the HRI-Hsp90 complex

Heme Regulated Inhibitor (HRI) is known to get activated in various stresses such as heme deficiency, heat shock, heavy metal toxicity etc. Heat shock protein 90 (Hsp90), a ubiquitous cytoplasmic protein interacts with HRI in order to regulate protein synthesis. However, it still remains to establish this interaction of HRI and Hsp90 at cellular levels and how this modulation of HRI activity is mediated by Hsp90 during stress. In the present report, using co-immunoprecipitation analysis we show that HRI interacts with Hsp90 and this association is independent of other co-chaperones in in vitro conditions. Further, analysis using truncated domains of HRI revealed that the K1 subdomain is essential for HRI - Hsp90 complex formation. Our in silico protein - protein interaction studies also indicated interaction of Hsp90 with K1 subdomain of HRI. Mammalian two hybrid assay validated this HRI - Hsp90 interaction at cellular levels. When the in vitro kinase assay was carried out with the co-immunoprecipitated complex of HRI - Hsp90, an increase in the kinase activity was observed resulting elevated levels of eIF2α phosphorylation upon heavy metal stress and heat shock. Thus, our results clearly indicate modulation of HRI kinase activity with simultaneous Hsp90 association under stress conditions.

Elucidation of molecular mechanism of stability of the heme-regulated eIF2α kinase upon binding of its ligand, hemin in its catalytic kinase domain

The eIF2α kinase activity of the heme-regulated inhibitor (HRI) is regulated by heme which makes it a unique member of the family of eIF2α kinases. Since heme concentrations create an equilibrium for the kinase to be active/inactive, it becomes important to study the heme binding effects upon the kinase and understanding its mechanism of functionality. In the present study, we report the thermostability achieved by the catalytic kinase domain of HRI (HRI.CKD) upon ligand (heme) binding. Our CD data demonstrates that the HRI.CKD retains its secondary structure at higher temperatures when it is in ligand bound state. HRI.CKD when incubated with hemin loses its monomeric state and attains a higher order oligomeric form resulting in its stability. The HRI.CKD fails to refold into its native conformation upon mutation of H377A/H381A, thereby confirming the necessity of these His residues for correct folding, stability, and activity of the kinase. Though our in silico study demonstrated these His being the ligand binding sites in the kinase insert region, the spectra-based study did not show significant difference in heme affinity for the wild type and His mutant HRI.CKD.