Central Role of Glycogen Synthase Kinase-3β in Endoplasmic Reticulum Stress-induced Caspase-3 Activation

Abstract

Stress of the endoplasmic reticulum (ER), which is associated with many neurodegenerative conditions, can lead to the elimination of affected cells by apoptosis through only partially understood mechanisms. Thapsigargin, which causes ER stress by inhibiting the ER Ca 2؉ -ATPase, was found to not only activate the apoptosis effector caspase-3 but also to cause a large and prolonged increase in the activity of glycogen synthase kinase-3␤ (GSK3␤). Activation of GSK3␤ was obligatory for thapsigargin-induced activation of caspase-3, because inhibition of GSK3␤ by expression of dominantnegative GSK3␤ or by the GSK3␤ inhibitor lithium blocked caspase-3 activation. Thapsigargin treatment activated GSK3␤ by inducing dephosphorylation of phospho-Ser-9 of GSK3␤, a phosphorylation that normally maintains GSK3␤ inactivated. Caspase-3 activation induced by thapsigargin was blocked by increasing the phosphorylation of Ser-9-GSK3␤ with insulin-like growth factor-1 or with the phosphatase inhibitors okadaic acid and calyculin A, but the calcineurin inhibitors FK506 and cyclosporin A were ineffective. Insulin-like growth factor-1, okadaic acid, calyculin A, and lithium also protected cells from two other inducers of ER stress, tunicamycin and brefeldin A. Thus, ER stress activates GSK3␤ through dephosphorylation of phospho-Ser-9, a prerequisite for caspase-3 activation, and this process is amenable to pharmacological intervention.

Glucocorticoid induces apoptosis of osteoblast cells through the activation of glycogen synthase kinase 3β

“…Thapsigargin induced apoptosis of neuronal cells through activation of GSK3b, and this was prevented by LiCl [9]. Overexpression of GSK3b was also found to be sufficient to facilitate staurosporine-and heat shock-induced apoptosis, and LiCl attenuated caspase-3 activation [11].…”

“…A percentage of the total cell number was represented. The values are expressed as mean ± SD intracellular signaling factor in the canonical Wnt/b-catenin pathway in osteoblasts [2,5], and has previously been found to induce apoptosis [9,10,21]. Furthermore the activation and regulation of GSK3b are dependent on phosphorylation [22].…”

“…Phosphorylation at Tyr-216 of GSK3b is shown to be required for GSK3b activity and function [8]. Indeed, inhibition of GSK3b leads to protection of many apoptotic conditions [9][10][11]. Nuclear GSK3b participates in regulation of many transcription factors, which influence many signaling pathway, and result in control of expression level of many genes [7].…”

Cellular response to endoplasmic reticulum stress: a matter of life or death

“…169 However, other experiments suggest that caspase-12 cleavage precedes caspase-7 cleavage during ER stress (our unpublished observations), implying that the order of activation may be the opposite in some contexts. In addition, glycogen synthase kinase 3b may influence caspase-3 activation specifically during ER stress, 170 although whether this is a direct effect or a far-upstream event remains unclear. A systematic search for caspase-12 substrates should provide additional insight into how ER stress ultimately leads to caspase-dependent cell death.…”

The protein phosphatase-1/inhibitor-2 complex differentially regulates GSK3 dephosphorylation and increases sarcoplasmic/endoplasmic reticulum calcium ATPase 2 levels

“…The regulation of Ser21/9 phosphorylation has been attributed to the actions of protein phosphatase-2A (PP2A) [40,43,27,38] and protein phosphatase-1 (PP1) [46,34,41]. The PP1 holoenzyme is comprised of a 37 kDa catalytic subunit which associates with inhibitory subunits and targeting subunits.…”