2 COOH NH2 S9 Basic domain Y216 p53 association Figure 1: Glycogen synthase kinase-3β (GSK-3β) structure. GSK- 3β is a 433 residue protein consisting of 3 distinct structural domains. The N-terminal domain (yellow) consists of the ﬁrst 134 residues and forms a 7-strand β-barrel. A short linker from the N-terminal domain, residues 135–151 connect the N-terminal domain to the α-helical domain (magenta). The α-helical domain is composed of residues 152–342. Sandwiched between the N- terminal and α-helical domain is the ATP-binding site. The C- terminal domain consists of residues 343–433 (blue). A strand diagram of GSK-3β. Phosphorylation of Ser-9 inactivates the enzyme, while phosphorylation of Tyr-216 activates. The p53 association region and basic domain region are both located in the N-terminal domain. Image was made using PyMol Molecular Graphics Software version 1.3 with the PDB structure 1UV5. Dysregulation of GSK-3β expression leads to many pathological conditions, including diabetes (or insulin resis- tance), neuronal dysfunction, Alzheimer’s disease [15–18], schizophrenia [19], Dopamine-associated behaviors [20], bipolar disorders [21], Parkinson’s disease [22], and cancer. Of special interest is the involvement of GSK-3β in cancer with data supporting a role as a tumor suppressor and tumor promoter, a discrepancy that at least in part depends on both cell type and signaling environment. For example, GSK- 3β has been shown to inhibit androgen receptor-stimulated cell growth in prostate cancer, thus acting as a tumor suppressor [23]. In contrast, GSK-3β is highly expressed in colorectal cancer [24, 25] and has been shown to participate in nuclear factor-κB (NF-κB) mediated cell survival in pancreatic cancer [26], thus behaving as a tumor promoter. Moreover, the kinase has dual functions in the regulation of cell survival, where it can either activate or inhibit apoptosis [3, 27], further complicating its involvement in cancer. This paper will focus on how GSK-3β can both activate as well as protect from apoptosis with a focus on oncology. Regulation of β-catenin levels is a critical step in Wnt signaling. β-Catenin is phosphorylated by GSK-3β and then degraded through the ubiquitin-proteasome system [28– 30]. Inhibition of GSK-3β activity leads to stabilization and accumulation of β-catenin in the cytosol, which is shuttled into the nucleus and regulates gene expression (Figure 2). GSK-3β is also involved in cell cycle regulation through the International Journal of Cell Biology phosphorylation of cyclin D1, which results in the rapid proteolytic turnover of cyclin D1 protein [1, 31] (Figure 2). Direct overexpression of wild-type GSK-3β is known to induce apoptosis in various cell types in culture, and speciﬁc inhibitors of GSK-3β are able to stop this apoptotic signaling [6, 7, 9, 32]. The detailed molecular mechanism of GSK- 3β’s proapoptotic eﬀect is as yet unknown, but it involves regulation of metabolic and signaling proteins, transcription factors, and gene expression [4, 33]. GSK-3β is required for proper development [4] and is ubiquitously expressed in the animal kingdom. GSK-3β protein was originally isolated from skeletal muscle, but though widely expressed, the protein is most abundant in brain tissue, especially neurons. The high level of expression in brain tissue is likely due to its vital role in neuronal signaling. In neuronal cells, GSK-3β is required for dendrite extension and synapse formation in newborns. 2. Regulation of Apoptosis by GSK-3β GSK-3β has been shown to induce apoptosis in a wide variety of conditions including DNA damage [34], hypoxia [35], endoplasmic reticulum stress [36], and Huntington’s disease-associated polyglutamine toxicity [37]. In cell culture studies, apoptosis was either attenuated or fully abrogated by inhibiting GSK-3β in primary neurons [38], HT-22 cells [39], PC12 cells [40], and human SH-SY5Y neuroblastoma cells [36, 41]. GSK-3β promotes apoptosis by inhibiting prosurvival transcription factors, such as CREB and heat shock factor-1 [42], and facilitating proapoptotic transcription factors such as p53 [34]. A list of some alternative conditions where GSK- 3β facilitates apoptosis is given in Table 1. A large number of proteins have been shown to interact with the tumor suppressor transcription factor p53 to regulate its actions [43, 44], which has been implicated in the proapoptotic actions of GSK-3β in several studies. Following DNA dam- age, the normally short-lived p53 protein is stabilized and modiﬁed by a complex array of posttranslational modiﬁ- cations, such as phosphorylation, acetylation, methylation, ubiquitination, sumoylation, glycosylation, and neddylation. One of these regulatory proteins is GSK-3β, which forms a complex with nuclear p53 to promote p53-induced apoptosis [34, 45, 46]. GSK-3β binds directly to p53, and the C- terminal region of p53 is necessary for this interaction [45]. GSK-3β was shown to directly phosphorylate p53 at Ser- 33 [47] and to mediate p53 phosphorylation at Ser-315 and Ser-376 [48, 49]. GSK-3β also promotes p53-mediated transcription of speciﬁc genes and regulates the intracellular localization of p53 [45, 46, 49]. In addition to GSK-3β regulating p53, GSK-3β is also regulated by p53. The activity of GSK-3β is increased by a phosphorylation-independent mechanism of direct binding of p53 to GSK-3β [34]. Nuclear localization of GSK-3β may also be regulated by binding of activated p53 [50]. In addition to direct interaction, GSK-3β can regulate p53 levels through the phosphorylation of the p53-speciﬁc E3 ubiquitin ligase MDM2 [69]. Regulation of p53 by MDM2