International Journal of Cell Biology Table 3: List of known positive regulators of GSK-3β. Mode of activation Notes Inhibits PDK phosphorylation of Akt COX-2 inhibitor [100]. General kinase inhibitor (including PKA/PKC) Inhibits PDK phosphorylation of Akt [101, 103, 104]. Induces Akt dephosphorylation HDAC inhibitor acts through PP1 [105]. Akt dephosphorylation Direct target not known [102]. Suppresses Akt kinase activity and Does not aﬀect upstream Akt activators [106]. activation Inhibits PI3-Kinase Indirect eﬀect on GSK-3β [97, 98]. Inhibits PI3-Kinase Likely aﬀects ATP binding to kinase [98, 99]. Potentially inhibits S6K1 [m1T07O, R10p8a]t.hway can also inhibit GSK3 pErnohmanocteess GnuScKle-a3rβlkoicnaaliszeaaticotnivity and [eR1ne1hd1au,nc1ce1es2si]na.hctiibviatotirnygpphhoosspphhoorryyllaattiioonnand Reduces inhibitory phosphorylation of Promotes GSK-3β-dependent cyclin D1 GSK-3β degradation [80, 109]. 6 Activator Celecoxib Staurosporine Trichostatin A Unknown Curcumin Unknown sAigknt/aplrinogteiinnhkiibnitaoser-B2 (API-2) Unknown Wortmannin IC50 = 5 nM LY294002 IC50 = 1.4 μM Rapamycin Diﬀerentiation-inducing factors (DIFs) Retinoids Unknown Unknown Unknown Activation potency IC50 = 3.5 μM IC50 = 0.22 μM Table 4: Selected list of known GSK-3β inhibitors. IC50 = 6 mM Ki =2 mM Inhibitor Inhibition potency Mode of inhibition Notes Beryllium Mg competitor Also inhibits cdc2 Lithium Mg competitor Anilino maleimides (SB216763, SB415286) Ki = 10–30 nM ATP competitor Does not inhibit a range of other kinases Arylpyrazolopyridazines IC50 = 0.8–150 nM ATP competitor Also inhibits CDK2 (e.g., 6-aryl pyrazole [3,4-b] pyridine 4) Bisindole maleimides (e.g., Ro 31-8220, GF 109203x) IC50 = 5–170 nM ATP competitor Also inhibits PKC Indirubins (6-bromoindirubin-3 -oxime, aka BIO) IC50 = 5–50 nM ATP competitor Also inhibits CDKs Paullones (alsterpaullone) IC50 = 4–80 nM ATP competitor Also inhibits CDKs Pseudosubstrate peptide Ki = 0.7 mM Substrate competitor Speciﬁc selection and GSK3-β inhibition [119–125]. Beryllium was shown to compete with both ATP and Mg2+, while lithium competed only with Mg2+ [126]. The small molecule inhibitors of GSK-3 SB-216763 and SB-415286 are structurally distinct maleimides that inhibit GSK-3α/β in vitro, with Kis of 9 nM and 31 nM, respectively, in an ATP competitive manner [127]. Hymenialdisine [128] and paullones [129] also inhibit GSK-3β in an ATP com- petitive manner. Indirubins inhibit GSK-3β in an ATP com- petitive manner with a IC50 of 50–100 nM [130–132]. Small molecule inhibitors like TZDZ8 that are thiadiazolidinones inhibit GSK-3β with a IC50 of 2 μM in a noncompetitive manner [133, 134]. The other type of GSK-3β inhibitors is represented by cell-permeable, phosphorylated substrate- competitive peptides which interact with the phospho- recognition motif comprising R96, R180, and K205 to prevent substrate access to the active site. There are also GSK-3β-inhibiting peptides that contain GSK-3β interacting domains, block the interaction between Axin and GSK-3, and prevent β-catenin phosphorylation [135]. In the recent decade small molecule inhibitors of GSK-3β are emerging as a promising drug for treatments against neurodegenerative diseases, radiation damage, Alzheimer’s disease, diabetes, and cancer [116]. 6. Exploiting the GSK-3β Conundrum GSK-3β signaling is a complex process inﬂuenced not only by cellular type and transformation status, but by environmental and cellular conditions. Survival signals have been mainly determined by studies involving GSK-3β inhi- bition, through gene silencing or pharmacologic inhibition. The resulting inhibition of apoptosis is complex, and requires further elucidation. However several studies suggest that the eﬀects may at least in part be mediated by the eﬀect of GSK- 3β on NF-κB levels. In addition, it is clear that subcellular localization is important, as only cytosolic GSK-3β seems to be able to mediate the survival signals. Notably, the role in promotion of apoptosis by GSK- 3β has been more clearly delineated. It performs this task by both facilitating proapoptotic signals while inhibiting anti-apoptotic molecules. This signal interplay occurs mostly at the level of the mitochondria, and combined with the association with primarily nuclear GSK-3β, suggests a downstream role of GSK-3β in modulation. So how do we exploit these paradoxical roles of GSK- 3β? In healthy cells, the shift to pro-survival modes is important for cell survival under conditions of cellular stress. In these cases, the upstream signals seem to override