Can a Two-Faced Kinase be Exploited for Osteosarcoma?
Authors: J. R. Woodgett
Journal: JNCI Journal of the National Cancer Institute (2012)

There are 518 protein kinases in the human genome, many of which act as important nodes in signal transduction pathways (1). Members of this class of important regulatory enzymes are also, unsurprisingly, frequently mutated or dysregulated in cancer and many other diseases. They have thus become the target of therapeutic interventions, with many successes. What is often overlooked, however, is that there are fewer than 15 signaling pathways for transmitting extracellular information into the cell. This paucity of communication routes is responsible for extracting appropriate cellular responses to a myriad of external cues. If it sounds as though too many eggs are in a limited number of baskets, the situation is further exacerbated by sharing of several transduction components between pathways. The most egregious example is that of glycogen synthase kinase-3 (GSK-3), a protein kinase first identified as a regulator of glycogen synthesis (2). This innocuously named protein is anything but because it plays a central role in at least four of these signaling pathways—the Wnt, Notch, Hedgehog, and nuclear factor-kB (NF-kB) pathways—with important roles in at least six more—the ras/mitogen-associated protein kinase (RAS/MAPK), cyclic-AMP, transforming growth factor-b/activin (TGF-b), phosphatidylinositol -3-kinase (PI3K), jun kinase/stressactivated protein kinase (JNK/SAPK), and janus kinase/signal transducer and activator of transcription (JAK/STAT) pathways. There are two highly related isoforms of GSK-3 (termed a and b) encoded by distinct genes, but that is still a substantial responsibility assigned to a particular protein kinase begging the question of why and how pathways maintain the authenticity of their signals if relying on the same molecules (3). Only the cyclic GMP, p38 mitogen-activated protein kinase (p38 MAPK), Ca 2+ , calmodulin, and Hippo pathways, and the intracellular DNA damage response and unfolded protein response pathways currently lack known roles for GSK-3. In this issue of the Journal, Tang et al. (4) report that suppression of GSK-3b interferes with the growth and viability of osteosarcoma cells, likely by inhibition of NF-kB. This transcription factor plays a critical role in inflammatory responses and has been directly implicated in the promotion of tumorigenesis (5). An important connection between GSK-3 and NF-kB was revealed upon the generation of mice that were genetically engineered to lack GSK-3b (6). GSK-3b null embryos frequently died before birth of extensive apoptosis in the liver, a phenotype reminiscent of mice that were engineered to lack RelA, a subunit of NF-kB (7). Mouse embryo fibroblasts derived from the GSK-3 b null mice displayed a substantial deficit in the ability of tumor necrosis factor alpha (TNF-a) to induce activation of NF-kB and its regulation of anti-apoptotic gene expression. (TNF-a-mediated induction of the caspase pathway was unaffected; hence, the hepatocytes were sensitized to programmed cell death.) Interestingly, mice engineered to lack GSK-3a are viable and NF-kB signaling is normal (8). The molecular mechanism by which GSK-3b selectively regulates and is required for NF-kB function remains unclear, but this kinase modifies the spectrum of genes regulated by NF- kB (9). Both isoforms of GSK-3 are negatively regulated by serine phosphorylation (ie, Serine 21 of GSK-3a and Serine 9 of GSK-3b), and this is one mechanism by which the cyclic-AMP and phosphatidylinositol-3- kinase pathways suppress GSK-3 activity. Tang et al. (4) observed that the level of inhibitory phosphorylation of GSK-3b at Serine 9 was low in several osteosarcoma lines compared with that in a normal osteoblast cell line, suggesting that GSK-3 activity was higher than normal, although this was not directly measured. They also found that b-catenin levels (a target of the Wnt pathway) were increased in some lines, but this finding is unlikely to be related to GSK-3b phosphorylation for several reasons. First, agonists that induce serine phosphorylation of GSK-3 do not typically affect b-catenin (10,11), probably because