The stability of the Wnt pathway transcription factor beta-catenin is tightly regulated by the multi-subunit destruction complex. Deregulated Wnt pathway activity has been implicated in many cancers, making this pathway an attractive target for anticancer therapies. However, the development of targeted Wnt pathway inhibitors has been hampered by the limited number of pathway components that are amenable to small molecule inhibition. Here, we used a chemical genetic screen to identify a small molecule, XAV939, which selectively inhibits beta-catenin-mediated transcription. XAV939 stimulates beta-catenin degradation by stabilizing axin, the concentration-limiting component of the destruction complex. Using a quantitative chemical proteomic approach, we discovered that XAV939 stabilizes axin by inhibiting the poly-ADP-ribosylating enzymes tankyrase 1 and tankyrase 2. Both tankyrase isoforms interact with a highly conserved domain of axin and stimulate its degradation through the ubiquitin-proteasome pathway. Thus, our study provides new mechanistic insights into the regulation of axin protein homeostasis and presents new avenues for targeted Wnt pathway therapies.

Key Points• CD25 is a predictive biomarker for sensitivity to PIM inhibitors in AML cells.• PIM inhibitors may prolong overall/relapse-free survival through attenuating STAT5 activation and destabilizing MYC in CD25 1 AML cells.Postchemotherapy relapse presents a major unmet medical need in acute myeloid leukemia (AML), where treatment options are limited. CD25 is a leukemic stem cell marker and a conspicuous prognostic marker for overall/relapse-free survival in AML. Rare occurrence of genetic alterations among PIM family members imposes a substantial hurdle in formulating a compelling patient stratification strategy for the clinical development of selective PIM inhibitors in cancer. Here we show that CD25, a bona fide STAT5 regulated gene, is a mechanistically relevant predictive biomarker for sensitivity to PIM kinase inhibitors. Alone or in combination with tyrosine kinase inhibitors, PIM inhibitors can suppress STAT5 activation and significantly shorten the half-life of MYC to achieve substantial growth inhibition of high CD25-expressing AML cells. Our results highlight the importance of STAT5 and MYC in rendering cancer cells sensitive to PIM inhibitors. Because the presence of a CD25-positive subpopulation in leukemic blasts correlates with poor overall or relapse-free survival, our data suggest that a combination of PIM inhibitors with chemotherapy and tyrosine kinase inhibitors could improve long-term therapeutic outcomes in

Inhibitors of JAK2 kinase are emerging as an important treatment modality for myeloproliferative neoplasms (MPN). However, similar to other kinase inhibitors, resistance to JAK2 inhibitors may eventually emerge through a variety of mechanisms. Effective drug combination is one way to enhance therapeutic efficacy and combat resistance against JAK2 inhibitors. To identify potential combination partners for JAK2 compounds in MPN cell lines, we performed pooled shRNA screen targeting 5,000 genes in the presence or absence of JAK2 blockade. One of the top hits identified was MYC, an oncogenic transcription factor that is difficult to inhibit directly, but could be targeted by modulation of upstream regulatory elements such as kinases. We demonstrate herein that PIM kinase inhibitors efficiently suppress MYC protein levels in MPN cell lines. Overexpression of MYC restores the viability of PIM inhibitor-treated cells, revealing causal relationship between MYC down-regulation and cell growth inhibition by PIM compounds. Combination of various PIM inhibitors with a JAK2 inhibitor results in significant synergistic growth inhibition of multiple MPN cancer cell lines and induction of apoptosis. Mechanistic studies revealed strong downregulation of phosphorylated forms of S6 and 4EBP1 by JAK2/PIM inhibitor combination treatment. Finally, such combination was effective in eradicating in vitro JAK2 inhibitor-resistant MPN clones, where MYC is consistently up-regulated. These findings demonstrate that simultaneous suppression of JAK2 and PIM kinase activity by small molecule inhibitors is more effective than either agent alone in suppressing MPN cell growth. Our data suggest that JAK2 and PIM combination might warrant further investigation for the treatment of JAK2-driven hematologic malignancies.

Background:Little is known about the cancer dependence and druggability of PRP4. Results: Significance of PRP4 catalytic activity is demonstrated, novel substrates are identified, and features of kinase domain structure are revealed. Conclusion: PRP4 is required for cancer cell survival, displays substrate specificity, and is amenable to pharmacological inhibition. Significance: Our results indicate that PRP4 is a potential drug target to pursue in cancer.