Multiple myeloma (MM) is an incurable plasma cell malignancy for which novel treatment options are required. Signal Transducer and Activator of Transcription 3 (STAT3) overexpression in MM appears to be mediated by a variety of factors including interleukin-6 signaling and downregulation of Src homology phosphatase-1 (SHP-1). STAT3 overexpression in MM is associated with an adverse prognosis and may play a role in microenvironment-dependent treatment resistance. In addition to its pro-proliferative role, STAT3 upregulates anti-apoptotic proteins and leads to microRNA dysregulation in MM. Phosphatase of regenerating liver 3 (PRL-3) is an oncogenic phosphatase which is upregulated by STAT3. PRL-3 itself promotes STAT-3 phosphorylation resulting in a positive feedback loop. PRL-3 is overexpressed in a subset of MM patients and may cooperate with STAT3 to promote survival of MM cells. Indirectly targeting STAT3 via JAK (janus associated kinase) inhibition has shown promise in early clinical trials. Specific inhibitors of STAT3 showed in vitro efficacy but have failed in clinical trials while several STAT3 inhibitors derived from herbs have been shown to induce apoptosis of MM cells in vitro. Optimising the pharmacokinetic profiles of novel STAT3 inhibitors and identifying how best to combine these agents with existing anti-myeloma therapy are key questions to be addressed in future clinical trials.

Overexpression of PRL-3, an oncogenic phosphatase, was identified as a novel cluster in patients with newly diagnosed multiple myeloma. However, the regulation and oncogenic activities of PRL-3 in multiple myeloma warrant further investigation. Here, we report that IL6 activates STAT3, which acts as a direct transcriptional regulator of PRL-3. Upregulation of PRL-3 increased myeloma cell viability and rephosphorylated STAT3 in a biphasic manner through direct interaction and deactivation of SHP2, thus blocking the gp130 (Y759)-mediated repression of STAT3 activity. Abrogation of PRL-3 reduced myeloma cell survival, clonogenicity, and tumorigenesis, and detailed mechanistic studies revealed “deactivation” of effector proteins such as Akt, Erk1/2, Src, STAT1, and STAT3. Furthermore, loss of PRL-3 efficiently abolished nuclear localization of STAT3 and reduced its occupancy on the promoter of target genes c-Myc and Mcl-1, and antiapoptotic genes Bcl2 and Bcl-xL. PRL-3 also played a role in the acquired resistance of myeloma cells to bortezomib, which could be overcome by PRL-3 silencing. Of clinical relevance, STAT3 and PRL-3 expression was positively correlated in five independent cohorts, and the STAT3 activation signature was significantly enriched in patients with high PRL-3 expression. Furthermore, PRL-3 could be used as a biomarker to identify high-risk patients with multiple myeloma that exhibited poor prognosis and inferior outcome even when treated with novel combinational therapeutics (proteasome inhibitors and immunomodulatory imide drugs). Conclusively, our results support a feedforward mechanism between STAT3 and PRL-3 that prolongs prosurvival signaling in multiple myeloma, and suggest targeting PRL-3 as a valid therapeutic opportunity in multiple myeloma.Significance: IL6 promotes STAT3-dependent transcriptional upregulation of PRL-3, which in turn re-phosphorylates STAT3 and aberrantly activates STAT3 target genes, leading to bortezomib resistance in multiple myeloma.

Current conventional chemotherapy for acute myeloid leukemia (AML) can achieve remission in over 70% of patients, but a majority of them will relapse within 5 years despite continued treatment. The relapse is postulated to be due to leukemia stem cells (LSCs), which are different from normal hematopoietic stem cells (HSCs). LIN28B is microRNA regulator and stem cell reprogramming factor. Overexpression of LIN28B has been associated with advance human malignancies and cancer stem cells (CSCs), including AML. However, the molecular mechanism by which LIN28B contributes to the development of AML remains largely elusive. We modulated LIN28B expression in AML and non-leukemic cells and investigated functional consequences in cell proliferation, cell cycle, and colony-forming assays. We performed a microarray-based analysis for LIN28B-silencing cells and interrogated gene expression data with different bioinformatic tools. AML mouse xenograft model was used to examine the in vivo function of LIN28B. We demonstrated that targeting LIN28B in AML cells resulted in cell cycle arrest, inhibition of cell proliferation and colony formation, which was induced by de-repression of let-7a miRNA. On the other hand, overexpression of LIN28B promoted cell proliferation. Data point to a mechanism where that inhibition of LIN28B induces metabolic changes in AML cells. IGF2BP1 was confirmed to be a novel downstream target of LIN28B via let-7 miRNA in AML. Notably, ectopic expression of LIN28B increased tumorigenicity, while silencing LIN28B led to slow tumor growth in vivo. In sum, these results uncover a novel mechanism of an important regulatory signaling, LIN28B/let-7/IGF2BP1, in leukemogenesis and provide a rationale to target this pathway as effective therapeutic strategy.

Protein tyrosine phosphatase of regenerating liver 3 (PRL-3) is overexpressed in a subset of AML patients with inferior prognosis, representing an attractive therapeutic target. However, due to relatively shallow pocket of the catalytic site of PRL-3, it is difficult to develop selective small molecule inhibitor. In this study, we performed whole-genome lentiviral shRNA library screening to discover synthetic lethal target to PRL-3 in AML. We used specific small molecule inhibitors to validate the synthetic lethality in human PRL-3 high vs PRL-3 low human AML cell lines and primary bone marrow cells from AML patients. AML mouse xenograft model was used to examine the in vivo synergism. The list of genes depleted in TF1-hPRL3 cells was particularly enriched for members involved in WNT/β-catenin pathway and AKT/mTOR signaling. These findings prompted us to explore the impact of AKT/mTOR signaling inhibition in PRL-3 high AML cells in combination with WNT/β-catenin inhibitor. VS-5584, a novel, highly selective dual PI3K/mTOR inhibitor, and ICG-001, a WNT inhibitor, were used as a combination therapy. A synthetic lethal interaction between mTOR/AKT pathway inhibition and WNT/β-catenin was validated by a variety of cellular assays. Notably, we found that treatment with these two drugs significantly reduced leukemic burden and prolonged survival of mice transplanted with human PRL-3 high AML cells, but not with PRL-3 low AML cells. In summary, our results support the existence of cooperative signaling networks between AKT/mTOR and WNT/β-catenin pathways in PRL-3 high AML cells. Simultaneous inhibition of these two pathways could achieve robust clinical efficacy for this subtype of AML patient with high PRL-3 expression and warrant further clinical investigation.