IRF8 as a Novel Marker to Differentiate Between CD30-Positive Large Cell Lymphomas.

OBJECTIVES: Interferon regulatory factor 8 (IRF8) is a new biomarker shown to be positive in monocytic leukemias as well as in B cells. As a transcription factor, it plays a critical role in pre-B-cell differentiation and induction of tolerance pathways, among other functions. Given the frequent diagnostic dilemma in CD30-positive large cell lymphomas that could resemble both Hodgkin lymphoma and anaplastic large cell lymphoma (ALCL), we sought to determine whether IRF8 can be useful in distinguishing between these neoplasms that require different treatment strategies. METHODS: In this retrospective study, 74 cases of classic Hodgkin lymphoma (CHL) and 7 cases of nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL) on a tissue microarray (TMA), as well as 15 individual cases of ALK-negative ALCL and 4 cases of ALK-positive ALCL, were stained for IRF8. Paired box 5 (PAX5) immunostaining of the TMA was also performed and compared alongside since that marker is occasionally the only marker to help clinically differentiate between T- and B-cell lymphomas with anaplastic/Hodgkin-like cytology. RESULTS: None (0%) of the ALCLs were positive for IRF8 while all (100%) of the NLPHLs and 85% of the CHLs were positive for IRF8. Six (8%) cases of CHL were PAX5 negative but IRF8 positive. Conversely, seven (10%) cases of CHL were PAX5 positive but IRF8 negative. Four (6%) cases of CHL were negative for both PAX5 and IRF8. CONCLUSIONS: There is significant morphologic and immunophenotypic (CD30 positive and CD45 and CD20 negative) overlap between CHL and ALCL. Since many ALCLs show downregulation of lineage-specific T-cell markers or are "null cell" type, only PAX5 has been a reliable marker to differentiate between borderline cases. This is further confounded by positivity of PAX5 in some ALCLs due to amplification of PAX5. On the basis of recent discoveries of IRF8 function as well as performance as an immunostain, we tested this marker in human lymphoma samples and found that it aids in the discrimination between these tumors.

Global assessment of IRF8 as a novel cancer biomarker.

Interferon regulatory factor 8 (IRF8) is a member of the IRF family that is specific to the hematopoietic cell and is involved in regulating the development of human monocytic and dendritic-lineage cells, as well as B-cells. Because its utility as a sensitive and specific monoblast marker in the context of acute monocytic leukemias has been recently demonstrated, we hypothesized that it may also be useful as a novel immunohistochemical marker in myeloid sarcomas and blastic plasmacytoid dendritic cell neoplasms (BPDCNs) with respect to their differential diagnoses. In this retrospective study, we analyzed the IHC expression pattern of IRF8 in 385 patient samples across 30 types of cancers, referenced to their mRNA expression data available through The Cancer Genome Atlas. In addition, we assessed IRF8 in 35 myeloid sarcomas and 15 BPDCNs. Twenty-four of 35 cases of myeloid sarcomas (68.5%) showed positivity for IRF8, with six cases (17.1%) demonstrating IRF8 expression in the absence of CD34 and MPO. All 15 of 15 BPDCNs (100%) showed strong uniform expression of IRF8 and were occasionally more definitive than CD123. IRF8 was negative in all desmoplastic small round cell tumors, Ewing sarcomas, synovial sarcomas, and undifferentiated pleomorphic sarcomas, as well as all epithelial malignancies tested except for 2 triple negative breast cancers that showed subset weak staining. In conclusion, IRF8 is a novel marker helpful in identifying extranodal hematopoietic tumors that can otherwise be difficult to diagnose given the broad differential diagnoses and frequent loss of more common lineage-defining markers.

Small-Molecule Activators of Protein Phosphatase 2A for the Treatment of Castration-Resistant Prostate Cancer.

Primary prostate cancer is generally treatable by androgen deprivation therapy, however, later recurrences of castrate-resistant prostate cancer (CRPC) that are more difficult to treat nearly always occur due to aberrant reactivation of the androgen receptor (AR). In this study, we report that CRPC cells are particularly sensitive to the growth-inhibitory effects of reengineered tricyclic sulfonamides, a class of molecules that activate the protein phosphatase PP2A, which inhibits multiple oncogenic signaling pathways. Treatment of CRPC cells with small-molecule activators of PP2A (SMAP) in vitro decreased cellular viability and clonogenicity and induced apoptosis. SMAP treatment also induced an array of significant changes in the phosphoproteome, including most notably dephosphorylation of full-length and truncated isoforms of the AR and downregulation of its regulatory kinases in a dose-dependent and time-dependent manner. In murine xenograft models of human CRPC, the potent compound SMAP-2 exhibited efficacy comparable with enzalutamide in inhibiting tumor formation. Overall, our results provide a preclinical proof of concept for the efficacy of SMAP in AR degradation and CRPC treatment.Significance: A novel class of small-molecule activators of the tumor suppressor PP2A, a serine/threonine phosphatase that inhibits many oncogenic signaling pathways, is shown to deregulate the phosphoproteome and to destabilize the androgen receptor in advanced prostate cancer. Cancer Res; 78(8); 2065-80. ©2018 AACR.

The Advantages of Targeted Protein Degradation Over Inhibition: An RTK Case Study.

Proteolysis targeting chimera (PROTAC) technology has emerged over the last two decades as a powerful tool for targeted degradation of endogenous proteins. Herein we describe the development of PROTACs for receptor tyrosine kinases, a protein family yet to be targeted for induced protein degradation. The use of VHL-recruiting PROTACs against this protein family reveals several advantages of degradation over inhibition alone: direct comparisons of fully functional, target-degrading PROTACs with target-inhibiting variants that contain an inactivated E3 ligase-recruiting ligand show that degradation leads to more potent inhibition of cell proliferation and a more durable and sustained downstream signaling response, and thus addresses the kinome rewiring challenge seen with many receptor tyrosine kinase inhibitors. Combined, these findings demonstrate the ability to target receptor tyrosine kinases for degradation using the PROTAC technology and outline the advantages of this degradation-based approach.

Activation of tumor suppressor protein PP2A inhibits KRAS-driven tumor growth.

Targeted cancer therapies, which act on specific cancer-associated molecular targets, are predominantly inhibitors of oncogenic kinases. While these drugs have achieved some clinical success, the inactivation of kinase signaling via stimulation of endogenous phosphatases has received minimal attention as an alternative targeted approach. Here, we have demonstrated that activation of the tumor suppressor protein phosphatase 2A (PP2A), a negative regulator of multiple oncogenic signaling proteins, is a promising therapeutic approach for the treatment of cancers. Our group previously developed a series of orally bioavailable small molecule activators of PP2A, termed SMAPs. We now report that SMAP treatment inhibited the growth of KRAS-mutant lung cancers in mouse xenografts and transgenic models. Mechanistically, we found that SMAPs act by binding to the PP2A Aα scaffold subunit to drive conformational changes in PP2A. These results show that PP2A can be activated in cancer cells to inhibit proliferation. Our strategy of reactivating endogenous PP2A may be applicable to the treatment of other diseases and represents an advancement toward the development of small molecule activators of tumor suppressor proteins.