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Karmacharya, Rakesh

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Karmacharya

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Rakesh

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Karmacharya, Rakesh

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2014 - 20152

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Author's Publications: search.filters.isAuthorOfPublication.10a7c5dd-1711-499f-abd9-6eaec45b7db1

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    Niche-Based Screening in Multiple Myeloma Identifies a Kinesin-5 Inhibitor with Improved Selectivity over Hematopoietic Progenitors
    (Elsevier BV, 2015) Chattopadhyay, Shrikanta; Stewart, Alison L.; Mukherjee, Siddhartha; Huang, Cherrie; Hartwell, Kimberly A.; Miller, Peter; Subramanian, Radhika; Carmody, Leigh C.; Yusuf, Rushdia; Sykes, David; Paulk, Joshiawa; Vetere, Amedeo; Vallet, Sonia; Santo, Loredana; Cirstea, Diana D.; Hideshima, Teru; Dan?ík, Vlado; Majireck, Max M.; Hussain, Mahmud M.; Singh, Shambhavi; Quiroz, Ryan; Iaconelli, Jonathan; Karmacharya, Rakesh; Tolliday, Nicola J.; Clemons, Paul A.; Moore, Malcolm A.S.; Stern, Andrew M.; Shamji, Alykhan; Ebert, Benjamin; Golub, Todd; Raje, Noopur; Scadden, David; Schreiber, Stuart
    Novel therapeutic approaches are urgently required for multiple myeloma (MM). We used a phenotypic screening approach using co-cultures of MM cells with bone marrow stromal cells to identify compounds that overcome stromal resistance. One such compound, BRD9876, displayed selectivity over normal hematopoietic progenitors and was discovered to be an unusual ATP non-competitive kinesin-5 (Eg5) inhibitor. A novel mutation caused resistance, suggesting a binding site distinct from known Eg5 inhibitors, and BRD9876 inhibited only microtubule-bound Eg5. Eg5 phosphorylation, which increases microtubule binding, uniquely enhanced BRD9876 activity. MM cells have greater phosphorylated Eg5 than hematopoietic cells, consistent with increased vulnerability specifically to BRD9876’s mode of action. Thus, differences in Eg5-microtubule binding between malignant and normal blood cells may be exploited to treat multiple myeloma. Additional steps are required for further therapeutic development, but our results indicate that unbiased chemical biology approaches can identify therapeutic strategies unanticipated by prior knowledge of protein targets.
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    HDAC6 Inhibitors Modulate Lys49 Acetylation and Membrane Localization of β-Catenin in Human iPSC-Derived Neuronal Cells
    (American Chemical Society, 2014) Iaconelli, Jonathan; Huang, Joanne H.; Berkovitch, Shaunna S.; Chattopadhyay, Shrikanta; Mazitschek, Ralph; Schreiber, Stuart; Haggarty, Stephen J.; Karmacharya, Rakesh
    We examined the effects of isoform-specific histone deacetylase (HDAC) inhibitors on β-catenin posttranslational modifications in neural progenitor cells (NPCs) derived from human induced pluripotent stem cells (iPSCs). β-catenin is a multifunctional protein with important roles in the developing and adult central nervous system. Activation of the Wnt pathway results in stabilization and nuclear translocation of β-catenin, resulting in activation of multiple target genes. In addition, β-catenin forms a complex with cadherins at the plasma membrane as part of the adherens junctions. The N-terminus of β-catenin has phosphorylation, ubiquitination, and acetylation sites that regulate its stability and signaling. In the absence of a Wnt signal, Ser33, Ser37, and Thr41 are constitutively phosphorylated by glycogen synthase kinase 3β (GSK3β). β-Catenin phosphorylated at these sites is recognized by β-transducin repeat-containing protein (βTrCP), which results in ubiquitination and degradation by the ubiquitin-proteasome pathway. The N-terminal regulatory domain of β-catenin also includes Ser45, a phosphorylation site for Casein Kinase 1α (CK1α) and Lys49, which is acetylated by the acetyltransferase p300/CBP-associated factor (PCAF). The relevance of Lys49 acetylation and Ser45 phosphorylation to the function of β-catenin is an active area of investigation. We find that HDAC6 inhibitors increase Lys49 acetylation and Ser45 phosphorylation but do not affect Ser33, Ser37, and Thr41 phosphorylation. Lys49 acetylation results in decreased ubiquitination of β-catenin in the presence of proteasome inhibition. While increased Lys49 acetylation does not affect total levels of β-catenin, it results in increased membrane localization of β-catenin.