The unexpected repurposing of nuclear transport proteins from their function in interphase to an equally vital and very different set of functions in mitosis was very surprising. The multi-talented cast when first revealed included the import receptors, importin alpha and beta, the small regulatory GTPase RanGTP, and a subset of nuclear pore proteins. In this review, we report that recent years have revealed new discoveries in each area of this expanding story in vertebrates: (a) The cast of nuclear transport receptors playing a role in mitotic spindle regulation has expanded: both transportin, a nuclear import receptor, and Crm1/Xpo1, an export receptor, are involved in different aspects of spindle assembly. Importin beta and transportin also regulate nuclear envelope and pore assembly. (b) The role of nucleoporins has grown to include recruiting the key microtubule nucleator the γ-TuRC complex and the exportin Crm1 to the mitotic kinetochores of humans. Together they nucleate microtubule formation from the kinetochores towards the centrosomes. (c) New research finds that the original importin beta/RanGTP team have been further co-opted by evolution to help regulate other cellular and organismal activities, ranging from the actual positioning of the spindle within the cell perimeter, to regulation of a newly discovered spindle microtubule branching activity, to regulation of the interaction of microtubule structures with specific actin structures. (d) Lastly, because of the multitudinous roles of karyopherins throughout the cell cycle, a recent large push toward testing their potential as chemotherapeutic targets has begun to yield burgeoning progress in the clinic.

Transportin-specific molecular tools are used to show that the mitotic cell contains importin β and transportin “global positioning system” pathways that are mechanistically parallel. Transportin works to control where the spindle, nuclear membrane, and nuclear pores are formed by directly affecting assembly factor function.

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Xenopus egg extracts are a powerful in vitro tool for studying complex biological processes, including nuclear reconstitution, nuclear membrane and pore assembly, and spindle assembly. Extracts have been further used to demonstrate a moonlighting regulatory role for nuclear import receptors or importins on these cell cycle assembly events. Here we show that exportins can also play a role in these events. Addition of Crm1, Exportin-t, or Exportin-5 decreased nuclear pore assembly in vitro. RanQ69L-GTP, a constitutively active form of RanGTP, ameliorated inhibition. Both Crm1 and Exportin-t inhibited fusion of nuclear membranes, again counteracted by RanQ69L-GTP. In mitotic extracts, Crm1 and Exportin-t negatively impacted spindle assembly. Pulldowns from the extracts using Crm1-or Exportin-t-beads revealed nucleoporins known to be essential for both nuclear pore and spindle assembly, with RanQ69L-GTP decreasing a subset of these target interactions. This study suggests a model where exportins, like importins, can regulate major mitotic assembly events.

e16297 Background: KRAS mutations are common in pancreatic ductal adenocarcinoma (PDAC). While 90% of PDAC tumors display activating mutations in KRAS, only ̃2% are G12C, a specific KRAS mutation targeted by inhibitors such as sotorasib or adagrasib. MEK, which lies downstream of KRAS, is an attractive target to more broadly counteract elevated MAPK signaling regardless of the upstream mutation. However, FDA registered MEK inhibitors are prone to pathway reactivation events, which limit their utility in RAS mutant disease and necessitate chronic pathway inhibition that contributes to on-target toxicity. In contrast, IMM-1-104 is a novel, allosteric dual-MEK inhibitor designed to block pathway reactivation by disrupting phosphorylation of both MEK and ERK and has a short plasma drug half-life. These characteristics enable IMM-1-104 to drive deep cyclic MAPK pathway inhibition, with the potential to inhibit tumors driven by diverse RAS mutations. Methods: IMM-1-104 was tested head-to-head versus sotorasib, adagrasib, selumetinib and binimetinib in a series of preclinical models to characterize differential activity of each compound against tumors driven by diverse KRAS mutations. Cell-based 2D biochemical and 3D growth assays were performed across nine PDAC models, and the Capan-2 PDAC xenograft animal model was used to evaluate single agent activity of IMM-1-104 (75, 100, 150 mg/kg BID p.o. or 150 mg/kg QD p.o.) vs. sotorasib or adagrasib (30 and 100 mg/kg QD p.o. each) for 21 days treatment after tumors had reached volumes of 150 to 200 mm3. Results: IMM-1-104 alone led to reductions in both pERK and pMEK across all 9 PDAC models tested (KRAS status shown), including Hs766T (Q61H), MIA PaCa-2 (G12C), Capan-2 (G12V), AsPC-1 (G12D), CFPAC-1 (G12V), BxPC3 (wild type), Panc 10.05 (G12D), Capan-1 (G12V) and PSN1 (G12R). A head-to-head comparison in vivo demonstrated no Tumor Growth Inhibition (TGI) by sotorasib and adagrasib in KRAS-G12V mutant Capan-2 PDAC tumors, while IMM-1-104 prompted TGIs of 49 to 84% across all doses and schedules tested. Conclusions: Despite multiple clinical studies, including Phase 2 studies for the MEK inhibitors trametinib and selumetinib, limited progress has been made in PDAC treatment since FOLFIRINOX’s approval in 2011. The Phase 2 KRYSTAL-1 and Phase 1/2 CodeBreaK 100 studies recently reported promising activity in KRAS-G12C PDAC, suggesting an opportunity for disruption of KRAS addiction. IMM-1-104 and sotorasib previously demonstrated comparable tumor regressions in vivo in a KRAS G12C mutant model, MIA PaCa-2 (2021 EORTC). Examining the broader activity of IMM-1-104 across 9 PDAC tumor models yielded data suggesting that deep, cyclic MEK inhibition by IMM-1-104 has the potential to offer a unique advantage over first generation MEK inhibitors and KRAS-G12C inhibitors in PDAC by inhibiting tumors driven by a broader range of more common KRAS mutations.