Exploring the role of casein kinase 1α splice variants across cancer cell lines.

Casein kinase 1α (CK1α) is a serine/threonine protein kinase that acts in various cellular processes affecting cell division and signal transduction. CK1α is present as multiple splice variants that are distinguished by the presence or absence of a long insert (L-insert) and a short carboxyl-terminal insert (S-insert). When overexpressed, zebrafish CK1α splice variants exhibit different biological properties, such as subcellular localization and catalytic activity. However, whether endogenous, alternatively spliced CK1α gene products also differ in their biological functions has yet to be elucidated. Here, we identify a panel of splice variant specific CK1α antibodies and use them to show that four CK1α splice variants are expressed in mammals. We subsequently show that the relative abundance of CK1α splice variants varies across distinct mouse tissues and between various cancer cell lines. Furthermore, we identify pathways whose expression is noticeably altered in cell lines enriched with select splice variants of CK1α. Finally, we show that the S-insert of CK1α promotes the growth of HCT 116 cells as cells engineered to lack the S-insert display decreased cell growth. Together, we provide tools and methods to identify individual CK1α splice variants, which we use to begin to uncover the differential biological properties driven by specific splice variants of mammalian CK1α.

Casein Kinase 1α as a Regulator of Wnt-Driven Cancer.

Wnt signaling regulates numerous cellular processes during embryonic development and adult tissue homeostasis. Underscoring this physiological importance, deregulation of the Wnt signaling pathway is associated with many disease states, including cancer. Here, we review pivotal regulatory events in the Wnt signaling pathway that drive cancer growth. We then discuss the roles of the established negative Wnt regulator, casein kinase 1α (CK1α), in Wnt signaling. Although the study of CK1α has been ongoing for several decades, the bulk of such research has focused on how it phosphorylates and regulates its various substrates. We focus here on what is known about the mechanisms controlling CK1α, including its putative regulatory proteins and alternative splicing variants. Finally, we describe the discovery and validation of a family of pharmacological CK1α activators capable of inhibiting Wnt pathway activity. One of the important advantages of CK1α activators, relative to other classes of Wnt inhibitors, is their reduced on-target toxicity, overcoming one of the major impediments to developing a clinically relevant Wnt inhibitor. Therefore, we also discuss mechanisms that regulate CK1α steady-state homeostasis, which may contribute to the deregulation of Wnt pathway activity in cancer and underlie the enhanced therapeutic index of CK1α activators.

Triptolide and its pro-drug Minnelide target high-risk MYC amplified medulloblastoma in preclinical models.

Most children with medulloblastoma (MB) achieve remission, but some face very aggressive metastatic tumors. Their dismal outcome highlights the critical need to advance therapeutic approaches that benefit such high-risk patients. Minnelide, a clinically relevant analog of the natural product triptolide, has oncostatic activity in both preclinical and early clinical settings. Despite its efficacy and tolerable toxicity, this compound has not been evaluated in MB. Utilizing a bioinformatic dataset that integrates cellular drug response data with gene expression, we predicted that Group 3 (G3) MB, which has a poor five-year survival, would be sensitive to triptolide/Minnelide. We subsequently showed that both triptolide and Minnelide attenuate the viability of G3 MB cells ex vivo. Transcriptomic analyses identified MYC signaling, a pathologically relevant driver of G3 MB, as a downstream target of this class of drugs. We validated this MYC dependency in G3 MB cells and showed that triptolide exerts its efficacy by reducing both MYC transcription and MYC protein stability. Importantly, Minnelide acted on MYC to reduce tumor growth and leptomeningeal spread, which resulted in improved survival of G3 MB animal models. Moreover, Minnelide improved the efficacy of adjuvant chemotherapy, further highlighting its potential for the treatment of MYC-driven G3 MB patients.

Multiprotein GLI Transcriptional Complexes as Therapeutic Targets in Cancer.

The Hedgehog signaling pathway functions in both embryonic development and adult tissue homeostasis. Importantly, its aberrant activation is also implicated in the progression of multiple types of cancer, including basal cell carcinoma and medulloblastoma. GLI transcription factors function as the ultimate effectors of the Hedgehog signaling pathway. Their activity is regulated by this signaling cascade via their mRNA expression, protein stability, subcellular localization, and ultimately their transcriptional activity. Further, GLI proteins are also regulated by a variety of non-canonical mechanisms in addition to the canonical Hedgehog pathway. Recently, with an increased understanding of epigenetic gene regulation, novel transcriptional regulators have been identified that interact with GLI proteins in multi-protein complexes to regulate GLI transcriptional activity. Such complexes have added another layer of complexity to the regulation of GLI proteins. Here, we summarize recent work on the regulation of GLI transcriptional activity by these novel protein complexes and describe their relevance to cancer, as such GLI regulators represent alternative and innovative druggable targets in GLI-dependent cancers.

Protein arginine methyltransferase 5 regulates SHH-subgroup medulloblastoma progression.

Background: Medulloblastoma (MB) is the most common pediatric brain tumor. Although standard-of-care treatment generally results in good prognosis, many patients exhibit treatment-associated lifelong disabilities. This outcome could be improved by employing therapies targeting the molecular drivers of this cancer. Attempts to do so in the SONIC HEDGEHOG MB subgroup (SHH-MB) have largely focused on the SHH pathway's principal activator, smoothened (SMO). While inhibitors targeting SMO have shown clinical efficacy, recurrence and resistance are frequently noted, likely resulting from mutations in or downstream of SMO. Therefore, identification of novel SHH regulators that act on the pathway's terminal effectors could be used to overcome or prevent such recurrence. We hypothesized that protein arginine methyltransferase 5 (PRMT5) is one such regulator and investigated its role and potential targeting in SHH-MB. Methods: PRMT5 expression in SHH-MB was first evaluated. Knockdown and pharmacological inhibitors of PRMT5 were used in SHH-MB sphere cultures to determine its effect on viability and SHH signaling. GLI1 arginine methylation was then characterized in primary SHH-MB tissue using LC-MS/MS. Finally, PRMT5 inhibitor efficacy was evaluated in vivo. Results: PRMT5 is overexpressed in SHH-MB tissue. Furthermore, SHH-MB viability and SHH activity is dependent on PRMT5. We found that GLI1 isolated from SHH-MB tissues is highly methylated, including three PRMT5 sites that affect SHH-MB cell viability. Importantly, tumor growth is decreased and survival increased in mice given PRMT5 inhibitor. Conclusions: PRMT5 is a requisite driver of SHH-MB that regulates tumor progression. A clinically relevant PRMT5 inhibitor represents a promising candidate drug for SHH-MB therapy.

Noncanonical activation of GLI signaling in SOX2+ cells drives medulloblastoma relapse.

SRY (sex determining region Y)-box 2 (SOX2)-labeled cells play key roles in chemoresistance and tumor relapse; thus, it is critical to elucidate the mechanisms propagating them. Single-cell transcriptomic analyses of the most common malignant pediatric brain tumor, medulloblastoma (MB), revealed the existence of astrocytic Sox2+ cells expressing sonic hedgehog (SHH) signaling biomarkers. Treatment with vismodegib, an SHH inhibitor that acts on Smoothened (Smo), led to increases in astrocyte-like Sox2+ cells. Using SOX2-enriched MB cultures, we observed that SOX2+ cells required SHH signaling to propagate, and unlike in the proliferative tumor bulk, the SHH pathway was activated in these cells downstream of Smo in an MYC-dependent manner. Functionally different GLI inhibitors depleted vismodegib-resistant SOX2+ cells from MB tissues, reduced their ability to further engraft in vivo, and increased symptom-free survival. Our results emphasize the promise of therapies targeting GLI to deplete SOX2+ cells and provide stable tumor remission.

A Druggable UHRF1/DNMT1/GLI Complex Regulates Sonic Hedgehog-Dependent Tumor Growth.

Dysregulation of Sonic hedgehog (SHH) signaling drives the growth of distinct cancer subtypes, including medulloblastoma (MB). Such cancers have been treated in the clinic with a number of clinically relevant SHH inhibitors, the majority of which target the upstream SHH regulator, Smoothened (SMO). Despite considerable efficacy, many of these patients develop resistance to these drugs, primarily due to mutations in SMO. Therefore, it is essential to identify druggable, signaling components downstream of SMO to target in SMO inhibitor resistant cancers. We utilized an integrated functional genomics approach to identify epigenetic regulators of SHH signaling and identified a novel complex of Ubiquitin-like with PHD and RING finger domains 1 (UHRF1), DNA methyltransferase 1 (DNMT1), and GLI proteins. We show that this complex is distinct from previously described UHRF1/DNMT1 complexes, suggesting that it works in concert to regulate GLI activity in SHH driven tumors. Importantly, we show that UHRF1/DNMT1/GLI complex stability is targeted by a repurposed FDA-approved therapy, with a subsequent reduction in the growth of SHH-dependent MB ex vivo and in vivo. IMPLICATIONS: This work describes a novel, druggable UHRF1/DNMT1/GLI complex that regulates SHH-dependent tumor growth, and highlights an FDA-approved drug capable of disrupting this complex to attenuate tumor growth.

The E3 ubiquitin ligase component, Cereblon, is an evolutionarily conserved regulator of Wnt signaling.

Immunomodulatory drugs (IMiDs) are important for the treatment of multiple myeloma and myelodysplastic syndrome. Binding of IMiDs to Cereblon (CRBN), the substrate receptor of the CRL4CRBN E3 ubiquitin ligase, induces cancer cell death by targeting key neo-substrates for degradation. Despite this clinical significance, the physiological regulation of CRBN remains largely unknown. Herein we demonstrate that Wnt, the extracellular ligand of an essential signal transduction pathway, promotes the CRBN-dependent degradation of a subset of proteins. These substrates include Casein kinase 1α (CK1α), a negative regulator of Wnt signaling that functions as a key component of the ß-Catenin destruction complex. Wnt stimulation induces the interaction of CRBN with CK1α and its resultant ubiquitination, and in contrast with previous reports does so in the absence of an IMiD. Mechanistically, the destruction complex is critical in maintaining CK1α stability in the absence of Wnt, and in recruiting CRBN to target CK1α for degradation in response to Wnt. CRBN is required for physiological Wnt signaling, as modulation of CRBN in zebrafish and Drosophila yields Wnt-driven phenotypes. These studies demonstrate an IMiD-independent, Wnt-driven mechanism of CRBN regulation and provide a means of controlling Wnt pathway activity by CRBN, with relevance for development and disease.