Cancer-associated fibroblast-derived acetate promotes pancreatic cancer development by altering polyamine metabolism via the ACSS2-SP1-SAT1 axis

The ability of tumour cells to thrive in harsh microenvironments depends on the utilization of nutrients available in the milieu. Here we show that pancreatic cancer-associated fibroblasts (CAFs) regulate tumour cell metabolism through the secretion of acetate, which can be blocked by silencing ATP citrate lyase (ACLY) in CAFs. We further show that acetyl-CoA synthetase short-chain family member 2 (ACSS2) channels the exogenous acetate to regulate the dynamic cancer epigenome and transcriptome, thereby facilitating cancer cell survival in an acidic microenvironment. Comparative H3K27ac ChIP-seq and RNA-seq analyses revealed alterations in polyamine homeostasis through regulation of SAT1 gene expression and enrichment of the SP1-responsive signature. We identified acetate/ACSS2-mediated acetylation of SP1 at the lysine 19 residue that increased SP1 protein stability and transcriptional activity. Genetic or pharmacologic inhibition of the ACSS2-SP1-SAT1 axis diminished the tumour burden in mouse models. These results reveal that the metabolic flexibility imparted by the stroma-derived acetate enabled cancer cell survival under acidosis via the ACSS2-SP1-SAT1 axis.

Structural Basis for Multivalent MUC16 Recognition and Robust Anti-Pancreatic Cancer Activity of Humanized Antibody AR9.6

Mucin-16 (MUC16) is a target for antibody-mediated immunotherapy in pancreatic ductal adenocarcinoma (PDAC) amongst other malignancies. The MUC16 specific monoclonal antibody AR9.6 has shown promise for PDAC immunotherapy and imaging. Here, we report the structural and biological characterization of the humanized AR9.6 antibody (huAR9.6). The structure of huAR9.6 was determined in complex with a MUC16 SEA (Sea urchin sperm, Enterokinase, Agrin) domain. Binding of huAR9.6 to recombinant, shed, and cell-surface MUC16 was characterized, and anti-PDAC activity was evaluated in vitro and in vivo. huAR9.6 bound a discontinuous, SEA domain epitope with an affinity of ~90 nM. Binding affinity depended on the specific SEA domain(s) present, and glycosylation enhanced affinity by 3-7-fold driven by favorable entropy and enthalpy and via distinct transition state thermodynamic pathways. Treatment with huAR9.6 reduced the in vitro growth, migration, invasion, and clonogenicity of MUC16-positive PDAC cells and patient-derived organoids (PDOs). HuAR9.6 blocked MUC16-mediated ErbB and AKT activation in PDAC cells, PDOs and patient-derived xenografts and induced antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity. More importantly, huAR9.6 treatment caused substantial PDAC regression in subcutaneous and orthotopic tumor models. The mechanism of action of huAR9.6 may depend on dense avid binding to homologous SEA domains on MUC16. The results of this study validate the translational therapeutic potential of huAR9.6 against MUC16-positive PDACs.

Cbl and Cbl-b Ubiquitin Ligases are Essential for Intestinal Epithelial Stem Cell Maintenance

Among the signaling pathways that control the stem cell self-renewal and maintenance vs. acquisition of differentiated cell fates, those mediated by receptor tyrosine kinase (RTK) activation are well established as key players. CBL family ubiquitin ligases are negative regulators of RTKs but their physiological roles in regulating stem cell behaviors are unclear. While hematopoietic Cbl/Cblb knockout (KO) leads to a myeloproliferative disease due to expansion and reduced quiescence of hematopoietic stem cells, mammary epithelial KO led to stunted mammary gland development due to mammary stem cell depletion. Here, we examined the impact of inducible Cbl/Cblb double-KO (iDKO) selectively in the Lgr5-defined intestinal stem cell (ISC) compartment. Cbl/Cblb iDKO led to rapid loss of the Lgr5 ^Hi ISC pool with a concomitant transient expansion of the Lgr5 ^Lo transit amplifying population. LacZ reporter-based lineage tracing showed increased ISC commitment to differentiation, with propensity towards enterocyte and goblet cell fate at the expense of Paneth cells. Functionally, Cbl/Cblb iDKO impaired the recovery from radiation-induced intestinal epithelial injury. In vitro , Cbl/Cblb iDKO led to inability to maintain intestinal organoids. Single cell RNAseq analysis of organoids revealed Akt-mTOR pathway hyperactivation in iDKO ISCs and progeny cells, and pharmacological inhibition of the Akt-mTOR axis rescued the organoid maintenance and propagation defects. Our results demonstrate a requirement for Cbl/Cblb in the maintenance of ISCs by fine tuning the Akt-mTOR axis to balance stem cell maintenance vs. commitment to differentiation.

UBXN2A suppresses the Rictor-mTORC2 signaling pathway, an established tumorigenic pathway in human colorectal cancer

The mTORC2 pathway plays a critical role in promoting tumor progression in human colorectal cancer (CRC). The regulatory mechanisms for this signaling pathway are only partially understood. We previously identified UBXN2A as a novel tumor suppressor protein in CRCs and hypothesized that UBXN2A suppresses the mTORC2 pathway, thereby inhibiting CRC growth and metastasis. We first used murine models to show that haploinsufficiency of UBXN2A significantly increases colon tumorigenesis. Induction of UBXN2A reduces AKT phosphorylation downstream of the mTORC2 pathway, which is essential for a plethora of cellular processes, including cell migration. Meanwhile, mTORC1 activities remain unchanged in the presence of UBXN2A. Mechanistic studies revealed that UBXN2A targets Rictor protein, a key component of the mTORC2 complex, for 26S proteasomal degradation. A set of genetic, pharmacological, and rescue experiments showed that UBXN2A regulates cell proliferation, apoptosis, migration, and colon cancer stem cells (CSCs) in CRC. CRC patients with a high level of UBXN2A have significantly better survival, and high-grade CRC tissues exhibit decreased UBXN2A protein expression. A high level of UBXN2A in patient-derived xenografts and tumor organoids decreases Rictor protein and suppresses the mTORC2 pathway. These findings provide new insights into the functions of an ubiquitin-like protein by inhibiting a dominant oncogenic pathway in CRC.

Crosstalk between protein kinase C α and transforming growth factor β signaling mediated by Runx2 in intestinal epithelial cells

Tight coordination of growth regulatory signaling is required for intestinal epithelial homeostasis. Protein kinase C α (PKCα) and transforming growth factor β (TGFβ) are negative regulators of proliferation with tumor suppressor properties in the intestine. Here, we identify novel crosstalk between PKCα and TGFβ signaling. RNA-Seq analysis of nontransformed intestinal crypt-like cells and colorectal cancer cells identified TGFβ receptor 1 (TGFβR1) as a target of PKCα signaling. RT-PCR and immunoblot analysis confirmed that PKCα positively regulates TGFβR1 mRNA and protein expression in these cells. Effects on TGFβR1 were dependent on Ras-extracellular signal-regulated kinase 1/2 (ERK) signaling. Nascent RNA and promoter-reporter analysis indicated that PKCα induces TGFβR1 transcription, and Runx2 was identified as an essential mediator of the effect. PKCα promoted ERK-mediated activating phosphorylation of Runx2, which preceded transcriptional activation of the TGFβR1 gene and induction of Runx2 expression. Thus, we have identified a novel PKCα→ERK→Runx2→TGFβR1 signaling axis. In further support of a link between PKCα and TGFβ signaling, PKCα knockdown reduced the ability of TGFβ to induce SMAD2 phosphorylation and cell cycle arrest, and inhibition of TGFβR1 decreased PKCα-induced upregulation of p21^Cip1 and p27^Kip1 in intestinal cells. The physiological relevance of these findings is also supported by The Cancer Genome Atlas data showing correlation between PKCα, Runx2, and TGFβR1 mRNA expression in human colorectal cancer. PKCα also regulated TGFβR1 in endometrial cancer cells, and PKCα, Runx2, and TGFβR1 expression correlates in uterine tumors, indicating that crosstalk between PKCα and TGFβ signaling may be a common mechanism in diverse epithelial tissues.

Combined inhibition of aurora kinases and Bcl-xL induces apoptosis through select BH3-only proteins

Aurora kinases (AURKs) are mitotic kinases important for regulating cell cycle progression. Small-molecule inhibitors of AURK have shown promising antitumor effects in multiple cancers; however, the utility of these inhibitors as inducers of cancer cell death has thus far been limited. Here, we examined the role of the Bcl-2 family proteins in AURK inhibition-induced apoptosis in colon cancer cells. We found that alisertib and danusertib, two small-molecule inhibitors of AURK, are inefficient inducers of apoptosis in HCT116 and DLD-1 colon cancer cells, the survival of which requires at least one of the two antiapoptotic Bcl-2 family proteins, Bcl-xL and Mcl-1. We further identified Bcl-xL as a major suppressor of alisertib- or danusertib-induced apoptosis in HCT116 cells. We demonstrate that combination of a Bcl-2 homology (BH)3-mimetic inhibitor (ABT-737), a selective inhibitor of Bcl-xL, Bcl-2, and Bcl-w, with alisertib or danusertib potently induces apoptosis through the Bcl-2 family effector protein Bax. In addition, we identified Bid, Puma, and Noxa, three BH3-only proteins of the Bcl-2 family, as mediators of alisertib-ABT-737-induced apoptosis. We show while Noxa promotes apoptosis by constitutively sequestering Mcl-1, Puma becomes associated with Mcl-1 upon alisertib treatment. On the other hand, we found that alisertib treatment causes activation of caspase-2, which promotes apoptosis by cleaving Bid into truncated Bid, a suppressor of both Bcl-xL and Mcl-1. Together, these results define the Bcl-2 protein network critically involved in AURK inhibitor-induced apoptosis and suggest that BH3-mimetics targeting Bcl-xL may help overcome resistance to AURK inhibitors in cancer cells.

MARK2 regulates chemotherapeutic responses through class IIa HDAC-YAP axis in pancreatic cancer

Despite paclitaxel's wide use in cancer treatment, patient response rate is still low and drug resistance is a major clinical obstacle. Through a Phos-tag-based kinome-wide screen, we identified MARK2 as a critical regulator for paclitaxel chemosensitivity in PDAC. We show that MARK2 is phosphorylated by CDK1 in response to antitubulin chemotherapeutics and in unperturbed mitosis. Phosphorylation is essential for MARK2 in regulating mitotic progression and paclitaxel cytotoxicity in PDAC cells. Mechanistically, our findings also suggest that MARK2 controls paclitaxel chemosensitivity by regulating class IIa HDACs. MARK2 directly phosphorylates HDAC4 specifically during antitubulin treatment. Phosphorylated HDAC4 promotes YAP activation and controls expression of YAP target genes induced by paclitaxel. Importantly, combination of HDAC inhibition and paclitaxel overcomes chemoresistance in organoid culture and preclinical PDAC animal models. The expression levels of MARK2, HDACs, and YAP are upregulated and positively correlated in PDAC patients. Inhibition of MARK2 or class IIa HDACs potentiates paclitaxel cytotoxicity by inducing mitotic abnormalities in PDAC cells. Together, our findings identify the MARK2-HDAC axis as a druggable target for overcoming chemoresistance in PDAC.

PKCα and PKCδ: Friends and Rivals

PKC comprises a large family of serine/threonine kinases that share a requirement for allosteric activation by lipids. While PKC isoforms have significant homology, functional divergence is evident among subfamilies and between individual PKC isoforms within a subfamily. Here, we highlight these differences by comparing the regulation and function of representative PKC isoforms from the conventional (PKCα) and novel (PKCδ) subfamilies. We discuss how unique structural features of PKCα and PKCδ underlie differences in activation and highlight the similar, divergent, and even opposing biological functions of these kinases. We also consider how PKCα and PKCδ can contribute to pathophysiological conditions and discuss challenges to targeting these kinases therapeutically.

A novel anti-proliferative PKCα-Ras-ERK signaling axis in intestinal epithelial cells

We have previously shown that the serine/threonine kinase PKCα triggers MAPK/ERK kinase (MEK)-dependent G₁→S cell cycle arrest in intestinal epithelial cells, characterized by downregulation of cyclin D1 and inhibitor of DNA-binding protein 1 (Id1) and upregulation of the cyclin-dependent kinase inhibitor p21^Cip1. Here, we use pharmacological inhibitors, genetic approaches, siRNA-mediated knockdown, and immunoprecipitation to further characterize anti-proliferative ERK signaling in intestinal cells. We show that PKCα signaling intersects the Ras-Raf-MEK-ERK kinase cascade at the level of Ras small GTPases, and that anti-proliferative effects of PKCα require active Ras, Raf, MEK and ERK, core ERK pathway components that are also essential for pro-proliferative ERK signaling induced by epidermal growth factor (EGF). However, PKCα-induced anti-proliferative signaling differs from EGF signaling in that it is independent of the Ras guanine nucleotide exchange factors (Ras-GEFs), SOS1/2, and involves prolonged rather than transient ERK activation. PKCα forms complexes with A-Raf, B-Raf and C-Raf that dissociate upon pathway activation, and all three Raf isoforms can mediate PKCα-induced anti-proliferative effects. At least two PKCα-ERK pathways that collaborate to promote growth arrest were identified: one pathway requiring the Ras-GEF, RasGRP3, and H-Ras, leads to p21^Cip1 upregulation, while additional pathway(s) mediate PKCα-induced cyclin D1 and Id1 downregulation. PKCα also induces ERK-dependent SOS1 phosphorylation, indicating possible negative crosstalk between anti-proliferative and growth-promoting ERK signaling. Importantly, the spatio-temporal activation of PKCα and ERK in the intestinal epithelium in vivo supports the physiological relevance of these pathways and highlights the importance of anti-proliferative ERK signaling to tissue homeostasis in the intestine.

KSR1- and ERK-dependent translational regulation of the epithelial-to-mesenchymal transition

The epithelial-to-mesenchymal transition (EMT) is considered a transcriptional process that induces a switch in cells from a polarized state to a migratory phenotype. Here, we show that KSR1 and ERK promote EMT-like phenotype through the preferential translation of Epithelial-Stromal Interaction 1 (EPSTI1), which is required to induce the switch from E- to N-cadherin and coordinate migratory and invasive behavior. EPSTI1 is overexpressed in human colorectal cancer (CRC) cells. Disruption of KSR1 or EPSTI1 significantly impairs cell migration and invasion in vitro, and reverses EMT-like phenotype, in part, by decreasing the expression of N-cadherin and the transcriptional repressors of E-cadherin expression, ZEB1 and Slug. In CRC cells lacking KSR1, ectopic EPSTI1 expression restored the E- to N-cadherin switch, migration, invasion, and anchorage-independent growth. KSR1-dependent induction of EMT-like phenotype via selective translation of mRNAs reveals its underappreciated role in remodeling the translational landscape of CRC cells to promote their migratory and invasive behavior.

The majority of cancer deaths result from tumor cells spreading to other parts of the body via a process known as metastasis. 90% of all cancers originate in epithelial cells that line the inner and outer surface of organs in our bodies. Epithelial cells, however, are typically stationary and must undergo various chemical and physical changes to transform in to migratory cells that can invade other tissues.

This transformation process alters the amount of protein cells use to interact with one another. For example, epithelial cells from the colon produce less of a protein called E-cadherin as they transition into migrating cancer cells and make another protein called N-cadherin instead. A protein called KSR1 is a key component of a signaling pathway that is responsible for generating the proteins colon cancer cells need to survive. But it is unknown which proteins KSR1 helps synthesize and whether it plays a role in the metastasis of colon cancer cells.

To investigate this, Rao et al. studied the proteins generated by cancerous colon cells cultured in the laboratory, in the presence and absence of KSR1. The experiment showed that KSR1 increases the levels of a protein called EPSTI1, which colon cancer cells need to transform into migratory cells. Depleting KSR1 caused cancer cells to generate less EPSTI1 and to share more features with healthy cells, such as higher levels of E-cadherin on their surface and reduced mobility. Adding EPSTI1 to the cancer cells that lacked KSR1 restored the traits associated with metastasis, such as high levels of N-cadherin, and allowed the cells to move more easily.

These findings suggest that KSR1 and EPSTI1 could be new drug targets for reducing, or potentially reversing, the invasive behavior of colon cancer cells. However, further investigation is needed to reveal how EPSTI1 is generated and how this protein helps colon cancer cells move and invade other tissues.

The complexities of PKCα signaling in cancer

Protein kinase C α (PKCα) is a ubiquitously expressed member of the PKC family of serine/threonine kinases with diverse functions in normal and neoplastic cells. Early studies identified anti-proliferative and differentiation-inducing functions for PKCα in some normal tissues (e.g., regenerating epithelia) and pro-proliferative effects in others (e.g., cells of the hematopoietic system, smooth muscle cells). Additional well documented roles of PKCα signaling in normal cells include regulation of the cytoskeleton, cell adhesion, and cell migration, and PKCα can function as a survival factor in many contexts. While a majority of tumors lose expression of PKCα, others display aberrant overexpression of the enzyme. Cancer-related mutations in PKCα are uncommon, but rare examples of driver mutations have been detected in certain cancer types (e. g., choroid gliomas). Here we review the role of PKCα in various cancers, describe mechanisms by which PKCα affects cancer-related cell functions, and discuss how the diverse functions of PKCα contribute to tumor suppressive and tumor promoting activities of the enzyme. We end the discussion by addressing mutations and expression of PKCα in tumors and the clinical relevance of these findings.

Abstract 307: KSR1-dependent modulation of the translational landscape in Ras-driven colorectal cancer

KRAS is mutated and activated in an estimated 40% of colon cancers but has proven difficult to target directly due to the lack of drug-binding pockets on the surface of Ras. Thus, identifying effectors that transmit signals from oncogenic Ras is a critical step in targeting vulnerabilities in the tumors of patients with colorectal cancer (CRC). Kinase Suppressor of Ras 1 (KSR1) is a molecular scaffold that coordinates the interaction of effectors in the Raf/MEK/ERK signaling cascade downstream of Ras. KSR1 is necessary for oncogenic transformation of cells expressing mutated Ras but is dispensable for normal cell growth making KSR1 an attractive therapeutic target for Ras-driven colon cancers. In CRC cells, KSR1 and ERK mediate Ras-dependent effects on protein translation, notably driving cap-dependent and cap-independent translation of the Myc oncogene. Depletion of KSR1 or treatment with ERK inhibitor (SCH772984) leads to a decrease in phophorylated 4EBP1 and a decrease in PDCD4, which collectively impair protein translation, notably leading to a decrease in Myc translation. Patient-derived CRC organoids with deletion of APC, p53, and SMAD4, and an activating G12D mutation in KRAS treated with ERK inhibitor show a marked decrease in Myc protein expression, coincident with changes in PDCD4 and 4EBP1 that impair protein translation. These observations indicate that the KSR1-dependent ERK activation observed in CRC cell lines is present in pre-clinical colon tumoroid models. Genome-wide polysome profiling in CRC cell lines HCT116 and HCT15 depleted of KSR1 identified mRNAs that were preferentially translated in a KSR1 and ERK-dependent manner. Several of these mRNAs were previously predicted by our Functional Signature Ontology (FUSION) screen to be critical to CRC viability but dispensable for normal cell growth. GSEA identified functional classes of mRNAs whose translation is KSR1-dependent, including mediators of oncogenic signaling in pathways that regulate mTOR, MAPK, WNT and JAK-STAT. Our data suggest that an essential component of oncogenic Ras-induced and KSR1-dependent signaling is the preferential translation of mRNAs supporting the transformed phenotype of CRC cells.

Citation Format: Heidi Vieira, Chaitra Rao, Adrian R. Black, Siddesh Southekal, Tomohiro Mizutani, Babu Guda, Hans Clevers, Jennifer D. Black, Robert E. Lewis. KSR1-dependent modulation of the translational landscape in Ras-driven colorectal cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 307.

Repair and regeneration of small intestine: A review of current engineering approaches

The small intestine (SI) is difficult to regenerate or reconstruct due to its complex structure and functions. Recent developments in stem cell research, advanced engineering technologies, and regenerative medicine strategies bring new hope of solving clinical problems of the SI. This review will first summarize the structure, function, development, cell types, and matrix components of the SI. Then, the major cell sources for SI regeneration are introduced, and state-of-the-art biofabrication technologies for generating engineered SI tissues or models are overviewed. Furthermore, in vitro models and in vivo transplantation, based on intestinal organoids and tissue engineering, are highlighted. Finally, current challenges and future perspectives are discussed to help direct future applications for SI repair and regeneration.

Crosstalk between PKCα and PI3K/AKT Signaling Is Tumor Suppressive in the Endometrium

Protein kinase C (PKC) isozymes are commonly recognized as oncoproteins based on their activation by tumor-promoting phorbol esters. However, accumulating evidence indicates that PKCs can be inhibitory in some cancers, with recent findings propelling a shift in focus to understanding tumor suppressive functions of these enzymes. Here, we report that PKCα acts as a tumor suppressor in PI3K/AKT-driven endometrial cancer. Transcriptional suppression of PKCα is observed in human endometrial tumors in association with aggressive disease and poor prognosis. In murine models, loss of PKCα is rate limiting for endometrial tumor initiation. PKCα tumor suppression involves PP2A-family-dependent inactivation of AKT, which can occur even in the context of genetic hyperactivation of PI3K/AKT signaling by coincident mutations in PTEN, PIK3CA, and/or PIK3R1. Together, our data point to PKCα as a crucial tumor suppressor in the endometrium, with deregulation of a PKCα→PP2A/PP2A-like phosphatase signaling axis contributing to robust AKT activation and enhanced endometrial tumorigenesis.

Abstract 189: Protein kinase C alpha suppresses AKT activation in endometrial cancer

Endometrial cancer (EC) is the most common gynecological malignancy and the fourth leading cancer in women in the United States. Hyperactivation of the PI3K/AKT pathway is observed in ∼90% of EC cases and this pathway is the most prominent driver of endometrial carcinogenesis. Mutations in multiple PI3K/AKT pathway components (including PTEN, PIK3CA, PIK3R1, AKT) often coexist in EC, indicating that EC development requires full, unopposed activation of PI3K/AKT signaling. In this study, we identified PKCα as a negative regulator of AKT activation that suppresses endometrial carcinogenesis. Our analysis revealed that PKCα protein and mRNA are reduced or lost in 50% of EC cell lines and ∼60% of human ECs. PKCα deficiency was also observed in hyperplastic endometrial lesions in murine models with allelic knock-in of mutant Pten (ptenΔ4-5/+, ptenC124R/+ and ptenC129E/+) or endometrial-specific Pten deletion (ptenpr-/-, ptenltf-/-), indicating that loss of PKCα is an early event in endometrial carcinogenesis.

Mechanistic analysis has further revealed that loss of PKCα is independent of changes in PI3K/AKT signaling; however, the enzyme acts as a potent negative regulator of the PI3K/AKT pathway in EC cells. PKC agonists reduced AKT phosphorylation/activity in EC cells that retain PKCα, via a PP2A-dependent mechanism. Conversely, PKCα knockdown led to increased AKT phosphorylation. Notably, while PKC agonists did not affect AKT in EC cells with loss of PKCα, exogenous expression of the enzyme in these cells restored the inhibitory effect of the agonists on PI3K/AKT signaling. Collectively these data indicate that PKCα has a restraining effect on PI3K/AKT signaling in EC cells, providing a basis for its loss during endometrial tumor progression.

Physiological relevance of these findings was established using cell lines, patient samples and mouse models. PKCα restoration in human EC cells decreased expression of oncogenes such as cyclin D1 and Id1 and abolished their anchorage-independent growth, supporting a tumor suppressive role for the isozyme in this tissue. Analysis of PKCα expression in patient specimens determined that loss of PKCα expression correlates with high grade disease. Finally, in mice heterozygous for mutant Pten, PKCα knockout led to a 3-fold increase in endometrial tumor burden at three months, confirming a role of PKCα as a tumor suppressor in the endometrium.

Taken together, our results provide evidence that PKCα 1) negatively regulates AKT signaling and the expression of important oncogenes in EC cells; and 2) functions as tumor suppressor in the endometrium whose loss is associated with disease progression. Thus, PKCα signaling represents a promising biomarker for risk stratification in early stage disease and may provide insight into therapeutic strategies for late stage disease.

Supported by NIH grants CA036727, CA016056, and DK60632.

Citation Format: Alice H. Hsu, Kathryn J. Curry, Kang-Sup Shim, Peter Frederick, Carl D. Morrison, Baojiang Chen, Subodh M. Lele, Takiko Daikoku, Sudhansu K. Dey, Gustavo Leone, Adrian R. Black, Jennifer D. Black. Protein kinase C alpha suppresses AKT activation in endometrial cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 189.

Protein Kinase Cα (PKCα) Is Resistant to Long Term Desensitization/Down-regulation by Prolonged Diacylglycerol Stimulation

Sustained activation of PKCα is required for long term physiological responses, such as growth arrest and differentiation. However, studies with pharmacological agonists (e.g. phorbol 12-myristate 13-acetate (PMA)) indicate that prolonged stimulation leads to PKCα desensitization via dephosphorylation and/or degradation. The current study analyzed effects of chronic stimulation with the physiological agonist diacylglycerol. Repeated addition of 1,2-dioctanoyl-sn-glycerol (DiC8) resulted in sustained plasma membrane association of PKCα in a pattern comparable with that induced by PMA. However, although PMA potently down-regulated PKCα, prolonged activation by DiC8 failed to engage known desensitization mechanisms, with the enzyme remaining membrane-associated and able to support sustained downstream signaling. DiC8-activated PKCα did not undergo dephosphorylation, ubiquitination, or internalization, early events in PKCα desensitization. Although DiC8 efficiently down-regulated novel PKCs PKCδ and PKCϵ, differences in Ca(2+) sensitivity and diacylglycerol affinity were excluded as mediators of the selective resistance of PKCα. Roles for Hsp/Hsc70 and Hsp90 were also excluded. PMA, but not DiC8, targeted PKCα to detergent-resistant membranes, and disruption of these domains with cholesterol-binding agents demonstrated a role for differential membrane compartmentalization in selective agonist-induced degradation. Chronic DiC8 treatment failed to desensitize PKCα in several cell types and did not affect PKCβI; thus, conventional PKCs appear generally insensitive to desensitization by sustained diacylglycerol stimulation. Consistent with this conclusion, prolonged (several-day) membrane association/activation of PKCα is seen in self-renewing epithelium of the intestine, cervix, and skin. PKCα deficiency affects gene expression, differentiation, and tumorigenesis in these tissues, highlighting the importance of mechanisms that protect PKCα from desensitization in vivo.

Abstract 4720: Protein kinase C alpha (PKCα) regulates PI3K/AKT signaling in endometrial cancer

Endometrial cancer is the most common gynecological malignancy and the fourth most common cancer in women in the United States. Although loss of PTEN and deregulation of PI3K/AKT signaling has been implicated in approximately 90% of endometrial cancer cases, understanding of the molecular etiology of the disease remains limited. Our analysis of tissue from 448 endometrial cancer patients has determined that ∼60% of human tumors show reduced expression or loss of the signal transduction molecule PKCα. Loss of this enzyme is also seen in endometrial hyperplasias arising in mouse models carrying germline mutations of PTEN (PtenΔ4-5, PtenC124R and PtenG129E) or endometrial specific deletion of PTEN (Ptenpr-/−), indicating that loss of PKCα can be an early event in development of the disease. In patients, reduced expression of PKCα correlates with markers of disease aggressiveness, such as increased myometrial invasion and lymph node involvement, supporting a tumor suppressive role for the enzyme in the endometrium. This correlation is particularly marked in endometrioid, PTEN-mutant disease, pointing to a potential link between PKCα and PI3K/AKT signaling.

The role of PKCα in endometrial cancer was further examined using a panel of 17 human endometrial cancer cell lines with varying expression of the kinase. Restoration of PKCα in cell lines lacking the enzyme blocked their ability to grow in soft agarose, further supporting a tumor suppressive role of the kinase in this tissue. Stimulation of cells that express PKCα with PKC agonists reduced phosphorylation/activation of AKT and promoted loss of downstream PI3K/AKT targets such as cyclin D1 and inhibitor of DNA binding 1 (Id1). Analysis of the PKC isozyme expression profile in these cells combined with the use of selective inhibitors pointed to PKCα as the mediator of the effect. The demonstration that PKC agonists failed to inhibit AKT signaling in cell lines that lack expression of PKCα, and that expression of exogenous PKCα restored the effect, confirmed the involvement of PKCα. The inhibition of AKT by PKCα is dependent on PP2A since okadaic acid and calyculin A, but not the PHLPP1/2 inhibitors NSC117079 and NSC45586, blocked the effect, a finding consistent with the ability of PKCα to inhibit AKT activity in PTEN mutant cell lines.

Taken together, our study provides evidence that 1) loss of PKCα is a common and early event in endometrial cancer; 2) PKCα plays a crucial role in regulating AKT activation and growth promoting signaling molecules in endometrial cancer, and 3) effects of PKCα on AKT in endometrial cancer are mediated by PP2A. Given the importance of PI3K/AKT in the disease, these findings highlight the potential of PKCα signaling as a potential biomarker for disease risk and as a potential therapeutic target in endometrial cancer.

Supported by NIH grants CA036727, CA016056, and DK60632.

Citation Format: Alice H. Hsu, Kathryn J. Curry, Kang-Sup Shim, Peter Frederick, Carl Morrison, Baojing Chen, Subodh M. Lele, Takiko Daikoku, Sudhansu K. Dey, Gustavo Leone, Adrian R. Black, Jennifer D. Black. Protein kinase C alpha (PKCα) regulates PI3K/AKT signaling in endometrial cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4720. doi:10.1158/1538-7445.AM2015-4720

Abstract 4211: Protein kinase C alpha (PKCα) signaling in endometrial cancer

Endometrial cancer is the most common gynecological malignancy in the US and the fourth most common cancer in women. Despite its high incidence, there is limited knowledge of the molecular etiology of this disease. With obesity being a major risk factor, the incidence of endometrial cancer has been on a steady incline and is expected to become an increasing cause of cancer mortality in future years. Thus, improved understanding of endometrial tumorigenesis is of critical importance. Our analysis of human endometrial tumors identified loss of the signaling molecule PKCα in approximately 60% of endometrioid (Type I) tumors. Loss of PKCα trends with increasing tumor grade and is associated with aggressive disease features such as lymphovascular involvement and myometrial invasion, pointing to an important role for this molecule in regulating endometrial cancer progression. Inactivation of the tumor suppressor PTEN and deregulation of the PI3K/AKT pathway are key drivers of Type I endometrial cancer. Thus, mice with allele-specific knock-in of cancer-related Pten mutants (i.e., PtenΔ4-5, PtenC124R, and PtenG120E) offer unique models for the disease. Precancerous endometrial hyperplasias arising in these mice show loss of PTEN and increased AKT activity. While PKCα is expressed in the normal murine endometrial epithelium at all phases of the estrus cycle, the enzyme is uniformly lost in these precancerous lesions. Thus, disruption of PKCα signaling can occur early in endometrial tumorigenesis. In human endometrial cancer cell lines, low PKCα levels also correlate with PTEN loss; however, shRNA-mediated knockdown of PTEN in PTEN expressing cells did not affect PKCα expression indicating that downregulation of the kinase is not the direct result of changes in PTEN activity. Notably, PKC agonists suppress AKT activity in cells expressing high levels PKCα (HEC-1-A, HEC-50) but not in PKCα-low cells (Ishikawa, RL95-2). Collectively, these data suggest that, in addition to PTEN inactivation, loss of PKCα is required for full activation of the PI3K/AKT signaling pathway during endometrial carcinogenesis. Analysis of the molecular basis for loss of PKCα expression in endometrial tumors identified multiple regulatory mechanisms. PKCα protein expression in tumors generally parallels that of its mRNA and promoter activity assays point to transcriptional mechanisms for PKCα downregulation. Differential mRNA and protein stability were identified as additional levels of regulation of PKCα expression in endometrial cancer. Taken together, our findings indicate that 1) PKCα signaling regulates the PI3K/AKT pathway in the endometrial epithelium; 2) concomitant loss of PKCα and PTEN may act in a co-operative manner in regulation of endometrial turmorigenesis; and 3) loss of PKCα can occur by multiple mechanisms, as often seen with tumor suppressive molecules during neoplastic progression. Supported by NIH grants CA036727, CA016056, and DK60632.

Citation Format: Alice H. Hsu, Kathryn J. Curry, Kang-Sup Shim, Peter Frederick, Carl D. Morrison, Baojing Chen, Subodh M. Lele, Gustavo Leone, Adrian R. Black, Jennifer D. Black. Protein kinase C alpha (PKCα) signaling in endometrial cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4211. doi:10.1158/1538-7445.AM2014-4211

Differential regulation of cyclin D1 expression by protein kinase C α and ϵ signaling in intestinal epithelial cells

Cellular accumulation of cyclin D1, a key regulator of cell proliferation and tumorigenesis, is subject to tight control. Our previous studies have identified PKCα as a negative regulator of cyclin D1 in the intestinal epithelium. However, treatment of non-transformed IEC-18 ileal crypt cells with PKC agonists has a biphasic effect on cyclin D1 expression. Initial PKCα-mediated down-regulation is followed by recovery and subsequent accumulation of the cyclin to levels markedly higher than those seen in untreated cells. Using protein overexpression strategies, siRNA, and pharmacological inhibitors, we now demonstrate that the recovery and hyperinduction of cyclin D1 reflect the combined effects of (a) loss of negative signals from PKCα due to agonist-induced PKCα down-regulation and (b) positive effects of PKCϵ. PKCϵ-mediated up-regulation of cyclin D1 requires sustained ERK stimulation and transcriptional activation of the proximal cyclin D1 (CCDN1) promoter, without apparent involvement of changes in protein stability or translation. PKCϵ also up-regulates cyclin D1 expression in colon cancer cells, through mechanisms that parallel those in IEC-18 cells. Although induction of cyclin D1 by PKCϵ is dependent on non-canonical NF-κB activation, the NF-κB site in the proximal promoter is not required. Instead, cyclin D1 promoter activity is regulated by a novel interaction between NF-κB and factors that associate with the cyclic AMP-response element adjacent to the NF-κB site. The differential effects of PKCα and PKCϵ on cyclin D1 accumulation are likely to contribute to the opposing tumor-suppressive and tumor-promoting activities of these PKC family members in the intestinal epithelium.

Heat shock proteins regulate activation-induced proteasomal degradation of the mature phosphorylated form of protein kinase C

Although alterations in stimulus-induced degradation of PKC have been implicated in disease, mechanistic understanding of this process remains limited. Evidence supports the existence of both proteasomal and lysosomal mechanisms of PKC processing. An established pathway involves rate-limiting priming site dephosphorylation of the activated enzyme and proteasomal clearance of the dephosphorylated protein. However, here we show that agonists promote down-regulation of endogenous PKCα with minimal accumulation of a nonphosphorylated species in multiple cell types. Furthermore, proteasome and lysosome inhibitors predominantly protect fully phosphorylated PKCα, pointing to this form as a substrate for degradation. Failure to detect substantive dephosphorylation of activated PKCα was not due to rephosphorylation because inhibition of Hsp70/Hsc70, which is required for re-priming, had only a minor effect on agonist-induced accumulation of nonphosphorylated protein. Thus, PKC degradation can occur in the absence of dephosphorylation. Further analysis revealed novel functions for Hsp70/Hsc70 and Hsp90 in the control of agonist-induced PKCα processing. These chaperones help to maintain phosphorylation of activated PKCα but have opposing effects on degradation of the phosphorylated protein; Hsp90 is protective, whereas Hsp70/Hsc70 activity is required for proteasomal processing of this species. Notably, down-regulation of nonphosphorylated PKCα shows little Hsp70/Hsc70 dependence, arguing that phosphorylated and nonphosphorylated species are differentially targeted for proteasomal degradation. Finally, lysosomal processing of activated PKCα is not regulated by phosphorylation or Hsps. Collectively, these data demonstrate that phosphorylated PKCα is a direct target for agonist-induced proteasomal degradation via an Hsp-regulated mechanism, and highlight the existence of a novel pathway of PKC desensitization in cells.

Agonist-induced down-regulation of endogenous protein kinase c α through an endolysosomal mechanism

Protein kinase C (PKC) isozymes undergo down-regulation upon sustained stimulation. Previous studies have pointed to the existence of both proteasome-dependent and -independent pathways of PKCα processing. Here we demonstrate that these down-regulation pathways are engaged in different subcellular compartments; proteasomal degradation occurs mainly at the plasma membrane, whereas non-proteasomal processing occurs in the perinuclear region. Using cholesterol depletion, pharmacological inhibitors, RNA interference, and dominant-negative mutants, we define the mechanisms involved in perinuclear accumulation of PKCα and identify the non-proteasomal mechanism mediating its degradation. We show that intracellular accumulation of PKCα involves at least two clathrin-independent, cholesterol/lipid raft-mediated pathways that do not require ubiquitination of the protein; one is dynamin-dependent and likely involves caveolae, whereas the other is dynamin- and small GTPase-independent. Internalized PKCα traffics through endosomes and is delivered to the lysosome for degradation. Supportive evidence includes (a) detection of the enzyme in EEA1-positive early endosomes, Rab7-positive late endosomes/multivesicular bodies, and LAMP1-positive lysosomes and (b) inhibition of its down-regulation by lysosome-disrupting agents and leupeptin. Only limited dephosphorylation of PKCα occurs during trafficking, with fully mature enzyme being the main target for lysosomal degradation. These studies define a novel and widespread mechanism of desensitization of PKCα signaling that involves endocytic trafficking and lysosome-mediated degradation of the mature, fully phosphorylated protein.

Protein kinase C signaling and cell cycle regulation

A link between T cell proliferation and the protein kinase C (PKC) family of serine/threonine kinases has been recognized for about 30 years. However, despite the wealth of information on PKC-mediated control of, T cell activation, understanding of the effects of PKCs on the cell cycle machinery in this cell type remains limited. Studies in other systems have revealed important cell cycle-specific effects of PKC signaling that can either positively or negatively impact proliferation. The outcome of PKC activation is highly context-dependent, with the precise cell cycle target(s) and overall effects determined by the specific isozyme involved, the timing of PKC activation, the cell type, and the signaling environment. Although PKCs can regulate all stages of the cell cycle, they appear to predominantly affect G0/G1 and G2. PKCs can modulate multiple cell cycle regulatory molecules, including cyclins, cyclin-dependent kinases (cdks), cdk inhibitors and cdc25 phosphatases; however, evidence points to Cip/Kip cdk inhibitors and D-type cyclins as key mediators of PKC-regulated cell cycle-specific effects. Several PKC isozymes can target Cip/Kip proteins to control G0/G1 → S and/or G2 → M transit, while effects on D-type cyclins regulate entry into and progression through G1. Analysis of PKC signaling in T cells has largely focused on its roles in T cell activation; thus, observed cell cycle effects are mainly positive. A prominent role is emerging for PKCθ, with non-redundant functions of other isozymes also described. Additional evidence points to PKCδ as a negative regulator of the cell cycle in these cells. As in other cell types, context-dependent effects of individual isozymes have been noted in T cells, and Cip/Kip cdk inhibitors and D-type cyclins appear to be major PKC targets. Future studies are anticipated to take advantage of the similarities between these various systems to enhance understanding of PKC-mediated cell cycle regulation in T cells.

Protein kinase Cα signaling regulates inhibitor of DNA binding 1 in the intestinal epithelium

Increasing evidence supports a role for PKCα in growth arrest and tumor suppression in the intestinal epithelium. In contrast, the Id1 transcriptional repressor has pro-proliferative and tumorigenic properties in this tissue. Here, we identify Id1 as a novel target of PKCα signaling. Using a highly specific antibody and a combined morphological/biochemical approach, we establish that Id1 is a nuclear protein restricted to proliferating intestinal crypt cells. A relationship between PKCα and Id1 was supported by the demonstration that (a) down-regulation of Id1 at the crypt/villus junction coincides with PKCα activation, and (b) loss of PKCα in intestinal tumors is associated with increased levels of nuclear Id1. Manipulation of PKCα activity in IEC-18 nontransformed intestinal crypt cells determined that PKCα suppresses Id1 mRNA and protein via an Erk-dependent mechanism. PKCα, but not PKCδ, also inhibited Id1 expression in colon cancer cells. Id1 was found to regulate cyclin D1 levels in IEC-18 and colon cancer cells, pointing to a role for Id1 suppression in the antiproliferative/tumor suppressive activities of PKCα. Notably, Id1 expression was elevated in the intestinal epithelium of PKCα-knock-out mice, confirming that PKCα regulates Id1 in vivo. A wider role for PKCα in control of inhibitor of DNA binding factors is supported by its ability to down-regulate Id2 and Id3 in IEC-18 cells, although their suppression is more modest than that of Id1. This study provides the first demonstrated link between a specific PKC isozyme and inhibitor of DNA binding factors, and it points to a role for a PKCα → Erk ⊣ Id1 → cyclin D1 signaling axis in the maintenance of intestinal homeostasis.

Protein kinase C-mediated down-regulation of cyclin D1 involves activation of the translational repressor 4E-BP1 via a phosphoinositide 3-kinase/Akt-independent, protein phosphatase 2A-dependent mechanism in intestinal epithelial cells

We reported previously that protein kinase Calpha (PKCalpha), a negative regulator of cell growth in the intestinal epithelium, inhibits cyclin D1 translation by inducing hypophosphorylation/activation of the translational repressor 4E-BP1. The current study explores the molecular mechanisms underlying PKC/PKCalpha-induced activation of 4E-BP1 in IEC-18 nontransformed rat ileal crypt cells. PKC signaling is shown to promote dephosphorylation of Thr(45) and Ser(64) on 4E-BP1, residues directly involved in its association with eIF4E. Consistent with the known role of the phosphoinositide 3-kinase (PI3K)/Akt/mTOR pathway in regulation of 4E-BP1, PKC signaling transiently inhibited PI3K activity and Akt phosphorylation in IEC-18 cells. However, PKC/PKCalpha-induced activation of 4E-BP1 was not prevented by constitutively active mutants of PI3K or Akt, indicating that blockade of PI3K/Akt signaling is not the primary effector of 4E-BP1 activation. This idea is supported by the fact that PKC activation did not alter S6 kinase activity in these cells. Further analysis indicated that PKC-mediated 4E-BP1 hypophosphorylation is dependent on the activity of protein phosphatase 2A (PP2A). PKC signaling induced an approximately 2-fold increase in PP2A activity, and phosphatase inhibition blocked the effects of PKC agonists on 4E-BP1 phosphorylation and cyclin D1 expression. H(2)O(2) and ceramide, two naturally occurring PKCalpha agonists that promote growth arrest in intestinal cells, activate 4E-BP1 in PKC/PKCalpha-dependent manner, supporting the physiological significance of the findings. Together, our studies indicate that activation of PP2A is an important mechanism underlying PKC/PKCalpha-induced inhibition of cap-dependent translation and growth suppression in intestinal epithelial cells.

Protein kinase C alpha signaling inhibits cyclin D1 translation in intestinal epithelial cells

Cyclin D1 is a key regulator of cell proliferation, acting as a mitogen sensor and linking extracellular signaling to the cell cycle machinery. Strict control of cyclin D1 levels is critical for maintenance of tissue homeostasis. We have reported previously that protein kinase C alpha (PKCalpha), a negative regulator of cell growth in the intestinal epithelium, promotes rapid down-regulation of cyclin D1 (Frey, M. R., Clark, J. A., Leontieva, O., Uronis, J. M., Black, A. R., and Black, J. D. (2000) J. Cell Biol. 151, 763-778). The current study explores the mechanisms underlying PKCalpha-induced loss of cyclin D1 protein in non-transformed intestinal epithelial cells. Our findings exclude several mechanisms previously implicated in down-regulation of cyclin D1 during cell cycle exit/differentiation, including alterations in cyclin D1 mRNA expression and protein turnover. Instead, we identify PKCalpha as a novel repressor of cyclin D1 translation, acting at the level of cap-dependent initiation. Inhibition of cyclin D1 translation initiation is mediated by PKCalpha-induced hypophosphorylation/activation of the translational suppressor 4E-BP1, association of 4E-BP1 with the mRNA cap-binding protein eIF4E, and sequestration of cyclin D1 mRNA in 4E-BP1-associated complexes. Together, these post-transcriptional effects ensure rapid disappearance of the potent mitogenic molecule cyclin D1 during PKCalpha-induced cell cycle withdrawal in the intestinal epithelium.

Cell cycle- and protein kinase C-specific effects of resiniferatoxin and resiniferonol 9,13,14-ortho-phenylacetate in intestinal epithelial cells

We have previously reported that protein kinase C (PKC) signaling can trigger hallmark events of cell cycle withdrawal in intestinal epithelial cells, including downregulation of cyclin D1, induction of p21(Waf1/Cip1), and activation of the growth suppressor function of pocket proteins. In the current study, we compared the cell cycle- and PKC-specific effects of the vanilloid resiniferatoxin (RTX), its parent diterpene resiniferonol 9,13,14-ortho-phenylacetate (ROPA), and the PKC agonist PMA in the IEC-18 non-transformed intestinal crypt cell line. ROPA and PMA were found to produce strikingly similar alterations in cell cycle progression and PKC activity in IEC-18 cells, although PMA was approximately 1000-fold more potent in producing these effects. Both agents induced a transient PKC-dependent blockade in G1---> S progression associated with transient downregulation of cyclin D1 and induction of p21(Waf1/Cip1). In contrast, RTX produced a prolonged PKC-independent cell cycle arrest in G(0)/G(1) phase which was maintained for longer than 24h. This arrest was vanilloid receptor-independent and associated with prolonged downregulation of cyclin D1 mRNA and protein, with little effect on levels of p21(Waf1/Cip1). Combined exposure to RTX and ROPA produced a sustained and complete cell cycle blockade in IEC-18 cells, associated with depletion of cyclin D1 and sustained enhancement of p21(Waf1/Cip1) levels. PMA, ROPA, RTX and the RTX/ROPA combination were capable of activating ERK1/2 signaling in IEC-18 cells, albeit with different kinetics. In contrast, only PMA and ROPA activated JNK1/2 and p38 in this system. Notably, some preparations of commercially obtained RTX produced effects indistinguishable from those of the RTX/ROPA combination, suggesting that certain batches of the compound may contain significant amounts of ROPA (or another PKC agonist activity). Together, these data demonstrate that structurally related compounds can produce similar cell cycle-specific effects but through distinct mechanisms. In addition, they add to a growing body of evidence that vanilloids can have antiproliferative effects in a variety of cell types.

Involvement of the ERK Signaling Cascade in Protein Kinase C-mediated Cell Cycle Arrest in Intestinal Epithelial Cells

We have reported previously that protein kinase C (PKC) signaling can mediate a program of cell cycle withdrawal in IEC-18 nontransformed intestinal crypt cells, involving rapid disappearance of cyclin D1, increased expression of Cip/Kip cyclin-dependent kinase inhibitors, and activation of the growth suppressor function of pocket proteins. In the current study, we present evidence to support a requisite role for PKC alpha in mediating these effects. Furthermore, analysis of the signaling events linking PKC/PKC alpha activation to changes in the cell cycle regulatory machinery implicate the Ras/Raf/MEK/ERK cascade. PKC/PKC alpha activity promoted GTP loading of Ras, activation of Raf-1, and phosphorylation/activation of ERK. ERK activation was found to be required for critical downstream effects of PKC/PKC alpha activation, including cyclin D1 down-regulation, p21(Waf1/Cip1) induction, and cell cycle arrest. PKC-induced ERK activation was strong and sustained relative to that produced by proliferative signals, and the growth inhibitory effects of PKC agonists were dominant over proliferative events when these opposing stimuli were administered simultaneously. PKC signaling promoted cytoplasmic and nuclear accumulation of ERK activity, whereas growth factor-induced phospho-ERK was localized only in the cytoplasm. Comparison of the effects of PKC agonists that differ in their ability to sustain PKC alpha activation and growth arrest in IEC-18 cells, together with the use of selective kinase inhibitors, indicated that the length of PKC-mediated cell cycle exit is dictated by the magnitude/duration of input signal (i.e. PKC alpha activity) and of activation of the ERK cascade. The extent/duration of phospho-ERK nuclear localization may also be important determinants of the duration of PKC agonist-induced growth arrest in this system. Taken together, the data point to PKC alpha and the Ras/Raf/MEK/ERK cascade as key regulators of cell cycle withdrawal in intestinal epithelial cells.

Intestinal tumor progression is associated with altered function of KLF5

Krüppel-like transcription factors have been linked to cell growth regulation and tumorigenesis in a number of systems. In the intestinal epithelium, expression of KLF5 (IKLF/BTEB2) is limited to proliferating crypt cells, indicating a growth-promoting role. Consistent with this role, we demonstrate that expression of KLF5 in non-transformed intestinal epithelial cells (ileal IEC-18 and Immorto-Min Colon Epithelial (IMCE) cells) enhances colony formation, cyclin D1 transcription, and cell growth. However, in contrast to these effects in non-transformed cells, KLF5 reduced colony number, failed to enhance cyclin D1 transcription, and was negatively correlated with cell growth in colon cancer cell lines. The relationship between tumor progression and KLF5 was further investigated using Ras-mediated transformation of IEC-18 and IMCE cells as syngeneic models. Ras-transformation recapitulated differences in the effects of KLF5 on cell growth and cyclin D1 transcription, providing a direct link between intestinal tumor progression and altered function of KLF5. Ras-transformation also markedly down-regulated KLF5; further analysis indicated that reduced expression of KLF5 mRNA and destabilization of KLF5 protein occur in intestinal tumors. Reduced levels of KLF5 mRNA were also detected in APC(min) mouse and human familial adenomatous polyposis adenomas compared with normal crypt epithelium, indicating that down-regulation of KLF5 is an early event in intestinal tumorigenesis in vivo. Collectively, these data indicate that intestinal tumor progression is associated with a change in the growth-related functions of KLF5 and that intestinal tumors down-regulate KLF5 expression by multiple mechanisms.

Hydrology in Scotland: towards a scientific basis for the sustainable management of freshwater resources--foreword to thematic issue

The physical background to hydrology in Scotland is briefly reviewed. The main scientific events associated with understanding the science of hydrology in Scotland are summarized and major contemporary research themes are outlined. A brief overview of each paper in the current volume is given.

Hydrological science, society and the sustainable management of Scottish freshwaters resources in the 21st century

Hydrology in Scotland has emerged as a diverse and maturing discipline in recent years following its origins in engineering and the environmental sciences. Despite significant progress in understanding the physical, chemical and biological aspects of the hydrological cycle in Scotland, hydrologists face a number of significant challenges. These include: improved basic process understanding and modelling of catchment functioning; increased understanding of climatic variability and change; the collection of more extensive and well-integrated data sets; improved understanding of the role of hydrology in maintaining good ecological status in managed rivers; and a rapidly evolving policy agenda both within Scotland and the EU. So far, the response of the scientific community to these challenges has been encouraging. However, it is concluded that in the future, hydrologists need to be increasingly engaged in interdisciplinary research projects and communicate better with environmental planners and various stakeholder groups if the discipline is going to make its full contribution to sustainable water resource management in Scotland.

SUMO-1 modification of human cytomegalovirus IE1/IE72

Human cytomegalovirus (HCMV) immediate-early protein IE1/IE72 is involved in undermining many cellular processes including cell cycle regulation, apoptosis, nuclear architecture, and gene expression. The multifunctional nature of IE72 suggests that posttranslational modifications may modulate its activities. IE72 is a phosphoprotein and has intrinsic kinase activity (S. Pajovic, E. L. Wong, A. R. Black, and J. C. Azizkhan, Mol. Cell. Biol. 17:6459-6464, 1997). We now demonstrate that IE72 is covalently conjugated to the small ubiquitin-like modifier (SUMO-1). SUMO-1 is an 11.5-kDa protein that is conjugated to multiple proteins and has been reported to exhibit multiple effects, including modulation of protein stability, subcellular localization, and gene expression. A covalently modified protein migrating at approximately 92 kDa, which is stabilized by a SUMO-1 hydrolase inhibitor, is revealed by Western blotting with anti-IE72 of lysates from cells infected with HCMV or cells expressing IE72. SUMO modification of IE72 was confirmed by immunoprecipitation with anti-IE72 and anti-SUMO-1 followed by Western blotting with anti-SUMO-1 and anti-IE72, respectively. Lysine 450 is within a sumoylation consensus site (I,V,L)KXE; changing lysine 450 to arginine by point mutation abolishes SUMO-1 modification of IE72. Inhibition of protein phosphatase 1 and 2A, which increases the phosphorylation of IE72, suppresses the formation of SUMO-1-IE72 conjugates. Both wild-type IE72 and IE72(K450R) localize to nuclear PML oncogenic domains and disrupt them. Studies of protein stability, transactivation, and complementation of IE72-deficient HCMV (CR208) have revealed no significant differences between wild-type IE72 and IE72(K450R).

Autologous endometrial coculture in patients with in vitro-fertilization (IVF) failure: correlations of outcome with leukemia inhibiting factor (LIF) production

PROBLEM

To determine if LIF produced by autologous endometrial co-culture (ECC) was associated with outcome in 46 patients with a history of multiple IVF failures.

METHOD OF STUDY

The conditioned media (CM) from ECC cells exposed or non-exposed to human embryos was analyzed for LIF.

RESULTS

Exposure or non-exposure to an embryo did not result in differing levels of LIF in the CM. LIF levels were significantly greater in the CM than in the serum controls (LIF was not found in the serum controls). Embryos grown on ECC demonstrated a significant improvement in number of blastomeres and fragmentation when compared to embryos grown in conventional media without ECC (6.7 +/- 1.3 vs. 5.6 +/- 1.2 blastomeres and 17.6% +/- 9.3 vs. 26.4% +/- 9.8 fragmentation; P < 0.05). When LIF levels were detectable in the CM, the embryos grown in ECC were of improved quality as compared to the embryos grown only in conventional media and demonstrated a non-significant increase in pregnancy rates (60 vs. 48%, P = 0.50).

CONCLUSIONS

We have demonstrated a significant improvement in embryo quality with ECC. The cells in the ECC express LIF. The presence of LIF in the CM was associated with embryonic development and clinical pregnancy.

Polyamine depletion in human melanoma cells leads to G1 arrest associated with induction of p21WAF1/CIP1/SDI1, changes in the expression of p21-regulated genes, and a senescence-like phenotype

The cell cycle regulatory events that interface with polyamine requirements for cell growth have not yet been clearly identified. Here we use specific inhibitors of polyamine biosynthetic enzymes to investigate the effect of polyamine pool depletion on cell cycle regulation. Treatment of MALME-3M cells with either the ornithine decarboxylase inhibitor alpha-difluoromethylornithine or the S-adenosylmethionine decarboxylase inhibitor MDL-73811 lowered specific polyamine pools and slowed cell growth but did not induce cell cycle arrest. By contrast, treatment with the combination of inhibitors halted cell growth and caused a distinct G1 arrest. The latter was associated with marked reduction of all three polyamine pools, a strong increase in p21(WAF1/CIP1/SDI1) (p21), and hypophosphorylation of retinoblastoma protein. All effects were fully prevented by exogenous polyamines. p21 induction preceded p53 stabilization in MALME-3M cells and also occurred in a polyamine-depleted, p53-nonfunctional melanoma cell line, indicating that p21 is induced at least in part through p53-independent mechanisms. Conditional overexpression of p21 in a fibrosarcoma cell line was shown previously to inhibit the expression of multiple proliferation-associated genes and to induce the expression of genes associated with various aspects of cell senescence and organism aging. Polyamine depletion in MALME-3M cells was associated with inhibition of seven of seven tested p21-inhibited genes and with induction of 13 of 14 tested p21-induced genes. p21 expression is also known to induce a senescence-like phenotype, and phenotypic features of senescence were observed in polyamine-depleted MALME-3M cells. Cells increased in size, appeared more granular, and expressed senescence-associated beta-galactosidase. Cells released from the polyamine inhibition lost the ability to form colonies, failed to replicate their DNA, and approximately 25% became bi- or multinucleated. These events parallel the outcome of prolonged p21 induction in fibrosarcoma cells. The results of this study indicate that polyamine pool depletion achieved by specific biosynthetic enzyme inhibitors causes p21-mediated G1 cell cycle arrest followed by p21-mediated changes in gene expression, development of a senescence-like phenotype, and loss of cellular proliferative capacity.

Sp1 and krüppel-like factor family of transcription factors in cell growth regulation and cancer

The Sp/KLF family contains at least twenty identified members which include Sp1-4 and numerous krüppel-like factors. Members of the family bind with varying affinities to sequences designated as 'Sp1 sites' (e.g., GC-boxes, CACCC-boxes, and basic transcription elements). Family members have different transcriptional properties and can modulate each other's activity by a variety of mechanisms. Since cells can express multiple family members, Sp/KLF factors are likely to make up a transcriptional network through which gene expression can be fine-tuned. 'Sp1 site'-dependent transcription can be growth-regulated, and the activity, expression, and/or post-translational modification of multiple family members is altered with cell growth. Furthermore, Sp/KLF factors are involved in many growth-related signal transduction pathways and their overexpression can have positive or negative effects on proliferation. In addition to growth control, Sp/KLF factors have been implicated in apoptosis and angiogenesis; thus, the family is involved in several aspects of tumorigenesis. Consistent with a role in cancer, Sp/KLF factors interact with oncogenes and tumor suppressors, they can be oncogenic themselves, and altered expression of family members has been detected in tumors. Effects of changes in Sp/KLF factors are context-dependent and can appear contradictory. Since these factors act within a network, this diversity of effects may arise from differences in the expression profile of family members in various cells. Thus, it is likely that the properties of the overall network of Sp/KLF factors play a determining role in regulation of cell growth and tumor progression.

Protein kinase C signaling mediates a program of cell cycle withdrawal in the intestinal epithelium

Members of the protein kinase C (PKC) family of signal transduction molecules have been widely implicated in regulation of cell growth and differentiation, although the underlying molecular mechanisms involved remain poorly defined. Using combined in vitro and in vivo intestinal epithelial model systems, we demonstrate that PKC signaling can trigger a coordinated program of molecular events leading to cell cycle withdrawal into G(0). PKC activation in the IEC-18 intestinal crypt cell line resulted in rapid downregulation of D-type cyclins and differential induction of p21(waf1/cip1) and p27(kip1), thus targeting all of the major G(1)/S cyclin-dependent kinase complexes. These events were associated with coordinated alterations in expression and phosphorylation of the pocket proteins p107, pRb, and p130 that drive cells to exit the cell cycle into G(0) as indicated by concomitant downregulation of the DNA licensing factor cdc6. Manipulation of PKC isozyme levels in IEC-18 cells demonstrated that PKCalpha alone can trigger hallmark events of cell cycle withdrawal in intestinal epithelial cells. Notably, analysis of the developmental control of cell cycle regulatory molecules along the crypt-villus axis revealed that PKCalpha activation is appropriately positioned within intestinal crypts to trigger this program of cell cycle exit-specific events in situ. Together, these data point to PKCalpha as a key regulator of cell cycle withdrawal in the intestinal epithelium.

Regulation of E2F: a family of transcription factors involved in proliferation control

Members of the E2F family of transcription factors are key participants in orchestration of the cell cycle, cell growth arrest and apoptosis. Therefore, an understanding of the regulation of E2F activity is essential for an understanding of the control of cellular proliferation. E2F activity is regulated by the retinoblastoma family of tumor suppressors and by multiple other mechanisms. This review will describe our current knowledge of these mechanisms which together constitute a highly complex network by which the cell cycle and cellular proliferation can be controlled.

Growth/cell cycle regulation of Sp1 phosphorylation

Sp1 sites can mediate growth/cell cycle induction of dihydrofolate reductase in late G1 (Jensen, D. E., Black, A. R. Swick, A. G., and Azizkhan, J. C. (1997) J. Cell. Biochem. 67, 24-31). To investigate mechanisms underlying this induction, effects of serum stimulation on regulation of Sp1 were examined. In Balb/c 3T3 cells, serum stimulation did not affect Sp1 synthesis or the relative binding of Sp1 family members to DNA; however, it did result in a rapid, approximately 2-fold increase in Sp1 levels and an approximately 3-fold increase in specific Sp1 phosphorylation in mid-G1. In normal human diploid fibroblasts, serum stimulation also increased Sp1 phosphorylation in mid-G1 but did not affect Sp1 levels. Therefore, Sp1 phosphorylation is regulated in a growth/cell cycle-dependent manner which correlates temporally with induction of dihydrofolate reductase transcription. Further studies revealed a kinase activity specifically associated with Sp1 in a growth-regulated manner. This activity is distinct from purified kinases previously shown to phosphorylate Sp1 in vitro and phosphorylates Sp1 between amino acids 612 and 678 in its C terminus, a region also phosphorylated in mid-G1 in vivo. Therefore, this study indicates that phosphorylation of the C terminus of Sp1 may play a role in the cell cycle regulation of its transcriptional activity.

Identification of a viral kinase that phosphorylates specific E2Fs and pocket proteins

The transcription factor E2F and its regulation by pRB and related pocket proteins are central to cell cycle control in higher eukaryotes. Much of our knowledge of this regulation has come from studies using immediate-early proteins of DNA tumor viruses. Previously, we reported that the 72-kDa immediate-early region 1 gene product of the human cytomegalovirus, IE72, transactivates the dihydrofolate reductase promoter through the E2F site and that it physically interacts with E2F1 (M. J. Margolis, S. Pajovic, E. L. Wong, M. Wade, R. Jupp, J. A. Nelson, and J. C. Azizkhan, J. Virol. 69:7759-7767, 1995). In this study, we further characterized the mechanism by which IE72 modulates E2F-dependent transcription. In vitro phosphorylation reactions using gel-purified bacterially expressed proteins revealed that IE72 is a kinase that autophosphorylates and phosphorylates E2F1, -2, and -3 (but not E2F4 or -5) and the RB-related pocket proteins p130 and p107 (but not pRB). The region of IE72 spanning amino acids 173 to 197 shows a high level of homology to the ATP binding sites in over 500 kinases. The kinase-negative protein IE72deltaATP, from which this region has been deleted, cannot activate E2F-dependent transcription. The kinase activity of IE72 is also required for its ability to reduce the association of E2F4 with p107 and p130. Taken together, these data suggest that the kinase activity of IE72 is required for E2F-dependent transcriptional activation and that this is likely to result from phosphorylation of specific members of the E2F and pocket protein families by IE72.

Distinct roles for Sp1 and E2F sites in the growth/cell cycle regulation of the DHFR promoter

Dihydrofolate reductase activity is required for many biosynthetic pathways including nucleotide synthesis. Its expression is therefore central to cellular growth, and it has become a key target for cancer chemotherapy. Transcription of the dihydrofolate reductase gene is regulated with growth, being expressed maximally in late G1/early S phase following serum stimulation of quiescent cells. This regulation is directed by a promoter which contains binding sites for only the transcription factors Sp1 and E2F. In this study, the role of these promoter elements in growth/cell cycle regulation of dihydrofolate transcription was addressed directly by transient transfection of Balb/c 3T3 cells with mutant promoter-reporter gene constructs. The E2F sites were found to repress transcription in G0 and early G1 but did not contribute to the level of transcription in late G1/S phase. In contrast, Sp1 sites were able to mediate induction of transcription from the dihydrofolate reductase promoter, as well as a heterologous promoter, following serum stimulation of quiescent cells. These findings add dihydrofolate reductase to a growing list of genes at which E2F sites are primarily repressive elements and delineate a role for Sp1 sites in the growth/cell cycle regulation of transcription.

Alternate splicing of the rTS gene product and its overexpression in a 5-fluorouracil-resistant cell line

rTS is a gene with convergent and overlapping transcription relative to thymidylate synthase (TS). Screening of a cDNA library has identified a second rTS RNA (rTSbeta) which is related to a previously described rTS RNA (rTSalpha). rTSbeta differs from rTSalpha by the insertion of an extra 116 nucleotides after base 128 of rTSalpha and an altered 3'-noncoding region such that rTSbeta RNA is not complementary to TS mRNA. rTSalpha and rTSbeta RNAs have open reading frames predicted to encode proteins of 361 and 416 amino acids, respectively. Antibody to rTSbeta protein detects two proteins of the predicted molecular weight in the H630 colon tumor cell line. rTSbeta is overexpressed 40-70-fold in a 5-fluorouracil-resistant H630 subline compared with 7-8-fold for TS and rTSalpha.

Cell cycle-regulated association of E2F1 and Sp1 is related to their functional interaction

Because of the large number of growth-regulated genes containing binding sites for the transcription factors Sp1 and E2F and the reported ability of E2F to mediate cell cycle (growth) regulation, we studied interactions between E2F1 and Sp1. In transient transfection assays using Drosophila melanogaster SL2 cells, transfection with both Sp1 and E2F1 expression vectors resulted in greater than 85-fold activation of transcription from a hamster dihydrofolate reductase reporter construct, whereas cotransfection with either the Sp1 or E2F1 expression vector resulted in 30- or <2-fold activation, respectively. Therefore, these transcription factors act synergistically in activation of dihydrofolate reductase transcription. Transient transfection studies demonstrated that E2F1 could superactivate Sp1-dependent transcription in a promoter containing only Sp1 sites and that Sp1 could superactivate transcription of promoters through E2F sites, further demonstrating that these physically associated in Drosophila cells transfected with Sp1 and E2F1 expression vectors and in human cells, with maximal interaction detected in mid- to late G1. Additionally, E2F1 and Sp1 interact in vitro through specific domains of each protein, and the physical interaction and functional synergism appear to require the same regions. Taken together, these data demonstrate that E2F1 and Sp1 both functionally and physically interact; therefore this interaction, Sp1 and E2F1 may regulate transcription of genes containing binding sites for either or both factors.

Expression of rTS correlates with altered growth regulation of thymidylate synthase

The recently discovered rTS gene is convergent with and overlaps the thymidylate synthase gene in the antisense orientation. K562 B1A and KB 1BT are two methotrexate-resistant cell lines that have amplified genes for dihydrofolate reductase. K562 B1A cells have increased levels of rTS mRNA and protein compared with their parental K562 cells, whereas KB 1BT cells show unaltered rTS expression compared with their parental (KB) cells. Altered levels and loss of growth-regulation of in situ thymidylate synthase activity were found in methotrexate-resistant K562 B1A cells but not in K562, KB, and KB 1BT cells. These data point to a link between rTS and the regulation of TS activity.

rTS gene expression is associated with altered cell sensitivity to thymidylate synthase inhibitors

rTS is a recently discovered gene, phylogenetically conserved and found to be expressed in a wide variety of cell lines. rTS has also been found to be overexpressed in two cell lines resistant to FU and to MTX. The MTX-resistant cell line was found to have a high degree of cross resistance to several TS inhibitors. An apparent paradox to this correlation of rTS overexpression and resistance to TS inhibitors is the observation that expression of transfected rTS alpha results in enhanced sensitivity of cells to the TS inhibitor prodrug TFT and a modest increase in resistance to FUdR. Since immunoprecipitation of TS leads to the co-immunoprecipitation of two proteins within the expected molecular weight range of the two rTS proteins, it may be that both proteins bind to TS in vivo and modify its activity. Preliminary data substantiate this conclusion. It is conceivable that the ratio of the two rTS proteins associated with TS in vivo may differentially alter TS activity depending upon their stoichiometry or possibly posttranslational modification. Thus it may be possible for rTS to confer greater sensitivity to one pyrimidine analog (e.g., TFT) which is a product analog but to increase resistance or have a minor effect on a substrate analog (e.g., FdUMP) by stabilizing different conformations of TS. The structure of the rTS proteins suggests they are expected to have catalytic activity which involves proton abstraction from an alpha-carbon of a carboxyl group. Whether this enzyme activity is functional and related to pyrimidine metabolism awaits further study.

Involvement of rRNA synthesis in the enhanced survival and recovery of protein synthesis seen in thermotolerance

Although acquired thermotolerance has been linked to the induction of heat shock proteins, the molecular mechanism(s) by which cells become resistant to heat is unknown. The present study shows a strong correlation between the survival of cells following heat shock and the rate of recovery of protein, total RNA, and rRNA synthesis. Increasing exposure of CHO cells to 45 degrees C was found to decrease survival and cause a lengthening delay in these synthetic processes. The same reciprocal correlation was seen in thermotolerant cells. As thermotolerance develops, more cells survive a heat challenge and the delay in synthesis decreases. These data argue that enhanced recovery of protein and RNA synthesis is one factor which plays a key role in thermotolerance. The involvement of rRNA synthesis was further investigated by using actinomycin D at 0.1 microgram m1(-1), a concentration at which rRNA synthesis is selectively inhibited. When the drug was present during the recovery from a challenge heat treatment, the survival of thermotolerant cells was approximately 3-fold lower than expected from the mild toxicity of the drug. As this could not be accounted for by an interaction of the drug with the response of cells to single heat treatments, it is concluded that the drug inhibits the expression of thermotolerance in cells which would otherwise express a full degree of thermotolerance. The time and concentration dependence of this effect indicates that the drug acts though inhibition of rRNA synthesis. Therefore, enhanced recovery of RNA synthesis, presumably rRNA synthesis, is identified as one of the mechanisms responsible for enhanced survival of thermotolerant cells following heat shock.

Solubilization and physical characterization of acceptors for dendrotoxin and beta-bungarotoxin from synaptic membranes of rat brain

Dendrotoxin (DTX), an Mr 7000 convulsant polypeptide from the venom of Dendroaspis angusticeps, or its facilitatory homologues act through blockade of certain voltage-sensitive K+ currents in a variety of neurons. High-affinity acceptors for DTX have been demonstrated in synaptic plasma membranes of rat or chick brain, and a fraction of these avidly bind beta-bungarotoxin (beta-BuTX), a presynaptically active protein whose lighter B polypeptide is homologous to this toxin. Extraction of rat synaptic plasma membranes using Triton X-100 in K+-containing buffer yielded binding sites with KD values of approximately 0.5 and 0.7 nM for 125I-labeled DTX and beta-BuTX, respectively. The content of high-affinity sites obtained for beta-BuTX, including the contribution of a lower affinity component, approximates to the Bmax (approximately 1.3 pmol/mg of protein) obtained for the apparent single set of DTX acceptors. On solubilization, the pharmacological specificity of the acceptor for neurotoxic DTX congeners was retained. 125I-beta-BuTX binding (2.1 nM) was blocked efficaciously by DTX (IC50 = 1.6 nM) while the binding of 2.1 nM 125I-DTX was inhibited completely by beta-BuTX (IC50 = 25 nM); the lower potency of the latter could relate to the noncompetitive nature of the mutual competition and to the presence of high- and low-affinity sites for beta-BuTX. On gel filtration, or sedimentation analysis in H2O/sucrose and 2H2O/sucrose gradients, one peak of DTX binding activity was observed, and this was inhibitable by beta-BuTX. From the hydrodynamic properties of the acceptor/detergent/lipid complex (s20,w = 13.2 S; Stokes radius = 8.6 nm), a molecular weight of 405,000-465,000 was estimated.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of neuronal acceptors for dendrotoxin in its convulsive action in rat brain

Dendrotoxin, a snake-venom polypeptide, is a potent convulsant that facilitates transmitter release apparently by inhibition of voltage-sensitive K+ channels responsible for A-currents. A biologically active 125I-iodinated derivative of this toxin was prepared and used to characterize kinetically homogeneous non-interacting high-affinity acceptors in synaptic membranes from rat cerebral cortex and hippocampus. Binding of radiolabelled toxin from Dendroaspis angusticeps to its membrane acceptor protein was inhibitable by homologous polypeptides from other mamba snakes; most importantly, their rank order of potency was identical with that for their central neurotoxicities in rats, furnishing evidence for involvement of this binding component in the convulsive symptoms observed. Beta-Bungarotoxin, a presynaptically acting neurotoxin whose action on neurotransmitter release at the neuromuscular junction and effects on brain synaptosomes are antagonized by dendrotoxin, was only able to inhibit the binding of the 125I-labelled toxin with low efficacy, although dendrotoxin apparently interacts avidly with the acceptor sites for beta-bungarotoxin. This weak interaction of beta-bungarotoxin with the acceptor was not attributable to its phospholipolytic action. Other neurotoxins and ion-channel antagonists failed to affect the binding of dendrotoxin. The findings presented here, together with recent electrophysiological data, favour the interpretation that dendrotoxin binds to a membrane protein comprising, or closely associated with, this one group of voltage-dependent K+ channels.

Two acceptor sub-types for dendrotoxin in chick synaptic membranes distinguishable by beta-bungarotoxin

Using chick synaptic membranes, proteinaceous acceptors were characterized for dendrotoxin, a polypeptide from Dendroaspis angusticeps with convulsant activity due to its facilitation of transmitter release, resulting from inhibition of A-current K+ channels in brain. Both equilibrium and kinetic measurements of radioiodinated toxin binding showed that two populations of membraneous acceptors were discernible with different affinities (Kd approximately 0.5 nM and 15 nM; Bmax approximately 90 and 400 fmol/mg protein). Only the high-affinity component interacted avidly with beta-bungarotoxin, an inhibitory presynaptic neurotoxin whose lighter chain is homologous to dendrotoxin. Facilitatory homologues of dendrotoxin from Dendroaspis species antagonised its binding to both acceptor sub-types in proportion to their central neurotoxicities, whereas various other toxins (crotoxin, apamin), trypsin inhibitors and lectins proved ineffective. Cross-linking of toxin specifically bound to its membrane acceptors, using bis-imido esters followed by electrophoretic analysis in the presence of sodium dodecyl sulphate, revealed a polypeptide with Mr of 75,000 together with lesser amounts of a 69,000-Mr component. Notably, the covalent labelling of each of these bands was inhibited partially by low concentrations of beta-bungarotoxin, indicating that they are derived from both acceptor species. The demonstrated existence of an acceptor form shared by dendrotoxin and beta-bungarotoxin, together with another sub-type selective for dendrotoxin, is discussed in relation to the known pharmacological interactions of these toxins which exert opposite effects on transmitter release.