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Proc Natl Acad Sci U S A. Oct 26, 1999; 96(22): 12548–12552.
Cell Biology

Casein kinase Iepsilon in the Wnt pathway: Regulation of β-catenin function


Wnt and its intracellular effector β-catenin regulate developmental and oncogenic processes. Using expression cloning to identify novel components of the Wnt pathway, we isolated casein kinase Iepsilon (CKIepsilon). CKIepsilon mimicked Wnt in inducing a secondary axis in Xenopus, stabilizing β-catenin, and stimulating gene transcription in cells. Inhibition of endogenous CKIepsilon by kinase-defective CKIepsilon or CKIepsilon antisense-oligonucleotides attenuated Wnt signaling. CKIepsilon was in a complex with axin and other downstream components of the Wnt pathway, including Dishevelled. CKIepsilon appears to be a positive regulator of the pathway and a link between upstream signals and the complexes that regulate β-catenin.

Wnt regulates developmental and oncogenic processes through its downstream effector, β-catenin (13). Intracellular protein complexes, including Dishevelled (Dvl/Dsh), glycogen synthase kinase-3β (GSK-3β), axin and adenomatous polyposis coli (APC) protein, regulate cytosolic β-catenin protein levels. However, little is known about how Wnt or other upstream stimuli regulate these complexes. Most components of the Wnt pathway have been found by genetic approaches in Drosophila. Mutations in several molecules in the wingless (Wg, the Drosophila homologue of Wnt) pathway, such as Dvl/Dsh, β-catenin, and lymphoid enhancer factor-1 (Lef-1)/T cell factor (Tcf), caused segment polarity phenotypes in Drosophila similar to the Wg phenotype, suggesting that these molecules are positive regulators of the pathway (4, 5). Genetic studies in Drosophila also revealed that GSK-3β is a negative regulator of this pathway. GSK-3β is in a complex containing other negative regulators, axin and APC protein, and a positive regulator, β-catenin (610). β-catenin is an extensively studied effector in the pathway and has a pivotal role in both developmental processes and oncogenesis (2, 3). Upon Wnt stimulation, β-catenin protein is stabilized and moves to the nucleus where it forms a complex with and activates Lef-1/Tcf transcription factors (11, 12). Mutated forms of β-catenin appear to be involved in cancer and induce Lef-1/Tcf-dependent transcription even in the absence of Wnt stimulation (13, 14). The molecular mechanism by which Wnt regulates β-catenin is not yet fully understood. Here we show that casein kinase I (CKI)epsilon is an important regulator of β-catenin in the Wnt pathway and is a component of these complexes. CKIepsilon mimicked Wnt in inducing a secondary axis in Xenopus, stabilizing β-catenin, and stimulating β-catenin-dependent gene transcription. Inhibition of endogenous CKIepsilon by the kinase-defective form of CKIepsilon (KN-CKIepsilon) or antisense-oligonucleotide attenuated gene transcription stimulated by Wnt. CKIepsilon was found in a complex with axin and other downstream components of the Wnt pathway, including Dvl. We propose that CKIepsilon is a positive regulator of the Wnt pathway and is a possible functional link between upstream signals and the intracellular axin signaling complex that regulates β-catenin.

Materials and Methods


Human CKIδ cDNA was a gift from J. Kusuda (National Institute of Infectious Diseases, Tokyo). Human CKIα cDNA was isolated by PCR. Lysine-38 in mouse CKIepsilon was mutated to phenylalanine to make KN-CKIepsilon as described (15). CKIepsilon, KN-CKIepsilon, and ΔC-CKIepsilon were constructed in pCS2+ vector (16). CKIδ and CKIα were constructed in pcDNA3.1 vector (Invitrogen). Myc-tagged Axin construct has been described (10). Myc-taggd Dv13 was from D. Yan (Chiron).

Library Screening.

E14 mouse embryonic cDNA library [oligo(dT)-primed] was constructed in pCS2+ vector (16). Pools of RNA derived from the library were injected into the four-cell stage of the Xenopus embryos at the ventral side. A total of 6 × 105 independent clones were screened. Each pool for injection contains 25–50 clones.

Xenopus Experiments.

mRNAs were synthesized by using a mMESSAGE mMACHINE kit (Ambion, Austin, TX). The RNA samples (1–5 ng) were injected into the ventral side of the four-cell stage blastomeres. Embryos with secondary axis structure were counted 48–72 h after injection. β-catenin or XWnt-8 RNA was injected as a positive control (17, 18).

Reverse Transcription–PCR (RT-PCR).

mRNA for RT-PCR was prepared from ventral halves of the Xenopus embryos at stage 10–10 1/2. RT-PCRs and primers were as described (19).

Cell Culture, Immunoprecipitation, and Western Blotting.

S2 stable cell lines were generated by transfecting CKIepsilon and sgg under the control of methallothionein promoter with pMK33 vector that contains hygromycin-resistant gene for selection marker. S2 cells were lysed 24 h after induction by CuSO4. Transfection of 293 cells and immunoprecipitation was performed as described (10). Cytosolic fraction of 293 cells were prepared from supernatant by ultracentrifugation (100,000 g × 30 min) after lysis in hypotonic buffer.

Antisense Oligonucleotide Transfection.

Antisense oligonucleotides against human CKIepsilon (CK-ASa; 5′-gcggcagaagttgaggtatgttgag-3′, CK-ASb; 5′-cgtaggtaagagtagtcgggcttgt-3′) or control oligonucleotide (5′-cgccgtcttcaactccatacaactc-3′) (final concentration 100 nM) were transfected into 293 cells by using cationic peptoid reagents (20) followed by transfection with Lef-1, Lef-1 reporter, and Wnt-1 plasmids using Lipofectamine (Life Technologies, Grand Island, NY).

Results and Discussion

To find additional regulators in the Wnt pathway, we used a screen for molecules that could mimic the developmental effects of Wnt. In Xenopus embryos, ectopic expression of Wnt elicits formation of a secondary axis (21). We injected pools of RNA derived from a mouse embryonic cDNA library into the ventral side of Xenopus embryos and searched for a gene that caused secondary axis formation. In this screen, we isolated several clones including β-catenin and Wnt-1 that have been shown previously to induce a secondary axis, validating the efficacy of this approach in discovering genes in the Wnt pathway. We also isolated a full-length cDNA for mouse CKI, which is 98.8% identical to the human CKIepsilon isoform. The CKI gene family consists of seven different genes in mammals, CKIα, β, γ1, γ2, γ3, δ, and epsilon (15, 22). CKIepsilon and δ isoforms, the most closely related, share 98% identity in the kinase domain and are 53% identical in a C-terminal domain that is not present in other CKI isoforms. This C-terminal domain appears to negatively regulate kinase activity (15, 23). CKIα is 74% identical to CKIepsilon in the kinase domain and has no C-terminal extension. We showed that the ventral injection of CKIepsilon RNA induced a secondary axis in Xenopus embryos (Fig. (Fig.11 a and b). CKIδ, like CKIepsilon but not CKIα, induced a secondary axis when injected at the ventral side of the embryos (Fig. (Fig.11b). A point mutant of CKIepsilon that was defective in kinase activity (KN-CKIepsilon) did not induce a secondary axis (Fig. (Fig.11b). These data suggested that axis-inducing activity is specific for the CKIepsilon/δ isoform and depends on its kinase activity. Because the C-terminal domain is unique for CKIepsilon/δ isoforms, we tested whether deletion of this domain altered activity. When the truncated CKIepsilon (ΔC-CKIepsilon) RNA was injected into Xenopus embryos, we did not see any effects on axis formation (Fig. (Fig.11b). Both CKIα and ΔC-CKIepsilon had kinase activity in vitro comparable to or greater than that of wild-type CKIepsilon when they were expressed in mammalian cells (on a per-cell basis assessed by kinase assays of CKIepsilon immunoprecipitates) or in vitro-translated (data not shown) even though they were not effective in inducing a secondary axis. Therefore, CKI kinase activity was not sufficient for mimicking Wnt. The requirement for the C-terminal domain of CKIepsilon suggested that this part of the molecule is involved in linking CKIepsilon to the Wnt pathway. Indeed further studies (see below) confirmed that this domain is important for the interaction of CKIepsilon with a signaling complex.

Figure 1
CKIepsilon/δ induces a secondary axis in Xenopus embryos. (a) Examples of the embryos injected with CKIepsilon or β-galactosidase (β-gal) RNA. (b) Percentage of embryos with duplicated axis injected with β-gal, β-catenin, ...

To demonstrate that CKIepsilon activates Wnt signaling, we coinjected CKIepsilon with axin, which is a known inhibitor of the Wnt pathway that acts by linking β-catenin to GSK-3β (610, 17). Axin inhibited the induction of a secondary axis by CKIepsilon (Fig. (Fig.11c). This finding suggests that the CKIepsilon effect is mediated through β-catenin in a manner analogous to the effects of Wnt. One of the downstream target genes of β-catenin in Xenopus is Siamois (18, 19). Siamois is a homeobox gene induced by Wnt and responsible for its dorsalizing activity. CKIepsilon overexpression at the ventral side of Xenopus embryo also induced Siamois expression detected by RT-PCR (Fig. (Fig.11d and ref. 19). Our observations in the Xenopus experiments that CKIepsilon mimicked Wnt, both in its gene regulation and developmental effects, suggested that CKIepsilon might be a component of the Wnt pathway.

To understand the mechanism by which CKIepsilon activates the Wnt pathway, we examined the effect of CKIepsilon on β-catenin protein level. Wnt-1 stabilizes cytosolic β-catenin protein by suppressing GSK-3β (24, 25). We made Drosophila Schneider S2 cell lines that stably expressed CKIepsilon controlled by a metallothionein promoter. Overexpression of CKIepsilon caused accumulation of endogenous armadillo (arm) protein, the Drosophila homologue of β-catenin (Fig. (Fig.22a). We also showed that β-catenin protein level was increased by transiently overexpressing CKIepsilon in 293 cells (Fig. (Fig.22b). These results suggest that CKIepsilon activates the Wnt pathway by stabilizing β-catenin.

Figure 2
β-catenin stabilization induced by CKIepsilon. (a) Drosophila S2 Schneider cell lysates blotted with armadillo antibody and hemagglutinin (HA) antibody recognized transfected CKIepsilon. Tubulin was a loading control. (b) Cytosolic fraction ...

To further study the role of CKIepsilon in the Wnt pathway, we measured Lef-1 reporter gene activity in mammalian cells. The transcription factor Lef-1/Tcf forms a complex with β-catenin in response to Wnt stimulation (11, 12). When expressed in 293 cells, Wnt-1 stimulated the expression of a Lef-1 reporter gene transcription 4- to 6-fold over vector transfected cells (10). CKIepsilon and CKIδ activated the Lef-1 reporter gene about 10-fold (Fig. (Fig.33a). However CKIα and ΔC-CKIepsilon did not activate the Lef-1 reporter gene (Fig. (Fig.33a), consistent with the Xenopus injection experiments (Fig. (Fig.1).1). Coexpressing axin inhibited the Lef-1 reporter activation induced by CKIepsilon (Fig. (Fig.33b). These data confirm the results from Xenopus experiments, suggesting that CKIepsilon activates the Wnt pathway through an effect on the β-catenin-Lef-1/Tcf complex.

Figure 3
Lef-1 reporter gene activity induced by CKIepsilon. Lef-1 reporter gene assay was performed as described (10). Representative data from several independent experiments are shown. (a) The effects of CKI isoforms on Lef-1 activity. (b) Axin inhibits Lef-1 ...

KN-CKIepsilon inhibited the activation of Lef-1 reporter by Wnt-1 (Fig. (Fig.33c), suggesting the involvement of endogenous CKIepsilon during the Wnt signal. These data suggest that KN-CKIepsilon acts as a dominant negative kinase and blocks the upstream signal coming from Wnt-1. To further assess the physiological importance of CKIepsilon in the Wnt pathway, we used antisense-oligonucleotides to reduce the endogenous CKIepsilon protein level, which resulted in the inhibition of Lef-1 reporter activity induced by Wnt-1 (Fig. (Fig.33d). Taken together the Xenopus, Drosophila, and mammalian cell experiments showed that CKIepsilon activates the Wnt pathway and appears to be a significant positive regulatory component that is required for a full Wnt effect.

Some of the downstream molecules in the Wnt pathway have been shown to form complexes containing negative regulators (GSK-3β, axin, and adenomatous polyposis coli tumor suppressor protein) and a positive regulator (β-catenin) in vivo (610). We examined the possibility that CKIepsilon is also in a complex with these molecules. We found that axin, a negative regulator of the Wnt pathway, bound to CKIepsilon. Endogenous CKIepsilon was coimmunoprecipitated with overexpressed axin (Fig. (Fig.44a). Binding of axin to ΔC-CKIepsilon and ΔC-KN-CKIepsilon was much reduced compared to wild-type CKIepsilon and KN-CKIepsilon (Fig. (Fig.44b). These results suggest that the C-terminal domain of CKIepsilon is important for its interaction with axin, which may be the reason that ΔC-CKIepsilon and CKIα did not activate the Wnt pathway in Xenopus or mammalian cells (Figs. (Figs.11 and and3).3). To further study the complex of CKIepsilon with axin and GSK-3β, we showed that GSK-3β coimmunoprecipitated with CKIepsilon, but much less with ΔC-CKIepsilon, and only in the presence of axin (Fig. (Fig.44c). This finding suggests that CKIepsilon is a positive regulatory molecule in the Wnt pathway and its interaction through its C terminus with the axin-GSK-3β complex is likely to be important for its activity. Furthermore, we also detected endogenous CKIepsilon in the complex with overexpressed Dvl3 (Fig. (Fig.44d), an upstream molecule of the axin-GSK-3β complex.

Figure 4
CKIepsilon forms a complex with the other molecules in the Wnt pathway. (a) Endogenous CKIepsilon coimmunoprecipitated with transfected myc-tagged axin. (b) The C-terminus domain of CKIepsilon is required for binding to axin. Myc-axin and hemagglutinin ...

In this study, we cloned a CKIepsilon gene by using the Xenopus system as an indicator of Wnt-inducing effects. Recently a CKIepsilon gene of Drosophila was identified as the clock gene (26). Homozygous mutation in the Drosophila CKIepsilon gene produces embryonic lethality, which also suggests the involvement of CKIepsilon in an early process that is probably unrelated to its “clock” function. It will be interesting to examine in detail the embryonic phenotype of CKIepsilon-defective flies related to pathway genes.

In summary we report a signaling molecule in the Wnt pathway, CKIepsilon, that induces secondary axis formation in Xenopus, Lef-1-dependent transcription, and stabilization of β-catenin in both fly and mammalian cells. A critical role of CKIepsilon in Wnt signaling is supported by several observations: inhibition of endogenous CKIepsilon by KN-CKIepsilon or by antisense-oligonucleotides blocked Wnt-1 effects; endogenous CKIepsilon is present in a complex with axin, GSK-3β, and Dvl, known components of the Wnt pathway; and the C-terminal domain of CKIepsilon is required for its interaction with the axin complex and for the ability of CKIepsilon to mimic Wnt. It will be important to determine the substrates of CKIepsilon in this pathway and the upstream signals between Wnt receptors and CKIepsilon. It is also possible that stimuli other than Wnt regulate the ability of CKIepsilon to impinge on this pathway.


We thank J. Martin for his role in performing the Xenopus expression cloning and constructing plasmids, M. Wu for Xenopus injection assay, C. Reinhardt and M. Del Rosario for the advice in antisense-oligonucleotides technologies, R. Zuckermann for cationic peptoid reagents, and W. Fantl for reading the manuscript. This work was partially supported by the Howard Hughes Medical Institute and Bristol-Myers Squibb Company (C.S.).


casein kinase I
glycogen synthase kinase-3β
lymphoid enhancer factor-1
T cell factor
reverse transcription–PCR
kinase-defective form of CKIepsilon


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