Antagonism of Canonical Wnt/β-Catenin Signaling: Taking RAS Blockade to the Next Level?

Blockade of the renin-angiotensin system (RAS) is a universally accepted part of antihypertensive therapy in renal diseases and of therapies aiming to retard the progression of CKD. While this approach is undoubtedly effective, a considerable number of patients are still not being adequately treated and progress toward ESRD. One of the reasons for this progression are the many counter-regulatory mechanisms that come into play when single components of the RAS are blocked (such as upregulation of renin or so-called aldosterone breakthrough). In addition, the intrarenal RAS may still be activated when the systemic RAS is fully blocked; for example, renal tubular angiotensin II concentrations markedly exceed systemic concentrations.¹ Thus, more effective RAS blockade has long been investigated as a means of providing better renal protection in CKD.

In the present issue of JASN, Lili Zhou and colleagues address this issue.² They first performed a bioinformatics screen of the five RAS genes (angiotensinogen, renin, angiotensin-converting enzyme, and the two angiotensin II receptors), all of which were simultaneously upregulated in experimental proteinuric renal disease in the tubules. The screen yielded common promotor sequences in all five genes that can serve as binding sites for T cell factor/lymphoid enhancer factor (TCF/LEF). This observation laid the basis for investigating a potential link between Wnt/β-catenin and the RAS.

A few basics on the Wnt/β-catenin pathway: Wnt proteins are a family of secreted glycoproteins (about 20 exist in humans) that bind Frizzled receptors and induce three signaling pathways: the canonical pathway, the noncanonical planar cell polarity pathway, and the noncanonical Wnt/calcium pathway. Of these, it is the canonical Wnt pathway that causes the cytoplasmic accumulation of β-catenin by inhibiting β-catenin’s “destruction” complex, in particular glycogen synthase kinase-3β. Inhibition of glycogen synthase kinase-3β results in the accumulation of nonphosphorylated β-catenin in the cytoplasm, which is not easily degraded and translocates into the nucleus. Here it assembles a protein complex together with the transcriptional coactivator cAMP response element-binding protein (CREB)–binding protein to then activate transcription factors belonging to the TCF/LEF family. β-Catenin also performs a radically different cellular function, namely cell-cell adhesion in adherens junctions, and thus is an example of so-called moonlighting. Through this second role, β-catenin is involved in maintaining epithelial cell barriers, including the cellular contact inhibition necessary to maintain a single cell epithelial layer.

The next logical step in the study of Zhou et al. was to block the β-catenin pathway with a small molecule (ICG-001) that disrupts its binding to CREB-binding protein.² ICG-001, described 10 years ago, is one of many small-molecule inhibitors of β-catenin/TCF-mediated transcription, and it downregulates the expression of a subset of β-catenin/TCF-responsive genes.³ This effectively abolished the induction of all components of the activated RAS in the authors’ murine model of renal disease (doxorubicin-induced nephropathy).² Of high clinical relevance, both transient and late treatment with ICG-001 restored podocyte function and repressed proteinuria, renal inflammation, and fibrosis. The latter is consistent with the findings of many studies that documented potent antifibrotic effects of canonical Wnt signaling blockade in the kidney,⁴ lung,⁵ skin,⁶ and other organs. It also provides an exciting link between the RAS, TGF-β, and the Wnt/β-catenin pathway. In this scenario, RAS activation results in TGF-β activation, which is well documented in the kidney.⁷ Recent data also demonstrate that TGF-β stimulates canonical Wnt signaling by decreasing the expression of the Wnt antagonist Dickkopf-1, highlighting how both pathways interact to mediate fibrotic diseases.⁶ Together with TGF-β1, β-catenin also contributes to the epithelial cell phenotype switch known as epithelial-to-mesenchymal transition, which was previously considered central in renal fibrosis but has become controversial more recently.⁸ Independent of this latter issue, an amplification loop was thus established, wherein activation of RAS, TGF-β, and Wnt/β-catenin all feed into fibrosis and chronic tissue damage. Another very important stimulator of β-catenin signaling is Klotho deficiency,⁹ which typically characterizes CKD and is believed to be central in many CKD-associated complications. Interfering at any level in the RAS, TGF-β, and Wnt/β-catenin loop may therefore have broad consequences. With respect to RAS inhibition, indeed even at low levels that did not affect BP, an angiotensin II receptor blocker, telmisartan, had beneficial effects on renal matrix build-up; this was associated with downregulation of the Wnt/β-catenin pathway in a rat model of progressive GN.¹⁰

So, is the case solved and all we need to do is change from RAS blockers to Wnt/β-catenin blockers to actually prevent the progression of CKD? As is so often the case, the answer is “maybe.” As outlined above, ample evidence in experimental renal models now suggests that Wnt/β-catenin antagonism is beneficial in progressive renal disease models, in particular renal fibrosis. But this approach has several potential pitfalls. First, mediators of renal fibrosis often exert a bifunctional role (i.e., they promote chronic damage but at the same time they are essential in renal epithelial repair). Thus, renal tubular and lung epithelial repair is delayed following acute injury if β-catenin is blocked,^11,12 and, conversely, overexpression of Wnt-4 and β-catenin promoted the proliferation of renal tubular cells.¹³ Thus, it is unknown what the consequence of chronic β-catenin antagonism would be in CKD if AKI is superimposed. Obviously, this is not infrequent in clinical practice. Second, if blockade of Wnt/β-catenin indeed blocks almost all components of the RAS, this approach would be even more potent than dual RAS blockade, which is now discouraged in CKD given the high incidence of renal adverse effects, in particular AKI. Third, there is at present little to no information on the effects of Wnt/β-catenin inhibition on BP in CKD. Unfortunately, Zhou et al. did not report BP, either early or late after initiation of ICG-001; nor did a prior study of the same group in which ICG-001 reduced renal fibrosis in mice with ureteral obstruction.⁴ In particular, given the clinical problems of dual RAS blockade, such data will be essential in the future. Fourth, the potential for extrarenal adverse effects of Wnt/β-catenin antagonism is high, given the plethora of Wnt actions. For example, some evidence suggests that loss of Wnt signaling contributes to the progression of Alzheimer disease.¹⁴ Another well known target situation, in which stimulation rather than antagonism of Wnt signaling is effective, is the treatment of osteoporosis with antibodies to sclerostin.¹⁵ Finally, whereas many types of cancer depend on Wnt/β-catenin signaling for persistent growth, other high-turnover tissues, such as the gastric epithelium and hair follicles, unfortunately also rely on it.^16,17

At present no approved compounds exist for the specific antagonism of Wnt/β-catenin signaling. However, in oncology several phase I and II studies with specific compounds have been initiated, using, for example OMP-18R5, a monoclonal antibody targeting the Frizzled receptors to block association with Wnt ligands (NCT01345201); OMP-54F28, a fusion protein that sequesters soluble Wnt ligands (NCT01608867); PRI-724, a small-molecule inhibitor of the interaction between β-catenin and CBP (NCT01606579, NCT01302405); and LGK974, a drug that targets Porcupine, a Wnt-specific acyltransferase (NCT01351103).

While the study of Zhou et al. clearly offers important new insights and strengthens the rationale of Wnt/β-catenin inhibition in CKD, we should keep a close eye in particular on the adverse effect profile of the oncology trials and continue to do our homework, especially assessing the hemodynamic effects of Wnt/β-catenin inhibition in renal diseases.

Disclosure

None.