STRAP: a Bridge between Mutant APC and Wnt/ß-Catenin Signaling in Intestinal Cancer

The key role of Wnt/β-catenin pathway signaling in colon cancer initiation and progression has been well established.¹ APC deletions/mutations are involved in aberrant activation of Wnt/β-catenin signaling in about 80% of sporadic colon cancers.² APC inactivation leads to β-catenin stabilization, and consequently, to deregulation of the Wnt pathway through activation of TCF/LEF targets.³ However, attempts to target this pathway therapeutically are unsuccessful due to the remaining significant gaps in our knowledge of how this pathway is regulated. Importantly, crosstalk between Wnt/β-catenin signaling with other pathways complicates the possibility of targeting Wnt/β-catenin signaling in patients with APC mutation. In this issue of Gastroenterology, Vu et al demonstrates that STRAP functions as an upstream regulator of the link between MEK/ERK and Wnt/β-catenin signaling.⁴ This study shows that STRAP binds to MEK1/2 and increases the binding between MEK1/2 and ERK1/2 for phosphorylation of ERK1/2. Activation of MEK/ERK signaling increases LRP6 (a co-receptor of Wnt/β-catenin signaling) phosphorylation that results in nuclear localization and transcriptional activation of β-catenin without affecting its expression.⁴ This study identifies for the first time a regulator of both MEK/ERK and Wnt/β-catenin signaling, two major oncogenic pathways in colon cancer (Figure 1B). This study also contributes to our understanding of the mechanisms by which STRAP regulates Wnt/β-catenin signaling, indicating the functions of STRAP are highly dependent on the background. The Datta laboratory showed previously that STRAP binds with GSK-3β and reduces the phosphorylation, ubiquitination, and degradation of β-catenin through preventing its binding to GSK-3β in the destruction complex containing wild-type APC. Therefore, the effect of STRAP on β-catenin stabilization is lost in colon cancer cell lines with APC truncation/mutation.⁵ In this study, Vu et al determined a novel mechanism through which STRAP promotes Wnt signaling in the context of APC mutation.

Figure 1. Hypothetical models for functions of STRAP in embryonic development and intestinal tumorigenesis.

(A) STRAP dynamically regulates neuronal exon splicing during embryonic differentiation. Genetic deletion of Strap in mouse leads to defective alternate splicing which results in impeded differentiation of embryonic stem cells (ESC) and early embryonic lethality.¹⁰ Representative litters of E7.5–10.5 embryos from intercrosses of C57BL/6 Strap^+/− mice are shown. Compared to wild-type littermate, Strap^−/− embryo shows size and morphological defects with delayed development. For E7.5–8.5 and E9.5 Strap^−/− embryos, scale bar equals 300 μm. For E9.5 WT and E10.5 embryos, scale bar equals 1 mm. ec, ectoplacental cone; cd, chorionic dome; fn, frontonasal region; h, heart; pnf, posterior neural folds; fb, front brain; fl, front limb; hl, hind limb; t, tail. (B) STRAP promotes CRC stem-like cells (CSC) self-renewal and proliferation through regulating the Wnt/β-catenin and Notch pathways. STRAP functions as a positive modulator of NOTCH signaling by effectively repressing the formation of PRC2 complex in CSCs. STRAP promotes Wnt/β-catenin signaling by activating MEK/ERK pathway. Activation of MEK/ERK signaling promotes the nuclear translocation of β-catenin by inducing LRP6 phosphorylation. Together with transcription factors TCF/LEF, nuclear β-catenin induces the expression of the target genes including Cyclin D1, c-Myc, Lgr5, and Survivin, leading to cell proliferation and maintenance of CSCs. Mutant APC and activated Wnt/ß-catenin signaling upregulate STRAP expression. TGF-β-mediated growth inhibition and apoptosis is inhibited by STRAP by preventing phosphorylation and activation of Smad2/3. Altogether, STRAP upregulation in colon cancer significantly promotes intestinal tumor initiation and progression by regulating different signaling pathways.

The current publication in this edition of Gastroenterology also emphasizes the role of STRAP in the regulation of stemness. Previously the Datta laboratory showed that STRAP maintains the stem-like properties of colon cancer cells by activating Notch pathway.⁶ However, this is the first study determining the role of STRAP in regulation of intestinal stem cells (ISC) in an in vivo setting. Conditional deletion of Strap in the intestines of Apc^Min/+ mice resulted in decreased formation of intestinal adenomas and adenocarcinomas and downregulation of the intestinal stem-like gene signature. As previous studies suggested that ISCs represent the cells of origin for colon cancer,⁷ the reduced ISC signature observed in Strap-deficient adenomas contribute to the suppression of stemness and maintenance of differentiation in ISCs leading to reduced intestinal tumorigenesis. Importantly, analysis of human datasets showed that STRAP-related signaling pathways and gene signatures are conserved between humans and mice, emphasizing the clinical correlation of data collected from Strap conditional knockout mouse model. The results from the current study explain why upregulation of STRAP is associated with worse survival following adjuvant therapy and why patients with low STRAP expression benefit from the treatment.⁸ Vu et al also identifies STRAP as a novel target of mutant APC and Wnt/β-catenin signaling pathway, partially explaining the upregulation of STRAP in colon adenomas and adenocarcinomas. This is in agreement with the fact that inactivation of Apc in human and mouse intestinal tumors increase STRAP expression. The upregulation of STRAP in colon cancer correlates with major Wnt targets and regulators of stemness such as c-Myc, Lgr5, Survivin, Cyclin D1, etc. Previous studies established c-Myc as the mediator of early stages of neoplasia following APC loss as c-Myc deletion rescues all the phenotypes following APC loss and thus nearly blocks tumor formation.⁹ Therefore, it is possible that STRAP-mediated regulation of c-Myc and other Wnt/β-catenin targets is involved in APC mutation-induced tumor initiation. Although previous results suggest that c-Myc is a target for the treatment of cancers with APC mutations, given the difficulty in targeting a transcription factor such as c-Myc, it may be more effective to inhibit upstream regulators of c-Myc and other Wnt targets. In this respect, it is necessary to identify inhibitors that block the functions of STRAP and abolish its effects on the expression of c-Myc and other Wnt/β-catenin targets. Although deletion of Strap suppresses tumor initiation and progression following APC mutation, it does not show any toxic effects on the normal intestinal tissues. Altogether, these findings suggest that STRAP is a potential therapeutic target for the treatment of colon cancer, especially for cancers with APC mutation.

Importantly, upregulation of STRAP maintains the intestinal stemness in the background of APC mutation while it functions as a regulator of lineage differentiation in embryos (Figure 1). A recent study from Datta laboratory demonstrated STRAP as a putative spliceosome associated factor in early stages of embryonic development.¹⁰ STRAP preferably targets transcripts for nervous system development and regulates alternative splicing through preferred binding positions. Loss of Strap leads to impeded lineage differentiation in embryos, delayed neural tube closure, and altered exon skipping (Figure 1A). In cancer cells, STRAP has been shown to be involved in different signaling pathways, including TGF-β, NOTCH, MEK/ERK and Wnt/β-catenin signaling pathways (Figure 1B). STRAP, a member of the family of WD-40 repeat proteins, functions through interacting with other proteins (like chaperone) and inducing the formation or dissociation of protein complexes.¹¹ STRAP was originally identified as an inhibitor of TGF-β signaling as STRAP interacts with Smad7 and synergizes with it in suppression of the canonical TGF-β/Smad signaling.¹² Datta laboratory also showed that STRAP epigenetically promotes NOTCH signaling pathway by antagonizing the formation of the chromatin modifier PRC2. Mechanistically, STRAP competitively disrupts association of the PRC2 subunits EZH2 and SUZ12, thereby inhibiting PRC2 assembly. STRAP knockdown results in PRC2-mediated H3K27 trimethylation in the promoter regions of inactive transcripts of NOTCH genes, leading to suppression of NOTCH signaling.⁶ Finally, the current paper demonstrates that STRAP binds to MEK1/2 and increases the binding between MEK1/2 and ERK1/2 for phosphorylation of ERK1/2. Activation of MEK/ERK signaling by STRAP is required for the effect of STRAP on the induction of Wnt/β-catenin signaling.⁴ It remains to be determined how different oncogenic signaling regulated by STRAP are integrated together in APC-induced colon cancer initiation and progression. It is also possible that these oncogenic pathways are activated by STRAP in different stages of colon cancer progression depending on the genetic mutations. Taken together, with previous studies, this work adds to the diversity of targets functionally interacting with STRAP, rationalizing the involvements of STRAP in a variety of signaling pathways and biological processes.