Targeting the Wnt/beta-catenin pathway in cancer: Update on effectors and inhibitors

Highlights

• Wnt is a family of proteins involved in stem cell renewal and organogenesis.

• Wnt pathway activation occurs in many cancers and contributes to tumor recurrence.

• Wnt has significant cross talk with Notch and Sonic Hedgehog pathways.

• Update on clinical trials of agents targeting Wnt, Notch, and Sonic Hedgehog.

Abstract

The Wnt/beta-catenin pathway is a family of proteins that is implicated in many vital cellular functions such as stem cell regeneration and organogenesis. Several intra-cellular signal transduction pathways are induced by Wnt, notably the Wnt/beta-catenin dependent pathway or canonical pathway and the non-canonical or beta-catenin-independent pathway; the latter includes the Wnt/Ca2+ and Planar Cell Polarity pathway (PCP). Wnt activation occurs at the intestinal crypt floor, and is critical to optimal maintenance of stem cells. Colorectal cancers show evidence of Wnt signaling pathway activation and this is associated with loss of function of the tumor regulator APC. Wnt activation has been observed in breast, lung, and hematopoietic malignancies and contributes to tumor recurrence. The Wnt pathway cross talks with the Notch and Sonic Hedgehog pathways, which has implications for therapeutic interventions in cancers. There are significant challenges in targeting the Wnt pathway, including finding agents that are efficacious without damaging the system of normal somatic stem cell function in cellular repair and tissue homeostasis. Here, we comprehensively review the Wnt pathway and its interactions with the Notch and Sonic Hedgehog pathways. We present the state of the field in effectors and inhibitors of Wnt signaling, including updates on clinical trials in various cancers with inhibitors of Wnt, Notch, and Sonic Hedgehog.

Introduction

The Wnt family is a group of proteins implicated in many cellular functions: organ formation, stem cell renewal, and cell survival [1]. In humans, the Wnt family consists of cysteine rich glycoproteins that act as ligands for as many as fifteen receptors and co-receptors [2]. Extracellular Wnt can trigger varied intra-cellular signal transduction pathways, like the Wnt/beta-catenin dependent or canonical pathway and the beta-catenin-independent or non-canonical pathway (Figs. 1 and 2b). Examples of the beta-catenin-independent pathway include the Wnt/Ca 2 + pathway as well as the Planar Cell Polarity pathway (PCP) [3]. The beta-catenin-dependent signaling pathway is triggered by the binding of Wnt ligand to the LRP-5/6 receptors (low-density lipoprotein receptor) and Frizzled receptors. This in turn activates Disheveled (DVL), causing recruitment of the complex (Axin, GSK-3 beta, CK1, APC) to the receptor. [4], [5], [6]. The Wnt – Frizzled-Axin -LRP-5/6complex sequesters cytosolic GSK-3 beta rendering it incapable of phosphorylating beta-catenin. There is accumulation of un-phosphorylated beta-catenin in the cytosol which migrates to the nucleus, interacting there with T cell-specific factor (TCF)/lymphoid enhancer-binding factor (LEF) and co-activators, like Pygopus (Pygo) and Bcl-9, to turn on the Wnt target genes such as c-Myc, cyclin D1 and Cdkn1a [6].

Without Wnt, the beta-catenin in the cytosol undergoes phosphorylation by GSK-3 beta and CK1 and subsequent sequestration in the beta-catenin destruction complex, (APC, GSK-3 beta, CK1, Axin). This phosphorylated complex allows for the E3 ubiquitin ligase called beta-TrCP to attach to the beta-catenin at a binding site, that enhances its ubiquitination leading to subsequent proteasomal degradation [7], [8] (Fig. 2a). One of the Non-canonical Wnt pathways includes the PCP or Planar cell polarity pathway. This can be initiated by Wnt interaction with Frizzled receptors, with co-receptors RYK and ROR which control the activity of small GTPases such as RhoA that play a role in regulation of the remodeling of the cytoskeleton [8] (Fig. 2b). Wnt interaction with Frizzled leads to Dvl activation [8]. myosin and the Rho-associated kinase (ROCK) are activated by Rho GTPase, altering the mechanism of actin and cytoskeleton rearrangement. There is in tandem activation of Rac GTPase and activated Rac then stimulates JNK activity (c-Jun N-terminal kinase) [9].

In the Wnt/Ca 2 + pathway, activated by Wnt 5A, the frizzled FZD2 cleaves guanine nucleotide binding protein (G-protein), into protein beta/gamma subunits G-protein alpha-t2 causing Ca 2+ to be released into the cytosol promoting differentiation in the neuronal system. Calcium activates CaMK II and Calmodulin, enhancing phosphorylation of Tcf/Lef (T-cell factor and lymphoid enhancer factor) thus suppressing the canonical Wnt pathway. The mechanisms by which Wnt5a can also interact via the canonical pathway are not completely mapped out though it is speculated that the LRP5 co-receptor is activated along with FZD4 and FZD5 receptors [10], [11], [12], [13], [14].