Fig. 10: Link between oncogenic APC, loss of plasma membrane homeostasis, and colon cancer development.

The plasma membrane serves as a nexus integrating extra- and intracellular Wnt pathway modulators, which by means of their specific organization at the plasma membrane play an essential role in the homeostatic maintenance of the colonic crypt. (1) WT APC tightly regulates the levels of stabilized βcat by facilitating its degradation via the proteasome, (2) turning off upstream Wnt signaling activation. (3) In the presence of Wnt3a ligand, the canonical Wnt signaling pathway is activated. (4) Activation involves increased localization of IL free cholesterol and PI(4,5)P₂, Wnt receptor/effector clustering and translocation of Wnt-associated cytosolic proteins, e.g., Dvl1, at the plasma membrane. (5) Consequently, Wnt condensates drive an increase in the levels of stabilized βcat, which in turn, regulates the (6) transcription of Wnt target genes, (7) including those involved in maintaining membrane cholesterol homeostasis. Together, these processes ensure a normal/healthy colonic epithelium. In contrast, mutation of Apc leads to the expression of truncated APC protein, resulting in the dysregulation of key Wnt signaling-associated cellular steps. Firstly, truncated APC elicits (8) a decrease in βcat degradation, which leads to its (9) aberrant stabilization. (10) The increase in stabilized βcat modulates the transcription of (11) Wnt target genes, including those involved in cholesterol uptake, synthesis, and efflux. (12) The loss of cholesterol homeostasis increases the levels of plasma membrane cholesterol and rigidity, which in turn alters the spatial temporal dynamics of lipid rafts (Lo domains) as well as lipid raft localization and interactions of Wnt signaling-associated receptors and lipid/protein effectors within Wnt condensates in an unstimulated state (no Wnt3a). (13) In the presence of Wnt3a, truncated APC exacerbates oncogenic Wnt signaling by increasing IL cholesterol, PI(4,5)P₂ levels, Lo domain stability, and the number of Wnt receptors/effectors at the plasma membrane. This dysregulates Wnt receptor/effector spatial temporal dynamics and nanoclustering, thereby promoting feedforward activation of aberrant βcat signaling and tumorigenesis. “Off”, absence of Wnt3a ligand; “On”, presence of Wnt3a ligand.