The capacity for cancer cells to metastasize to distant organs depends on their ability to execute the carefully choreographed processes of cell adhesion and migration. As most human cancers are of epithelial origin (carcinoma), the transcriptional downregulation of adherent/tight junction proteins (e.g., E-cadherin, Claudin and Occludin) with the concomitant gain of adhesive and migratory phenotypes has been extensively studied. Most research and reviews on cell adhesion and migration focus on the actin cytoskeleton and its reorganization. However, metastasizing cancer cells undergo the extensive reorganization of their cytoskeletal system, specifically in originating/nucleation sites of microtubules and their orientation (e.g., from non-centrosomal to centrosomal microtubule organizing centers). The precise mechanisms by which the spatial and temporal reorganization of microtubules are linked functionally with the acquisition of an adhesive and migratory phenotype as epithelial cells reversibly transition into mesenchymal cells during metastasis remains poorly understood. In this Special Issue of “Molecular Mechanisms Underlying Cell Adhesion and Migration”, we highlight cell adhesion and migration from the perspectives of microtubule cytoskeletal reorganization, cell polarity and phosphoinositide signaling.

Cytoplasmic phosphoinositides (PI) are critical regulators of the membrane–cytosol interface that control a myriad of cellular functions despite their low abundance among phospholipids. The metabolic cycle that generates different PI species is crucial to their regulatory role, controlling membrane dynamics, vesicular trafficking, signal transduction, and other key cellular events. The synthesis of phosphatidylinositol (3,4,5)-triphosphate (PI3,4,5P₃) in the cytoplamic PI3K/Akt pathway is central to the life and death of a cell. This review will focus on the emerging evidence that scaffold proteins regulate the PI3K/Akt pathway in distinct membrane structures in response to diverse stimuli, challenging the belief that the plasma membrane is the predominant site for PI3k/Akt signaling. In addition, we will discuss how PIs regulate the recruitment of specific scaffolding complexes to membrane structures to coordinate vesicle formation, fusion, and reformation during autophagy as well as a novel lysosome repair pathway.

Reactive oxygen species (ROS) are generated by aerobic metabolism, and their deleterious effects are buffered by the cellular antioxidant response, which prevents oxidative stress. The nuclear factor erythroid 2-related factor 2 (NRF2) is a master transcriptional regulator of the antioxidant response. Basal levels of NRF2 are kept low by ubiquitin-dependent degradation of NRF2 by E3 ligases, including the Kelch-like ECH-associated protein 1 (KEAP1). Here, we show that the stability and function of NRF2 is regulated by the type I phosphatidylinositol phosphate kinase γ (PIPKIγ), which binds NRF2 and is required to stably couple phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) to NRF2 in response to oxidative stress. Stress also induces the interaction of the small heat shock protein HSP27 and NRF2 and this interaction is enhanced by PtdIns(4,5)P₂. Silencing PIPKIγ or HSP27 destabilizes NRF2, reduces expression of its target gene HO-1, and sensitizes cells to oxidative stress. These data demonstrate an unexpected collaboration between phosphoinositides, which are stably coupled to NRF2, and HSP27, which is recruited to NRF2 by a phosphoinositide-dependent mechanism to regulate NRF2 stability and function. These findings also point to PIPKIγ and HSP27 as drug targets to destabilize NRF2 in cancer.

Star-PAP is a noncanonical poly(A) polymerase that controls gene expression. Star-PAP was previously reported to bind PIPKI⍺ and its product PI(4,5)P₂, which regulate Star-PAP activity and expression of specific genes. Recent studies have revealed a nuclear p53-phosphoinositide signaling pathway in which the phosphatidylinositol (PI) transfer proteins (PITPs) and phosphoinositide kinases/phosphatases bind p53 to sequentially modify p53-linked phosphoinositides and regulate p53 function. Here we demonstrate that multiple phosphoinositides are also coupled to Star-PAP in response to stress. This pathway is initiated by PITP⍺/β binding to Star-PAP, and the Star-PAP-phosphoinositide complexes are sequentially modified by PI4KII⍺, PIPKI⍺, IPMK, and PTEN. The formation of Star-PAP-phosphoinositide complexes enhances the association of the small heat shock proteins HSP27 and ⍺B-crystallin with Star-PAP. Knockdown of the PITPs, PIP kinases, or HSP27 reduces the expression of Star-PAP targets. Our results demonstrate that PITP⍺/β play a key role in the assembly of Star-PAP-phosphoinositide complexes that are sequentially interconverted by PIP kinases/phosphatases and recruit the small heat shock proteins to these complexes to regulate Star-PAP activity in response to stress.

The tumor suppressor p53 maintains genome stability in the setting of cellular stress and is frequently mutated in cancer. The stability of p53 is regulated by its interaction with the oncoprotein MDM2, a ubiquitin E3 ligase. Recently, nuclear phosphoinositides were reported to bind and stabilize p53. Here, we report that genotoxic stress induces the type I phosphatidylinositol phosphate kinase (PIPKIα) and its product phosphatidylinositol 4,5-bisphosphate (PIP₂) to bind and regulate the stability and function of MDM2. Following genotoxic stress, nuclear PIPKIα binds to MDM2 to generate a complex of MDM2 and PIP₂. PIP₂ binding to MDM2 differentially regulates the recruitment of the small heat shock proteins (sHSPs) αB-crystallin (αBC) and HSP27 to the MDM2-PIP₂ complex, acting as an on-off switch that regulates MDM2 stability, ubiquitination activity and interaction with p53. Our results demonstrate an unexpected role for nuclear phosphoinositides conferring specificity to the MDM2-PIP₂-sHSPs association. Notably, the differential engagement of αBC and HSP27 reveals that sHSPs are not merely passive chaperones but play active, selective roles in fine-tuning MDM2 function and the MDM2-p53 nexus. These findings provide a previously unrecognized molecular framework for targeting this pathway in cancer.

Star-PAP is a noncanonical poly(A) polymerase that controls gene expression. Star-PAP was previously reported to bind the phosphatidylinositol 4-phosphate 5-kinase PIPKI⍺ and its product phosphatidylinositol 4,5-bisphosphate, which regulate Star-PAP poly(A) polymerase activity and expression of specific genes. Recent studies have revealed a nuclear PI signaling pathway in which the PI transfer proteins PITP⍺/β, PI kinases and phosphatases bind p53 to sequentially modify protein-linked phosphatidylinositol phosphates and regulate its function. Here we demonstrate that multiple phosphoinositides, including phosphatidylinositol 4-monophosphate and phosphatidylinositol 3,4,5-trisphosphate are also coupled to Star-PAP in response to stress. This is initiated by PITP⍺/β binding to Star-PAP, while the Star-PAP-linked phosphoinositides are modified by PI4KII⍺, PIPKI⍺, IPMK, and PTEN recruited to Star- PAP. The phosphoinositide coupling enhances the association of the small heat shock proteins HSP27/⍺B-crystallin with Star-PAP. Knockdown of the PITPs, kinases, or HSP27 reduce the expression of Star-PAP targets. Our results demonstrate that the PITPs generate Star-PAP-PIPn complexes that are then modified by PI kinases/phosphatases and small heat shock proteins that regulate the linked phosphoinositide phosphorylation and Star-PAP activity in response to stress.