The phosphoinositide coincidence detector Phafin2 promotes macropinocytosis by coordinating actin organisation at forming macropinosomes.

Uptake of large volumes of extracellular fluid by actin-dependent macropinocytosis has an important role in infection, immunity and cancer development. A key question is how actin assembly and disassembly are coordinated around macropinosomes to allow them to form and subsequently pass through the dense actin network underlying the plasma membrane to move towards the cell center for maturation. Here we show that the PH and FYVE domain protein Phafin2 is recruited transiently to newly-formed macropinosomes by a mechanism that involves coincidence detection of PtdIns3P and PtdIns4P. Phafin2 also interacts with actin via its PH domain, and recruitment of Phafin2 coincides with actin reorganization around nascent macropinosomes. Moreover, forced relocalization of Phafin2 to the plasma membrane causes rearrangement of the subcortical actin cytoskeleton. Depletion of Phafin2 inhibits macropinosome internalization and maturation and prevents KRAS-transformed cancer cells from utilizing extracellular protein as an amino acid source. We conclude that Phafin2 promotes macropinocytosis by controlling timely delamination of actin from nascent macropinosomes for their navigation through the dense subcortical actin network.

Intracellular Transport in Cancer Metabolic Reprogramming.

Tumor progression is a complex process consisting of several steps characterized by alterations in cellular behavior and morphology. These steps include uncontrolled cell division and proliferation, invasiveness and metastatic ability. Throughout these phases, cancer cells encounter a changing environment and a variety of metabolic stress. To meet their needs for energy while they proliferate and survive in their new environment, tumor cells need to continuously fine-tune their metabolism. The connection between intracellular transport and metabolic reprogramming during cancer progression is emerging as a central process of cellular adaptation to these changes. The trafficking of proteolytic enzymes, surface receptors, but also the regulation of downstream pathways, are all central to cancer progression. In this review, we summarize different hallmarks of cancer with a special focus on the role of intracellular trafficking in cell proliferation, epithelial to mesenchymal transition as well as invasion. We will further emphasize how intracellular trafficking contributes to the regulation of energy consumption and metabolism during these steps of cancer progression.

Tumor suppression by control of matrix metalloproteinase recycling.

Secretion of matrix metalloproteinases (MMPs) enables cancer cells to degrade extracellular matrix, thus promoting tumor invasion and metastasis. We have recently found that the endosomal protein WDFY2 serves as a gatekeeper for MMP recycling from endosomes and that deletion of WDFY2, which is frequently lost in metastatic cancers, causes increased matrix degradation and cell invasion.

WDFY2 restrains matrix metalloproteinase secretion and cell invasion by controlling VAMP3-dependent recycling.

Cancer cells secrete matrix metalloproteinases to remodel the extracellular matrix, which enables them to overcome tissue barriers and form metastases. The membrane-bound matrix metalloproteinase MT1-MMP (MMP14) is internalized by endocytosis and recycled in endosomal compartments. It is largely unknown how endosomal sorting and recycling of MT1-MMP are controlled. Here, we show that the endosomal protein WDFY2 controls the recycling of MT1-MMP. WDFY2 localizes to endosomal tubules by binding to membranes enriched in phosphatidylinositol 3-phosphate (PtdIns3P). We identify the v-SNARE VAMP3 as an interaction partner of WDFY2. WDFY2 knockout causes a strong redistribution of VAMP3 into small vesicles near the plasma membrane. This is accompanied by increased, VAMP3-dependent secretion of MT1-MMP, enhanced degradation of extracellular matrix, and increased cell invasion. WDFY2 is frequently lost in metastatic cancers, most predominantly in ovarian and prostate cancer. We propose that WDFY2 acts as a tumor suppressor by serving as a gatekeeper for VAMP3 recycling.

Class III phosphatidylinositol-3-OH kinase controls epithelial integrity through endosomal LKB1 regulation.

The molecular mechanisms underlying the interdependence between intracellular trafficking and epithelial cell polarity are poorly understood. Here we show that inactivation of class III phosphatidylinositol-3-OH kinase (CIII-PI3K), which produces phosphatidylinositol-3-phosphate (PtdIns3P) on endosomes, disrupts epithelial organization. This is caused by dysregulation of endosomally localized Liver Kinase B1 (LKB1, also known as STK11), which shows delocalized and increased activity accompanied by dysplasia-like growth and invasive behaviour of cells provoked by JNK pathway activation. CIII-PI3K inactivation cooperates with RasV12 to promote tumour growth in vivo in an LKB1-dependent manner. Strikingly, co-depletion of LKB1 reverts these phenotypes and restores epithelial integrity. The endosomal, but not autophagic, function of CIII-PI3K controls polarity. We identify the CIII-PI3K effector, WD repeat and FYVE domain-containing 2 (WDFY2), as an LKB1 regulator in Drosophila tissues and human organoids. Thus, we define a CIII-PI3K-regulated endosomal signalling platform from which LKB1 directs epithelial polarity, the dysregulation of which endows LKB1 with tumour-promoting properties.