Xiong, Xunhao

The mechanisms underlying E‐cadherin clustering and AJ maturation are not fully understood although the E‐cadherin‐mediated formation of adherens junctions (AJs) is essential for the morphogenesis of epithelial cells. Here we report that the association between type Iγ phosphatidylinositol phosphate kinase (PIPKIγ) and Exo70 regulated the transport and clustering of Ecadherin on the plasma membrane and, subsequently, the extension of nascent E‐cadherin adhesions. We also observed that PI4,5P(2) accumulation at E‐cadherin clusters during the assembly of E‐cadherin adhesions. PIPKIγ‐generated PI4,5P(2) is required for recruiting Exo70 to newly formed E‐cadherin junctions and facilitates the assembly and maturation of AJs. Additionally, Rho activity and actin structure were impaired by loss of Exo70 and/or PIPKIγ. These results support a model in which PIPKIγ and PIPKIγ‐generated PI4,5P(2) pools at nascent E‐cadherin contacts cue Exo70 targeting and orients the tethering of exocyst‐associated E‐cadherin. This could be an important mechanism that regulates E‐cadherin clustering and AJ maturation, and actin reorganization, which is essential for the establishment of solid, polarized epithelial structures.

Abstract Macropinocytosis supports the metabolic requirement of RAS‐transformed pancreatic ductal adenocarcinoma cells (PDACs). However, regulators of RAS‐transformation (activation) that lead to macropinocytosis have not been identified. Herein, we report that UBAP2 (ubiquitin‐binding associated protein 2), regulates the activation of KRAS and macropinocytosis in pancreatic cancer. We demonstrate that UBAP2 is highly expressed in both pancreatic cancer cell lines and tumor tissues of PDAC patients. The expression of UBAP2 is associated with poor overall survival in several cancers, including PDAC. Silencing UBAP2 decreases the levels of activated KRAS, and inhibits macropinocytosis, and tumor growth in vivo. Using a UBAP2‐deletion construct, we demonstrate that the UBA‐domain of UBAP2 is critical for the regulation of macropinocytosis and maintaining the levels of activated KRAS. In addition, UBAP2 regulates RAS downstream signaling and helps maintain RAS in the GTP‐bound form. However, the exact mechanism by which UBAP2 regulates KRAS activation is unknown and needs further investigation. Thus, UBAP2 may be exploited as a potential therapeutic target to inhibit macropinocytosis and tumor growth in activated KRAS‐driven cancers.

E-Cadherin–mediated formation of adherens junctions (AJs) is essential for the morphogenesis of epithelial cells. However, the mechanisms underlying E-cadherin clustering and AJ maturation are not fully understood. Here we report that type Iγ phosphatidylinositol-4-phosphate 5-kinase (PIPKIγ) associates with the exocyst via a direct interaction with Exo70, the exocyst subunit that guides the polarized targeting of exocyst to the plasma membrane. By means of this interaction, PIPKIγ mediates the association between E-cadherin and Exo70 and determines the targeting of Exo70 to AJs. Further investigation revealed that Exo70 is necessary for clustering of E-cadherin on the plasma membrane and extension of nascent E-cadherin adhesions, which are critical for the maturation of cohesive AJs. In addition, we observed phosphatidylinositol-4,5-bisphosphate (PI4,5P 2 ) accumulation at E-cadherin clusters during the assembly of E-cadherin adhesions. PIPKIγ-generated PI4,5P 2 is required for recruiting Exo70 to newly formed E-cadherin junctions and facilitates the assembly and maturation of AJs. These results support a model in which PIPKIγ and PIPKIγ-generated PI4,5P 2 pools at nascent E-cadherin contacts cue Exo70 targeting and orient the tethering of exocyst-associated E-cadherin. This could be an important mechanism that regulates E-cadherin clustering and AJ maturation, which is essential for the establishment of solid, polarized epithelial structures.