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BACKGROUND: ACAP1 plays a key role in endocytic recycling, which is essential for the normal function of lymphocytes. However, the expression and function of ACAP1 in lymphocytes have rarely been studied.
METHODS: Large-scale genomic data, including multiple bulk RNA-sequencing datasets, single-cell sequencing datasets, and immunotherapy cohorts, were exploited to comprehensively characterize ACAP1 expression, regulation, and function. Gene set enrichment analysis (GSEA) was used to uncover the pathways associated with ACAP1 expression. Eight algorithms, including TIMER, CIBERSORT, CIBERSORT-ABS, QUANTISEQ, xCELL, MCPCOUNTER, EPIC, and TIDE, were applied to estimate the infiltrating level of immune cells. Western blotting, qPCR, and ChIP-PCR were used to validate the findings from bioinformatic analyses. A T-cell co-culture killing assay was used to investigate the function of ACAP1 in lymphocytes.
RESULTS: ACAP1 was highly expressed in immune-related tissues and cells and minimally in other tissues. Moreover, single-cell sequencing analysis in tumor samples revealed that ACAP1 is expressed primarily in tumor-infiltrating lymphocytes (TILs), including T, B, and NK cells. ACAP1 expression is negatively regulated by promoter DNA methylation, with its promoter hypo-methylated in immune cells but hyper-methylated in other cells. Furthermore, SPI1 binds to the ACAP1 promoter and positively regulates its expression in immune cells. ACAP1 levels positively correlate with the infiltrating levels of TILs, especially CD8+ T cells, across a broad range of solid cancer types. ACAP1 deficiency is associated with poor prognosis and immunotherapeutic response in multiple cancer types treated with checkpoint blockade therapy (ICT). Functionally, the depletion of ACAP1 by RNA interference significantly impairs the T cell-mediated killing of tumor cells.
CONCLUSIONS: Our study demonstrates that ACAP1 is essential for the normal function of TILs, and its deficiency indicates an immunologically \"cold\" status of tumors that are resistant to ICT.

ADP-ribosylation factor (Arf)-GTPase-activating protein (GAP) with coiled-coil, ankyrin repeat and PH domains 1 (ACAP1) has been reported to serve as an adaptor for clathrin coat complex playing a role in endocytic recycling and cellular migration. The potential role of ACAP1 in lung adenocarcinoma (LUAD) has not been yet completely defined. We performed the comprehensive analyses, including gene expression, survival analysis, genetic alteration, function enrichment, and immune characteristics. ACAP1 was remarkably downregulated in tumor tissues, and linked with the clinicopathologic features in LUAD patients. Prognostic analysis demonstrated that low ACAP1 expression was correlated with unsatisfactory overall survival (OS) and disease specific survival (DSS) in LUAD patients. Moreover, ACAP1 could be determined as a prognostic biomarker according to Cox proportional hazard model and nomogram model. We also confirmed that ACAP1 was downregulated in two LUAD cell lines, comparing to normal lung cell. Overexpression of ACAP1 caused a profound attenuation in cell proliferation, migration, invasion, and promoted cell apoptosis. Additionally, functional enrichment analyses confirmed that ACAP1 was highly correlated with T cell activation and immune response. Then, we further conducted immune landscape analyses, including single cell RNA sequencing, immune cells infiltration, and immune checkpoints. ACAP1 expression was positively associated with the infiltrating level of immune cells in TME and the expression of immune checkpoint molecules. This study first comprehensively analyzed molecular expression, clinical implication, and immune landscape features of ACAP1 in LUAD, suggesting that ACAP1 was predictive of prognosis and could serve as a potential biomarker predicting immunotherapy response for LUAD patients.

Remodeling of host cellular membrane transport pathways is a common pathogenic trait of many intracellular microbes that is essential to their intravacuolar life cycle and proliferation. The bacterium Brucella abortus generates a host endoplasmic reticulum-derived vacuole (rBCV) that supports its intracellular growth, via VirB Type IV secretion system-mediated delivery of effector proteins, whose functions and mode of action are mostly unknown. Here, we show that the effector BspF specifically promotes Brucella replication within rBCVs by interfering with vesicular transport between the trans-Golgi network (TGN) and recycling endocytic compartment. BspF targeted the recycling endosome, inhibited retrograde traffic to the TGN, and interacted with the Arf6 GTPase-activating Protein (GAP) ACAP1 to dysregulate Arf6-/Rab8a-dependent transport within the recycling endosome, which resulted in accretion of TGN-associated vesicles by rBCVs and enhanced bacterial growth. Altogether, these findings provide mechanistic insight into bacterial modulation of membrane transport used to promote their own proliferation within intracellular vacuoles.

ADP-ribosylation factor (Arf) GTPase-activating protein (GAP) with coiled-coil, ankyrin repeat and PH domains 1 (ACAP1) is an Arf6 GAP that regulates membrane trafficking and is critical for the migratory potential of cells. However, the roles of ACAP1 have not been fully explored and its association with clinicopathological features in ovarian cancer is still unknown. In this study, we systematically analyzed multiple databases, including TISIDB, Tumor Immune Estimation Resource (TIMER2.0), Gene Expression Omnibus (GEO), CORTECON, Kaplan-Meier Plotter and LinkedOmics platforms to reveal the clinical significance and function of ACAP1 in ovarian cancer. We found that the expression of ACAP1 was upregulated in ovarian cancer and high ACAP1 expression predicted poor prognosis. Our data demonstrated that the expression and methylation status of ACAP1 were strongly correlated with immune infiltration levels, immunomodulators, and chemokines. Gene set enrichment analysis (GSEA) analysis also showed that the mechanism of ACAP1 in regulating ovarian cancer was related to a variety of immune-related pathways. In addition, we also revealed that the expression of ACAP1 was altered during cell differentiation and associated with cancer cell stemness markers. Our study highlights the clinical significance of ACAP1 in ovarian cancer and provides insight into the novel function of ACAP1 in regulation of tumor immune microenvironment and cancer cell stemness.

A key function of coat proteins is the sorting of protein cargoes into intracellular transport pathways. For many years, however, it has been unclear whether this role of coat proteins would apply to pathways of endocytic recycling. This issue has been clarified in recent years through the identification of multiple coat complexes acting in the recycling pathways. Leading this charge have been studies on a coat complex defined by ACAP1 (adenosine diphosphate ribosylation factor GTPase-activating proteins with Coiled-coil, Ankryin repeat and PH domains 1), which acts in the sorting of cargoes at the recycling endosome for their return to the plasma membrane. This chapter describes the methods used to characterize this role of ACAP1.

Arf6 and the Arf6 GTPase-activating protein (GAP) ACAP1 are established regulators of integrin traffic important to cell adhesion and migration. However, the function of Arf6 with ACAP1 cannot explain the range of Arf6 effects on integrin-based structures. We propose that Arf6 has different functions determined, in part, by the associated Arf GAP. We tested this idea by comparing the Arf6 GAPs ARAP2 and ACAP1. We found that ARAP2 and ACAP1 had opposing effects on apparent integrin β1 internalization. ARAP2 knockdown slowed, whereas ACAP1 knockdown accelerated, integrin β1 internalization. Integrin β1 association with adaptor protein containing a pleckstrin homology (PH) domain, phosphotyrosine-binding (PTB) domain, and leucine zipper motif (APPL)-positive endosomes and EEA1-positive endosomes was affected by ARAP2 knockdown and depended on ARAP2 GAP activity. ARAP2 formed a complex with APPL1 and colocalized with Arf6 and APPL in a compartment distinct from the Arf6/ACAP1 tubular recycling endosome. In addition, although ACAP1 and ARAP2 each colocalized with Arf6, they did not colocalize with each other and had opposing effects on focal adhesions (FAs). ARAP2 overexpression promoted large FAs, but ACAP1 overexpression reduced FAs. Taken together, the data support a model in which Arf6 has at least two sites of opposing action defined by distinct Arf6 GAPs.