Caprin-1 influences autophagy-induced tumor growth and immune modulation in pancreatic cancer

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is characterized by rapid progression and poor prognosis. Understanding the genetic mechanisms that affect cancer properties and reprogram tumor immune microenvironment will develop new strategies to maximize the benefits for cancer therapies.

METHODS

Gene signatures and biological processes associated with advanced cancer and unfavorable outcome were profiled using bulk RNA sequencing and spatial transcriptome sequencing, Caprin-1 was identified as an oncogenesis to expedite pancreatic cancer growth by activating autophagy. The mechanism of Caprin-1 inducing autophagy activation was further explored in vitro and in vivo. In addition, higher level of Caprin-1 was found to manipulate immune responses and inflammatory-related pathways. The immune profiles associated with increased levels of Caprin-1 were identified in human PDAC samples. The roles of CD4+T cells, CD8+T cells and tumor associated macrophages (TAMs) on clinical outcomes prediction were investigated.

RESULTS

Caprin-1 was significantly upregulated in advanced PDAC and correlated with poor prognosis. Caprin-1 interacted with both ULK1 and STK38, and manipulated ULK1 phosphorylation which activated autophagy and exerted pro-tumorigenic phenotypes. Additionally, the infiltrated CD4+T cells and tumor associated macrophages (TAMs) were increased in Caprin-1High tissues. The extensive CD4+T cells determined poor clinical outcome in Caprin-1high patients, arguing that highly expressed Caprin-1 may assist cancer cells to escape from immune surveillance.

CONCLUSIONS

Our findings establish causal links between the upregulated expression of Caprin-1 and autophagy activation, which may manipulate immune responses in PDAC development. Our study provides insights into considering Caprin-1 as potential therapeutic target for PDAC treatment.

A novel polypeptide CAPG-171aa encoded by circCAPG plays a critical role in triple-negative breast cancer

BACKGROUND

The treatment of Triple-negative breast cancer (TNBC) has always been challenging due to its heterogeneity and the absence of well-defined molecular targets. The present study aims to elucidate the role of protein-coding circRNAs in the etiology and carcinogenesis of TNBC.

METHODS

CircRNA expression data in TNBC (GEO: GSE113230, GSE101123) were reanalyzed and then circCAPG was selected for further study. To identify the polypeptide-coding function of circCAPG, a series of experiments, such as Mass spectrometry and dual-luciferase reporter assays were conducted. Cell proliferation, apoptosis and metastasis parameters were determined to investigate the cancerous functions CAPG-171aa plays in both TNBC organoids and nude mice. Mechanistically, the relation between CAPG-171aa and STK38 in TNBC was verified by immunoprecipitation analyses and mass spectrometry. The interactions between SLU7 and its binding site on circCAPG were validated by RIP-qPCR experiments.

RESULTS

In both TNBC clinical samples and cell lines, the expression level of circCAPG was identified to be higher compared with normal ones and positively correlated with the overall survival (n = 132) in a 10-year follow-up study, in which the area under the curve of receiver operating characteristic was 0.8723 with 100% specificity and 80% sensitivity. In addition, we found that circCAPG knockdown (KD) significantly inhibited the growth of TNBC organoids. Intriguingly, circCAPG can be translated into a polypeptide named CAPG-171aa which promotes tumor growh by disrupting the binding of serine/threonine kinase 38 (STK38) to SMAD-specific E3 ubiquitin protein ligase 1 (SMURF1) and thereby preventing MEKK2 ubiquitination and proteasomal degradation. Furthermore, we found that SLU7 Homolog- Splicing Factor (SLU7) can regulate the bio-generation of circCAPG through binding to the flanking Alu sequences of circRNA transcripts.

CONCLUSIONS

circCAPG significantly enhances the proliferation and metastasis of TNBC cells by encoding a novel polypeptide CAPG-171aa and afterwards activates MEKK2-MEK1/2-ERK1/2 pathway. Additionally, the formation of circCAPG is found to be mediated by SLU7. The present study provides innovative insight into the role of protein-coding circRNAs CAPG-171aa in TNBC, and its capacity to serve as a promising prognostic biomarker and potential therapeutic target in TNBC.

Prognostic and Immunological Role of STK38 across Cancers: Friend or Foe?

Although STK38 (serine-threonine kinase 38) has been proven to play an important role in cancer initiation and progression based on a series of cell and animal experiments, no systemic assessment of STK38 across human cancers is available. We firstly performed a pan-cancer analysis of STK38 in this study. The expression level of STK38 was significantly different between tumor and normal tissues in 15 types of cancers. Meanwhile, a prognosis analysis showed that a distinct relationship existed between STK38 expression and the clinical prognosis of cancer patients. Furthermore, the expression of STK38 was related to the infiltration of immune cells, such as NK cells, memory CD4+ T cells, mast cells and cancer-associated fibroblasts in a few cancers. There were three immune-associated signaling pathways involved in KEGG analysis of STK38. In general, STK38 shows a significant prognostic value in different cancers and is closely associated with cancer immunity.

STK38 is a PPARγ-interacting protein promoting adipogenesis

Peroxisome proliferator-activated receptor-γ (PPARγ) is the master regulator of adipogenesis, but knowledge about how PPARγ is regulated at the protein level is very limited. We aimed to identify PPARγ-interacting proteins which modulate PPARγ’s protein levels and transactivating activities in human adipocytes. We expressed Flag-tagged PPARγ in human preadipocytes as bait to capture PPARγ-associated proteins, followed by mass spectroscopy and proteomics analysis, which identified serine/threonine kinase 38 (STK38) as a major PPARγ-associated protein. Protein pulldown studies confirmed this protein–protein interaction in transfected cells, and reporter assays demonstrated that STK38 enhanced PPARγ’s transactivating activities without requiring STK38’s kinase activity. In cell-based assays, STK38 increased PPARγ protein stability, extending PPARγ’s half-life from ~1.08 to 1.95 h. Notably, in human preadipocytes, the overexpression of STK38 enhanced adipogenesis, whereas knockdown impaired the process in a PPARγ-dependent manner. Thus, we discovered that STK38 is a novel PPARγ-cofactor promoting adipogenesis, likely through stabilization of PPARγ

Phosphorylated LASS2 inhibits prostate carcinogenesis via negative regulation of Wnt/β-catenin signaling

LASS2 is a novel tumor-suppressor gene and has been characterized as a ceramide synthase, which synthesizes very-long acyl chain ceramides. However, LASS2 function and pathway-related activity in prostate carcinogenesis are still largely unexplored. Here, we firstly report that LASS2 promotes β-catenin degradation through physical interaction with STK38, SCYL2, and ATP6V0C via the ubiquitin-proteasome pathway, phosphorylation of LASS2 is essential for β-catenin degradation, and serine residue 248 of LASS2 is illustrated to be a key phosphorylation site. Furthermore, we find that dephosphorylation of LASS2 at serine residue 248 significantly enhances prostate cancer cell growth and metastasis in vivo, indicating that phosphorylated LASS2 inhibits prostate carcinogenesis through negative regulation of Wnt/β-catenin signaling. Thus, our findings implicate LASS2 as a potential biomarker and therapeutic target of prostate cancer.

The STK38-XPO1 axis, a new actor in physiology and cancer

The Hippo signal transduction pathway is an essential regulator of organ size during developmental growth by controlling multiple cellular processes such as cell proliferation, cell death, differentiation, and stemness. Dysfunctional Hippo signaling pathway leads to dramatic tissue overgrowth. Here, we will briefly introduce the Hippo tumor suppressor pathway before focusing on one of its members and the unexpected twists that followed our quest of its functions in its multifarious actions beside the Hippo pathway: the STK38 kinase. In this review, we will precisely discuss the newly identified role of STK38 on regulating the nuclear export machinery by phosphorylating and activating, the major nuclear export receptor XPO1. Finally, we will phrase STK38's role on regulating the subcellular distribution of crucial cellular regulators such as Beclin1 and YAP1 with its implication in cancer.

The hippo kinase STK38 ensures functionality of XPO1

The proper segregation of basic elements such as the compartmentalization of the genome and the shuttling of macromolecules between the nucleus and the cytoplasm is a crucial mechanism for homeostasis maintenance in eukaryotic cells. XPO1 (Exportin 1) is the major nuclear export receptor and is required for the export of proteins and RNAs out of the nucleus. STK38 (also known as NDR1) is a Hippo pathway serine/threonine kinase with multifarious functions in normal and cancer cells. In this review, we summarize the history of the discovery of the nucleo/cytoplasmic shuttling of proteins and focus on the major actor of nuclear export: XPO1. After describing the molecular events required for XPO1-mediated nuclear export of proteins, we introduce the Hippo pathway STK38 kinase, synthetize its regulation mechanisms as well as its biological functions in both normal and cancer cells, and finally its intersection with XPO1 biology. We discuss the recently identified mechanism of XPO1 activation by phosphorylation of XPO1_S1055 by STK38 and contextualize this finding according to the biological functions previously reported for both XPO1 and STK38, including the second identity of STK38 as an autophagy regulator. Finally, we phrase this newly identified activation mechanism into the general nuclear export machinery and examine the possible outcomes of nuclear export inhibition in cancer treatment.

STK38 kinase acts as XPO1 gatekeeper regulating the nuclear export of autophagy proteins and other cargoes

STK38 (also known as NDR1) is a Hippo pathway serine/threonine protein kinase with multifarious functions in normal and cancer cells. Using a context-dependent proximity-labeling assay, we identify more than 250 partners of STK38 and find that STK38 modulates its partnership depending on the cellular context by increasing its association with cytoplasmic proteins upon nutrient starvation-induced autophagy and with nuclear ones during ECM detachment. We show that STK38 shuttles between the nucleus and the cytoplasm and that its nuclear exit depends on both XPO1 (aka exportin-1, CRM1) and STK38 kinase activity. We further uncover that STK38 modulates XPO1 export activity by phosphorylating XPO1 on serine 1055, thus regulating its own nuclear exit. We expand our model to other cellular contexts by discovering that XPO1 phosphorylation by STK38 regulates also the nuclear exit of Beclin1 and YAP1, key regulator of autophagy and transcriptional effector, respectively. Collectively, our results reveal STK38 as an activator of XPO1, behaving as a gatekeeper of nuclear export. These observations establish a novel mechanism of XPO1-dependent cargo export regulation by phosphorylation of XPO1's C-terminal auto-inhibitory domain.

DIAPH3 promotes the tumorigenesis of lung adenocarcinoma

Aberrant activation of MEKK-MEK-ERK signaling is frequently observed in lung cancer. Several inhibitors, which target this pathway, have shown clinical potential for the lung cancer treatment. Better understanding the regulation of this pathway would help the development of treatment strategies. In this study, we have identified the DIAPH3 as an up-regulated gene in lung adenocarcinoma. DIAPH3 promoted the growth of lung cancer cells both in the liquid culture and in the soft agar, and knockdown DIAPH3 inhibited the tumorigenesis both in the nude mice and in the de novo mouse model. In the molecular mechanism study, DIAPH3 was identified as the binding protein of STK38, impaired the interaction between STK38 and MEKK, and activated ERK signaling. Taken together, this study demonstrated the oncogenic roles of DIAPH3 in the tumorigenesis of lung cancer by interacting with STK38.

Grass carp STK38 regulates IFN I expression by decreasing the phosphorylation level of GSK3β

As a member of NDR protein kinase family and a novel protein kinase of Hippo signal pathway, Serine/threonine kinase 38 (STK38) plays a very significant role in the innate immune. In mammals, STK38 performs its function by combining with GSK3β. Nowadays, there are few reports of STK38 in fish. In order to explain the function of fish STK38 in the innate immunity, we cloned the ORF of grass carp (Ctenopharyngodon idella) STK38 (CiSTK38) and the related kinase GSK3β (CiGSK3β). Phylogenetic trees revealed that CiSTK38 and CiGSK3β evolved closer kinship with sinocyclocheilus grahami STK38 and siniperca chuatsi GSK3β respectively. CiSTK38 and CiGSK3β can respond to the intradermal injection of poly (I:C) in grass carp different tissues and the transfection of poly I:C in CIK cells. Subcellular localization revealed the CiGSK3β were broadly distributed through the cytoplasm, whereas CiSTK38 were observed both in cytoplasm and nucleus. However, when they were co-transferred into cells, the two proteins were found to aggregate in the nucleus. GST-pulldown and co-immunoprecipitation analysis revealed that CiSTK38 can physically interact with CiGSK3β. Phos-tag PAGE illustrated CiSTK38 can decrease the phosphorylation and auto-phosphorylation level of CiGSK3β at Ser9 and at Tyr216. To investigate the functional correlation between CiSTK38 and CiGSK3β, we overexpressed CiSTK38 and CiGSK3β in CIK cells and found that they can up-regulate the expression of IFN I. In short, we demonstrated that CiSTK38 can confer CiGSK3β kinase activity by reducing its phosphorylation level. Result from this study strongly suggested that the anti-viral immune effects elicited by poly (I:C) in part were mediated through activation of CiGSK3β. The findings provided scientific basis for the anti-viral immune mechanism of STK38 and GSK3β in fish.

MOB (Mps one Binder) Proteins in the Hippo Pathway and Cancer

The family of MOBs (monopolar spindle-one-binder proteins) is highly conserved in the eukaryotic kingdom. MOBs represent globular scaffold proteins without any known enzymatic activities. They can act as signal transducers in essential intracellular pathways. MOBs have diverse cancer-associated cellular functions through regulatory interactions with members of the NDR/LATS kinase family. By forming additional complexes with serine/threonine protein kinases of the germinal centre kinase families, other enzymes and scaffolding factors, MOBs appear to be linked to an even broader disease spectrum. Here, we review our current understanding of this emerging protein family, with emphases on post-translational modifications, protein-protein interactions, and cellular processes that are possibly linked to cancer and other diseases. In particular, we summarise the roles of MOBs as core components of the Hippo tissue growth and regeneration pathway.

Measuring the Kinase Activities of the LATS/NDR Protein Kinases

The Hippo tissue growth control and regeneration pathway is a main regulator of the YAP/TAZ effectors. In this regard, the LATS/NDR serine/threonine protein kinases can function as central components of the Hippo core module. More specifically, LATS/NDR-mediated phosphorylation of YAP/TAZ on different residues can regulate the subcellular localization and/or stability of YAP/TAZ. Therefore, the assessment of LATS/NDR activities can serve as readout for the activity status of the Hippo pathway. Here, we describe our preferred methodology regarding the measurement of the activities of LATS/NDR kinases.

Mitochondrial clearance by the STK38 kinase supports oncogenic Ras-induced cell transformation

Oncogenic Ras signalling occurs frequently in many human cancers. However, no effective targeted therapies are currently available to treat patients suffering from Ras-driven tumours. Therefore, it is imperative to identify downstream effectors of Ras signalling that potentially represent promising new therapeutic options. Particularly, considering that autophagy inhibition can impair the survival of Ras-transformed cells in tissue culture and mouse models, an understanding of factors regulating the balance between autophagy and apoptosis in Ras-transformed human cells is needed. Here, we report critical roles of the STK38 protein kinase in oncogenic Ras transformation. STK38 knockdown impaired anoikis resistance, anchorage-independent soft agar growth, and in vivo xenograft growth of Ras-transformed human cells. Mechanistically, STK38 supports Ras-driven transformation through promoting detachment-induced autophagy. Even more importantly, upon cell detachment STK38 is required to sustain the removal of damaged mitochondria by mitophagy, a selective autophagic process, to prevent excessive mitochondrial reactive oxygen species production that can negatively affect cancer cell survival. Significantly, knockdown of PINK1 or Parkin, two positive regulators of mitophagy, also impaired anoikis resistance and anchorage-independent growth of Ras-transformed human cells, while knockdown of USP30, a negative regulator of PINK1/Parkin-mediated mitophagy, restored anchorage-independent growth of STK38-depleted Ras-transformed human cells. Therefore, our findings collectively reveal novel molecular players that determine whether Ras-transformed human cells die or survive upon cell detachment, which potentially could be exploited for the development of novel strategies to target Ras-transformed cells.

HOXD8 exerts a tumor-suppressing role in colorectal cancer as an apoptotic inducer

Homeobox (HOX) genes are conserved transcription factors which determine the anterior-posterior body axis patterning. HOXD8 is a member of HOX genes deregulated in several tumors such as lung carcinoma, neuroblastoma, glioma and colorectal cancer (CRC) in a context-dependent manner. In CRC, HOXD8 is downregulated in cancer tissues and metastatic foci as compared to normal tissues. Whether HOXD8 acts as a tumor suppressor of malignant progression and metastasis is still unclear. Also, the underlying mechanism of its function including the downstream targets is totally unknown. Here, we clarified the lower expression of HOXD8 in clinical colorectal cancer vs. normal colon tissues. Also, we showed that stable expression of HOXD8 in colorectal cancer cells significantly reduced the cell proliferation, anchorage-independent growth and invasion. Further, using The Cancer Genome Atlas (TCGA), we identified the genes associated with HOXD8 in order to demonstrate its function as a suppressor or a promoter of colorectal carcinoma. Among inversely related genes, apoptotic inhibitors like STK38 kinase and MYC were shown to be negatively associated with HOXD8. We demonstrated the ability of HOXD8 to upregulate executioner caspases 6 & 7 and cleaved PARP, thus inducing the apoptotic events in colorectal cancer cells.

HEY1 functions are regulated by its phosphorylation at Ser-68

HEY1-dependent activation of the p53 tumour suppressor pathway can be inhibited through direct phosphorylation of HEY1 at Ser-68 located in the bHLH domain. STK38 and STK38L serine/threonine kinases can phosphorylate HEY1 Ser-68 and could modulate its biological function.

HEY1 (hairy/enhancer-of-split related with YRPW motif 1) is a member of the basic helix–loop–helix-orange (bHLH-O) family of transcription repressors that mediate Notch signalling. HEY1 acts as a positive regulator of the tumour suppressor p53 via still unknown mechanisms. A MALDI-TOF/TOF MS analysis has uncovered a novel HEY1 regulatory phosphorylation event at Ser-68. Strikingly, this single phosphorylation event controls HEY1 stability and function: simulation of HEY1 Ser-68 phosphorylation increases HEY1 protein stability but inhibits its ability to enhance p53 transcriptional activity. Unlike wild-type HEY1, expression of the phosphomimetic mutant HEY1-S68D failed to induce p53-dependent cell cycle arrest and it did not sensitize U2OS cells to p53-activating chemotherapeutic drugs. We have identified two related kinases, STK38 (serine/threonine kinase 38) and STK38L (serine/threonine kinase 38 like), which interact with and phosphorylate HEY1 at Ser-68. HEY1 is phosphorylated at Ser-68 during mitosis and it accumulates in the centrosomes of mitotic cells, suggesting a possible integration of HEY1-dependent signalling in centrosome function. Moreover, HEY1 interacts with a subset of p53-activating ribosomal proteins. Ribosomal stress causes HEY1 relocalization from the nucleoplasm to perinucleolar structures termed nucleolar caps. HEY1 interacts physically with at least one of the ribosomal proteins, RPL11, and both proteins cooperate in the inhibition of MDM2-mediated p53 degradation resulting in a synergistic positive effect on p53 transcriptional activity. HEY1 itself also interacts directly with MDM2 and it is subjected to MDM2-mediated degradation. Simulation of HEY1 Ser-68 phosphorylation prevents its interaction with p53, RPL11 and MDM2 and abolishes HEY1 migration to nucleolar caps upon ribosomal stress. Our findings uncover a novel mechanism for cross-talk between Notch signalling and nucleolar stress.

The Roles of NDR Protein Kinases in Hippo Signalling

The Hippo tumour suppressor pathway has emerged as a critical regulator of tissue growth through controlling cellular processes such as cell proliferation, death, differentiation and stemness. Traditionally, the core cassette of the Hippo pathway includes the MST1/2 protein kinases, the LATS1/2 protein kinases, and the MOB1 scaffold signal transducer, which together regulate the transcriptional co-activator functions of the proto-oncoproteins YAP and TAZ through LATS1/2-mediated phosphorylation of YAP/TAZ. Recent research has identified additional kinases, such as NDR1/2 (also known as STK38/STK38L) and MAP4Ks, which should be considered as novel members of the Hippo core cassette. While these efforts helped to expand our understanding of Hippo core signalling, they also began to provide insights into the complexity and redundancy of the Hippo signalling network. Here, we focus on summarising our current knowledge of the regulation and functions of mammalian NDR kinases, discussing parallels between the NDR pathways in Drosophila and mammals. Initially, we provide a general overview of the cellular functions of NDR kinases in cell cycle progression, centrosome biology, apoptosis, autophagy, DNA damage signalling, immunology and neurobiology. Finally, we put particular emphasis on discussing NDR1/2 as YAP kinases downstream of MST1/2 and MOB1 signalling in Hippo signalling.