Identification of annexin VI as a Ca2+-sensitive CRHSP-28-binding protein in pancreatic acinar cells

Plasma membrane integrity: implications for health and disease

Plasma membrane integrity is essential for cellular homeostasis. In vivo, cells experience plasma membrane damage from a multitude of stressors in the extra- and intra-cellular environment. To avoid lethal consequences, cells are equipped with repair pathways to restore membrane integrity. Here, we assess plasma membrane damage and repair from a whole-body perspective. We highlight the role of tissue-specific stressors in health and disease and examine membrane repair pathways across diverse cell types. Furthermore, we outline the impact of genetic and environmental factors on plasma membrane integrity and how these contribute to disease pathogenesis in different tissues.

P53 Activated by ER Stress Aggravates Caerulein-Induced Acute Pancreatitis Progression by Inducing Acinar Cell Apoptosis

BACKGROUND AND AIMS

Acute pancreatitis (AP) is a severe pancreatic disorder that remains associated with high mortality due to a lack of effective drugs and management strategies. This study aimed to investigate the molecular pathogenic mechanisms of AP involving p53 and endoplasmic reticulum (ER) stress pathways.

METHODS

Expression of PRSS1 and p53 in human AP tissues was detected by immunohistochemistry and Western blotting. AP was induced with caerulein in humanized PRSS1 transgenic mice, and its severity was verified by histological imaging, evaluation of edema, serum amylase, and trypsin activity assays. A transferase-mediated d-UTP nick end-labeling assay was performed to evaluate acinar cell apoptosis associated with AP. The expression of ER stress genes was assessed by quantitative RT-PCR (qRT-PCR) and Western blotting.

RESULTS

PRSS1 and p53 were highly expressed in human AP tissues. Expression of human PRSS1 in caerulein-treated mice induced significant acinar cell apoptosis and AP progression. P53 knockout significantly suppressed AP progression in humanized PRSS1 transgenic mice. The ER stress pathway was activated by PRSS1 and mediated the progression of AP in mouse pancreatic tissues. Application of a p53 inhibitor effectively ameliorated caerulein-induced AP in PRSS1 transgenic mice, while a p53 activator promoted the progression of AP.

CONCLUSION

P53, which was activated by the ER stress pathway, promoted the progression of AP in mice expressing PRSS1 by inducing acinar cell apoptosis.

Tumor protein D52 expression is post-transcriptionally regulated by T-cell intercellular antigen (TIA) 1 and TIA-related protein via mRNA stability

Although tumor protein D52 (TPD52) family proteins were first identified nearly 20 years ago, their molecular regulatory mechanisms remain unclear. Therefore, we investigated the post-transcriptional regulation of TPD52 family genes. An RNA immunoprecipitation (RIP) assay showed the potential binding ability of TPD52 family mRNAs to several RNA-binding proteins, and an RNA degradation assay revealed that TPD52 is subject to more prominent post-transcriptional regulation than are TPD53 and TPD54. We subsequently focused on the 3'-untranslated region (3'-UTR) of TPD52 as a cis-acting element in post-transcriptional gene regulation. Several deletion mutants of the 3'-UTR of TPD52 mRNA were constructed and ligated to the 3'-end of a reporter green fluorescence protein gene. An RNA degradation assay revealed that a minimal cis-acting region, located in the 78-280 region of the 5'-proximal region of the 3'-UTR, stabilized the reporter mRNA. Biotin pull-down and RIP assays revealed specific binding of the region to T-cell intracellular antigen 1 (TIA-1) and TIA-1-related protein (TIAR). Knockdown of TIA-1/TIAR decreased not only the expression, but also the stability of TPD52 mRNA; it also decreased the expression and stability of the reporter gene ligated to the 3'-end of the 78-280 fragment. Stimulation of transforming growth factor-β and epidermal growth factor decreased the binding ability of these factors, resulting in decreased mRNA stability. These results indicate that the 78-280 fragment and TIA-1/TIAR concordantly contribute to mRNA stability as a cis-acting element and trans-acting factor(s), respectively. Thus, we here report the specific interactions between these elements in the post-transcriptional regulation of the TPD52 gene.

Opposite effects of tumor protein D (TPD) 52 and TPD54 on oral squamous cell carcinoma cells

The tumor protein D52 (TPD52) protein family includes TPD52, -53, -54 and -55. Several reports have shown important roles for TPD52 and TPD53, and have also suggested the potential involvement of TPD54, in D52-family physiological effects. Therefore, we performed detailed expression analysis of TPD52 family proteins in oral squamous cell carcinoma (OSCC). Towards this end, TPD54-overexpressing or knocked-down cells were constructed using OSCC-derived SAS, HSC2 and HSC3 cells. tpd52 or tpd53 was expressed or co-expressed in these cells by transfection. The cells were then analyzed using cell viability (MTT), colony formation, migration, and invasion assays. In OSCC-xenograft experiments, the cells were transplanted into nude mice together with injection of anti-tpd siRNAs. MTT assay of cell monolayers showed little differences in growth of the transfected cells. tpd54 overexpression in SAS cells significantly decreased colony formation in an anchorage-independent manner. Additionally, knock-down of tpd54 enhanced the number of colonies formed and overexpression of tpd52 in tpd54 knock-down cells increased the size of the colonies formed. The chemotaxis assay showed that tpd54 overexpression decreased cell migration. In the OSCC-xenograft in vivo study, tpd54 overexpression slightly attenuated tumor volume in vivo, despite the fact that tumor metastasis or cell survival was not involved. Our results showed that TPD54 not only downregulated anchorage-independent growth and cell migration in vitro, but also attenuated tumor growth in vivo. Based on these results, it is considered that TPD54 might act as a negative regulator of tumor progression in OSCC cells.

Acute acinar pancreatitis blocks vesicle-associated membrane protein 8 (VAMP8)-dependent secretion, resulting in intracellular trypsin accumulation

Zymogen secretory granules in pancreatic acinar cells express two vesicle-associated membrane proteins (VAMP), VAMP2 and -8, each controlling 50% of stimulated secretion. Analysis of secretion kinetics identified a first phase (0-2 min) mediated by VAMP2 and second (2-10 min) and third phases (10-30 min) mediated by VAMP8. Induction of acinar pancreatitis by supramaximal cholecystokinin (CCK-8) stimulation inhibits VAMP8-mediated mid- and late-phase but not VAMP2-mediated early-phase secretion. Elevation of cAMP during supramaximal CCK-8 mitigates third-phase secretory inhibition and acinar damage caused by the accumulation of prematurely activated trypsin. VAMP8^-/- acini are resistant to secretory inhibition by supramaximal CCK-8, and despite a 4.5-fold increase in total cellular trypsinogen levels, are fully protected from intracellular trypsin accumulation and acinar damage. VAMP8-mediated secretion is dependent on expression of the early endosomal proteins Rab5, D52, and EEA1. Supramaximal CCK-8 (60 min) caused a 60% reduction in the expression of D52 followed by Rab5 and EEA1 in isolated acini and in in vivo The loss of D52 occurred as a consequence of its entry into autophagic vacuoles and was blocked by lysosomal cathepsin B and L inhibition. Accordingly, adenoviral overexpression of Rab5 or D52 enhanced secretion in response to supramaximal CCK-8 and prevented accumulation of activated trypsin. These data support that acute inhibition of VAMP8-mediated secretion during pancreatitis triggers intracellular trypsin accumulation and loss of the early endosomal compartment. Maintaining anterograde endosomal trafficking during pancreatitis maintains VAMP8-dependent secretion, thereby preventing accumulation of activated trypsin.

Early to Late Endosome Trafficking Controls Secretion and Zymogen Activation in Rodent and Human Pancreatic Acinar Cells

BACKGROUND & AIMS

Pancreatic acinar cells have an expanded apical endosomal system, the physiological and pathophysiological significance of which is still emerging. Phosphatidylinositol-3,5-bisphosphate (PI(3,5)P₂) is an essential phospholipid generated by PIKfyve, which phosphorylates phosphatidylinositol-3-phosphate (PI(3)P). PI(3,5)P₂ is necessary for maturation of early endosomes (EE) to late endosomes (LE). Inhibition of EE to LE trafficking enhances anterograde endosomal trafficking and secretion at the plasma membrane by default through a recycling endosome (RE) intermediate. We assessed the effects of modulating PIKfyve activity on apical trafficking and pancreatitis responses in pancreatic acinar cells.

METHODS

Inhibition of EE to LE trafficking was achieved using pharmacological inhibitors of PIKfyve, expression of dominant negative PIKfyve K1877E, or constitutively active Rab5-GTP Q79L. Anterograde endosomal trafficking was manipulated by expression of constitutively active and dominant negative Rab11a mutants. The effects of these agents on secretion, endolysosomal exocytosis of lysosome associated membrane protein (LAMP1), and trypsinogen activation in response to high-dose CCK-8, bile acids and cigarette toxin was determined.

RESULTS

PIKfyve inhibition increased basal and stimulated secretion. Adenoviral overexpression of PIKfyve decreased secretion leading to cellular death. Expression of Rab5-GTP Q79L or Rab11a-GTP Q70L enhanced secretion. Conversely, dominant-negative Rab11a-GDP S25N reduced secretion. High-dose CCK inhibited endolysosomal exocytosis that was reversed by PIKfyve inhibition. PIKfyve inhibition blocked intracellular trypsin accumulation and cellular damage responses to high CCK-8, tobacco toxin, and bile salts in both rodent and human acini.

CONCLUSIONS

These data demonstrate that EE-LE trafficking acutely controls acinar secretion and the intracellular activation of zymogens leading to the pathogenicity of acute pancreatitis.

Identification of PLP2 and RAB5C as novel TPD52 binding partners through yeast two-hybrid screening

Tumor protein D52 (TPD52) is overexpressed in different cancers, but its molecular functions are poorly defined. A large, low-stringency yeast two-hybrid screen using full-length TPD52 bait identified known partners (TPD52, TPD52L1, TPD52L2, MAL2) and four other preys that reproducibly bound TPD52 and TPD52L1 baits (PLP2, RAB5C, GOLGA5, YIF1A). PLP2 and RAB5 interactions with TPD52 were confirmed in pull down assays, with interaction domain mapping experiments indicating that both proteins interact with a novel binding region of TPD52. This study provides insights into TPD52 functions, and ways to maximise the efficiency of low-stringency yeast two-hybrid screens.

Tumor protein D52 controls trafficking of an apical endolysosomal secretory pathway in pancreatic acinar cells

Zymogen granule (ZG) formation in acinar cells involves zymogen cargo sorting from trans-Golgi into immature secretory granules (ISGs). ISG maturation progresses by removal of lysosomal membrane and select content proteins, which enter endosomal intermediates prior to their apical exocytosis. Constitutive and stimulated secretion through this mechanism is termed the constitutive-like and minor-regulated pathways, respectively. However, the molecular components that control membrane trafficking within these endosomal compartments are largely unknown. We show that tumor protein D52 is highly expressed in endosomal compartments following pancreatic acinar cell stimulation and regulates apical exocytosis of an apically directed endolysosomal compartment. Secretion from the endolysosomal compartment was detected by cell-surface antigen labeling of lysosome-associated membrane protein LAMP1, which is absent from ZGs, and had incomplete overlap with surface labeling of synaptotagmin 1, a marker of ZG exocytosis. Although culturing (16-18 h) of isolated acinar cells is accompanied by a loss of secretory responsiveness, the levels of SNARE proteins necessary for ZG exocytosis were preserved. However, levels of endolysosomal proteins D52, EEA1, Rab5, and LAMP1 markedly decreased with culture. When D52 levels were restored by adenoviral delivery, the levels of these regulatory proteins and secretion of both LAMP1 (endolysosomal) and amylase was strongly enhanced. These secretory effects were absent in alanine and aspartate substitutions of serine 136, the major D52 phosphorylation site, and were inhibited by brefeldin A, which does not directly affect the ZG compartment. Our results indicate that D52 directly regulates apical endolysosomal secretion and are consistent with previous studies, suggesting that this pathway indirectly regulates ZG secretion of digestive enzymes.

Expression, localization, and functional role for synaptotagmins in pancreatic acinar cells

Secretagogue-induced changes in intracellular Ca(2+) play a pivotal role in secretion in pancreatic acini yet the molecules that respond to Ca(2+) are uncertain. Zymogen granule (ZG) exocytosis is regulated by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes. In nerve and endocrine cells, Ca(2+)-stimulated exocytosis is regulated by the SNARE-associated family of proteins termed synaptotagmins. This study examined a potential role for synaptotagmins in acinar secretion. RT-PCR revealed that synaptotagmin isoforms 1, 3, 6, and 7 are present in isolated acini. Immunoblotting and immunofluorescence using three different antibodies demonstrated synaptotagmin 1 immunoreactivity in apical cytoplasm and ZG fractions of acini, where it colocalized with vesicle-associated membrane protein 2. Synaptotagmin 3 immunoreactivity was detected in membrane fractions and colocalized with an endolysosomal marker. A potential functional role for synaptotagmin 1 in secretion was indicated by results that introduction of synaptotagmin 1 C2AB domain into permeabilized acini inhibited Ca(2+)-dependent exocytosis by 35%. In contrast, constructs of synaptotagmin 3 had no effect. Confirmation of these findings was achieved by incubating intact acini with an antibody specific to the intraluminal domain of synaptotagmin 1, which is externalized following exocytosis. Externalized synaptotagmin 1 was detected exclusively along the apical membrane. Treatment with CCK-8 (100 pM, 5 min) enhanced immunoreactivity by fourfold, demonstrating that synaptotagmin is inserted into the apical membrane during ZG fusion. Collectively, these data indicate that acini express synaptotagmin 1 and support that it plays a functional role in secretion whereas synaptotagmin 3 has an alternative role in endolysosomal membrane trafficking.

Secretion of novel SEL1L endogenous variants is promoted by ER stress/UPR via endosomes and shed vesicles in human cancer cells

We describe here two novel endogenous variants of the human endoplasmic reticulum (ER) cargo receptor SEL1LA, designated p38 and p28. Biochemical and RNA interference studies in tumorigenic and non-tumorigenic cells indicate that p38 and p28 are N-terminal, ER-anchorless and more stable relative to the canonical transmembrane SEL1LA. P38 is expressed and constitutively secreted, with increase after ER stress, in the KMS11 myeloma line and in the breast cancer lines MCF7 and SKBr3, but not in the non-tumorigenic breast epithelial MCF10A line. P28 is detected only in the poorly differentiated SKBr3 cell line, where it is secreted after ER stress. Consistently with the presence of p38 and p28 in culture media, morphological studies of SKBr3 and KMS11 cells detect N-terminal SEL1L immunolabeling in secretory/degradative compartments and extracellularly-released membrane vesicles. Our findings suggest that the two new SEL1L variants are engaged in endosomal trafficking and secretion via vesicles, which could contribute to relieve ER stress in tumorigenic cells. P38 and p28 could therefore be relevant as diagnostic markers and/or therapeutic targets in cancer.

A role for tumor protein TPD52 phosphorylation in endo-membrane trafficking during cytokinesis

Tumor protein D52 is expressed at high levels in exocrine cells containing large secretory granules where it regulates Ca(2+)-dependent protein secretion; however, D52 expression is also highly induced in multiple cancers. The present study investigated a role for the Ca(2+)-dependent phosphorylation of D52 at the single major phospho-acceptor site serine 136 on cell division. Ectopic expression of wild type D52 (D52wt) and the phosphomutants serine 136/alanine (S136A) or serine 136/glutamate (S136/E) resulted in significant multinucleation of cells. D52wt and S136/E each resulted in a greater than 2-fold increase in multinucleated cells compared to plasmid-transfected controls whereas the S136/A phospho-null mutant caused a 9-fold increase in multinucleation at 48h post-transfection. Electron microscopy revealed D52 expression induced a marked accumulation of vesicles along the mid-line between nuclei where the final stages of cell abscission normally occurs. Supporting this, D52wt strongly colocalized on vesicular structures containing the endosomal regulatory protein vesicle associated membrane protein 8 (VAMP 8) and this colocalization significantly increased with elevations in cellular Ca(2+). As VAMP 8 is known to be necessary for the endo-membrane fusion reactions that mediate the final stages of cytokinesis, these data indicate that D52 expression and phosphorylation at serine 136 play an important role in supporting the Ca(2+)-dependent membrane trafficking events necessary for cytokinesis in rapidly proliferating cancer cells.

Complexin 2 modulates vesicle-associated membrane protein (VAMP) 2-regulated zymogen granule exocytosis in pancreatic acini

Complexins are soluble proteins that regulate the activity of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes necessary for vesicle fusion. Neuronal specific complexin 1 has inhibitory and stimulatory effects on exocytosis by clamping trans-SNARE complexes in a prefusion state and promoting conformational changes to facilitate membrane fusion following cell stimulation. Complexins are unable to bind to monomeric SNARE proteins but bind with high affinity to ternary SNARE complexes and with lower affinity to target SNARE complexes. Far less is understood about complexin function outside the nervous system. Pancreatic acini express the complexin 2 isoform by RT-PCR and immunoblotting. Immunofluorescence microscopy revealed complexin 2 localized along the apical plasma membrane consistent with a role in secretion. Accordingly, complexin 2 was found to interact with vesicle-associated membrane protein (VAMP) 2, syntaxins 3 and 4, but not with VAMP 8 or syntaxin 2. Introduction of recombinant complexin 2 into permeabilized acini inhibited Ca(2+)-stimulated secretion in a concentration-dependent manner with a maximal inhibition of nearly 50%. Mutations of the central α-helical domain reduced complexin 2 SNARE binding and concurrently abolished its inhibitory activity. Surprisingly, mutation of arginine 59 to histidine within the central α-helical domain did not alter SNARE binding and moreover, augmented Ca(2+)-stimulated secretion by 130% of control. Consistent with biochemical studies, complexin 2 colocalized with VAMP 2 along the apical plasma membrane following cholecystokinin-8 stimulation. These data demonstrate a functional role for complexin 2 outside the nervous system and indicate that it participates in the Ca(2+)-sensitive regulatory pathway for zymogen granule exocytosis.

Tumor protein D52 expression and Ca2+-dependent phosphorylation modulates lysosomal membrane protein trafficking to the plasma membrane

Tumor protein D52 (also known as CRHSP-28) is highly expressed in multiple cancers and tumor-derived cell lines; however, it is normally abundant in secretory epithelia throughout the digestive system, where it has been implicated in Ca(2+)-dependent digestive enzyme secretion (41). Here we demonstrate, using site-specific mutations, that Ca(2+)-sensitive phosphorylation at serine 136 modulates the accumulation of D52 at the plasma membrane within 2 min of cell stimulation. When expressed in Chinese hamster ovary CHO-K1 cells, D52 colocalized with adaptor protein AP-3, Rab27A, vesicle-associated membrane protein VAMP7, and lysosomal-associated membrane protein LAMP1, all of which are present in lysosome-like secretory organelles. Overexpression of D52 resulted in a marked accumulation of LAMP1 on the plasma membrane that was further enhanced following elevation of cellular Ca(2+). Strikingly, mutation of serine 136 to alanine abolished the Ca(2+)-stimulated accumulation of LAMP1 at the plasma membrane whereas phosphomimetic mutants constitutively induced LAMP1 plasma membrane accumulation independent of elevated Ca(2+). Identical results were obtained for endogenous D52 in normal rat kidney and HeLA cells, where both LAMP1 and D52 rapidly accumulated on the plasma membrane in response to elevated cellular Ca(2+). Finally, D52 induced the uptake of LAMP1 antibodies from the cell surface in accordance with both the level of D52 expression and phosphorylation at serine 136 demonstrating that D52 altered the plasma membrane recycling of LAMP1-associated secretory vesicles. These findings implicate both D52 expression and Ca(2+)-dependent phosphorylation at serine 136 in lysosomal membrane trafficking to and from the plasma membrane providing a novel Ca(2+)-sensitive pathway modulating the lysosome-like secretory pathway.

Calcium/calmodulin-dependent phosphorylation of tumor protein D52 on serine residue 136 may be mediated by CAMK2delta6

Tumor protein D52 is expressed at relatively high levels in cells within the gastrointestinal tract that undergo classical exocytosis and is overexpressed in several cancers. Current evidence supports a role for D52 in the regulation of vesicular trafficking. D52 function(s) are regulated by calcium-dependent phosphorylation; however, the intracellular mechanisms that mediate this process are not well characterized. The goal of this study was to identify the calcium-dependent phosphorylation site(s) in D52 and to characterize the protein kinase(s) that mediate this phosphorylation. Using mass spectrometry and site-directed mutagenesis, we identified a single amino acid residue, S(136), that undergoes increased phosphorylation upon elevation of intracellular Ca(2+) concentration. A phosphospecific antibody (pS(136)) was produced and used to characterize D52 kinase activity in gastric mucosal, colonic T84, and HEK293 cells. By using D52 as a substrate, a protein kinase with a molecular weight (M(r)) of approximately 50 kDa was identified with "in gel" assays. This kinase comigrated with rat brain calcium/calmodulin-dependent protein kinase (CAMK2)alpha cross-reacted with pan-specific CAMK2 antibodies as well as with anti-active CAMK2 (pT(286/287)) antibody when activated. Carbachol-stimulated phosphorylation of S(136) was inhibited by the CAMK2 inhibitor KN93 (IC(50) 38 microM) and by the calmodulin antagonist W7 (IC(50) 3.3 nM). A previously uncharacterized CAMK2 isoform, CAMK2delta6, which has the same domain structure and M(r) as CAM2alpha, was identified in gastric mucosa by RT-PCR. The cloned, expressed protein comigrated with D52 kinase and colocalized with D52 protein in T84 and HEK293 cells. These findings support a role for CAMK2delta6 in the mediation of D52 phosphorylation.

Nonredundant functions for tumor protein D52-like proteins support specific targeting of TPD52

PURPOSE

Tumor protein D52 (TPD52 or D52) is frequently overexpressed in breast and other cancers and present at increased gene copy number. It is, however, unclear whether D52 amplification and overexpression target specific functional properties of the encoded protein.

EXPERIMENTAL DESIGN

The expression of D52-like genes and MAL2 was compared in breast tissues using quantitative reverse transcription-PCR. The functions of human D52 and D53 genes were then compared by stable expression in BALB/c 3T3 fibroblasts and transient gene knockdown in breast carcinoma cell lines. In situ D52 and MAL2 protein expression was analyzed in breast tissue samples using tissue microarray sections.

RESULTS

The D52 (8q21.13), D54 (20q13.33), and MAL2 (8q24.12) genes were significantly overexpressed in breast cancer tissue (n = 95) relative to normal breast (n = 7; P </= 0.005) unlike the D53 gene (6q22.31; P = 0.884). Subsequently, D52-expressing but not D53-expressing 3T3 cell lines showed increased proliferation and anchorage-independent growth capacity, and reduced D52 but not D53 expression in SK-BR-3 cells significantly increased apoptosis. High D52 but not MAL2 expression was significantly associated with reduced overall survival in breast carcinoma patients (log-rank test, P < 0.001; n = 357) and was an independent predictor of survival (hazard ratio, 2.274; 95% confidence interval, 1.228-4.210; P = 0.009; n = 328).

CONCLUSION

D52 overexpression in cancer reflects specific targeting and may contribute to a more proliferative, aggressive tumor phenotype in breast cancer.

PrLZ is expressed in normal prostate development and in human prostate cancer progression

PURPOSE

We previously reported the isolation and characterization of PrLZ, a novel prostate-specific and androgen-responsive gene of the tumor protein D52 family at chromosome 8q21.1. PrLZ is the only known gene in this locus with prostate specificity. Expression level of PrLZ was elevated specifically in cancer cells, suggesting its association with malignancy.

EXPERIMENTAL DESIGN

To define its biological function in the morphogenesis, development, and functional maturation of the prostate gland and to gain further insight into its role in prostate cancer, we examined PrLZ expression in prostate specimens during early embryonic development and in adult tissue.

RESULTS

PrLZ first appears in the nuclei of the prostate epithelia at 16 weeks of gestation before its distribution in the cytoplasm at later ages. Its expression peaks at 24 years of age, declines at 31 years of age, and maintains a minimal level in later age. On prostate cancer development, PrLZ expression is reactivated, and its expression increases from primary localized tumor to bone metastasis. Overexpression of PrLZ in prostate cancer cells accelerates their growth in vitro and tumor formation in vivo.

CONCLUSION

This work identifies PrLZ as a marker for prostate cancer progression and metastasis, and its pattern of expression is suggestive of a proto-oncogene.

PC-1/PrLZ contributes to malignant progression in prostate cancer

PC-1/PrLZ gene overexpression has been identified to be associated with prostate cancer progression. Previous studies have revealed that PC-1 possesses transforming activity and confers malignant phenotypes to mouse NIH3T3 cells. However, the functional relevance of PC-1 expression changes during prostate cancer development and progression remains to be evaluated. In this study, gain-of-function and loss-of-function analyses in LNCaP and C4-2 cells, respectively, were implemented. Experimental data showed that PC-1 expression was in positive correlation with prostate cancer cell growth and anchor-independent colony formation in vitro, as well as tumorigenicity in athymic BALB/c mice. Moreover, PC-1 expression was also found to promote androgen-independent progression and androgen antagonist Casodex resistance in prostate cancer cells. These results indicate that PC-1 contributes to androgen-independent progression and malignant phenotypes in prostate cancer cells. Furthermore, molecular evidence revealed that PC-1 expression stimulated Akt/protein kinase B signaling pathway, which has been implicated to play important roles in promoting androgen refractory progression in prostate cancer. Increased PC-1 levels in C4-2 cells may represent an adaptive response in prostate cancer, mediating androgen-independent growth and malignant progression. Inhibiting PC-1 expression may represent a novel therapeutic strategy to delay prostate cancer progression.

Pancreatic acinar cells express vesicle-associated membrane protein 2- and 8-specific populations of zymogen granules with distinct and overlapping roles in secretion

Previous studies have demonstrated roles for vesicle-associated membrane protein 2 (VAMP 2) and VAMP 8 in Ca(2+)-regulated pancreatic acinar cell secretion, however, their coordinated function in the secretory pathway has not been addressed. Here we provide evidence using immunofluorescence microscopy, cell fractionation, and SNARE protein interaction studies that acinar cells contain two distinct populations of zymogen granules (ZGs) expressing either VAMP 2 or VAMP 8. Further, VAMP 8-positive granules also contain the synaptosome-associated protein 29, whereas VAMP 2-expressing granules do not. Analysis of acinar secretion by Texas red-dextran labeling indicated that VAMP 2-positive ZGs mediate the majority of exocytotic events during constitutive secretion and also participate in Ca(2+)-regulated exocytosis, whereas VAMP 8-positive ZGs are more largely involved in Ca(2+)-stimulated secretion. Previously undefined functional roles for VAMP and syntaxin isoforms in acinar secretion were established by introducing truncated constructs of these proteins into permeabilized acini. VAMP 2 and VAMP 8 constructs each attenuated Ca(2+)-stimulated exocytosis by 50%, whereas the neuronal VAMP 1 had no effects. In comparison, the plasma membrane SNAREs syntaxin 2 and syntaxin 4 each inhibited basal exocytosis, but only syntaxin 4 significantly inhibited Ca(2+)-stimulated secretion. Syntaxin 3, which is expressed on ZGs, had no effects. Collectively, these data demonstrate that individual acinar cells express VAMP 2- and VAMP 8-specific populations of ZGs that orchestrate the constitutive and Ca(2+)-regulated secretory pathways.

A testis-specific and testis developmentally regulated tumor protein D52 (TPD52)-like protein TPD52L3/hD55 interacts with TPD52 family proteins

Tumor protein D52-like proteins (TPD52) are small coiled-coil motif bearing proteins that were first identified in breast cancer. TPD52 and related proteins have been implicated in cell proliferation, apoptosis, and vesicle trafficking. To date, three human TPD52 members had been identified, named hD52 (TPD52), hD53 (TPD52L1), and hD54 (TPD52L2). The most important characteristic of the protein family is a highly conserved coiled-coil motif that is required for homo- and heteromeric interaction with other TPD52-like proteins. Herein, we identified a novel TPD52-like sequence (TPD52L3, or hD55) in human testis using cDNA microarray. Sequence analysis of the deduced protein suggests that hD55 contains a coiled-coil motif and is highly conserved compared with other TPD52-like sequences. Yeast two-hybrid and GST pull-down assays revealed that hD55 interacts with hD52, hD53, hD54, and itself. cDNA microarray detection found that hD55 was expressed at 5.6-fold higher levels in adult testis than in fetal testis. Additionally, the expression profile shows that hD55 is testis-specific, indicating a potential role for hD55 in testis development and spermatogenesis.

The tumor protein D52 family: many pieces, many puzzles

Tumor protein D52-like proteins are small coiled-coil motif bearing proteins which are conserved from lower organisms to human. The founding member of the family, human D52, has principally attracted research interest due to its frequent overexpression in cancer, often in association with D52 gene amplification. This review summarises published literature concerning this protein family since their discovery, which is highlighting an increasing diversity of functions for D52-like proteins. This in turn highlights a need for more comparative functional analyses, to determine which functions are conserved and which may be isoform-specific. This knowledge will be crucial for any future manipulation of D52 function in human disease, including cancer.

Receptor biology and signal transduction in pancreatic acinar cells

PURPOSE OF REVIEW

Secretagogue receptors and their intracellular signaling pathways regulate pancreatic physiology and may be altered in pathophysiology. Therefore, understanding of the continued progress into their nature and function is relevant to both biology and disease.

RECENT FINDINGS

The major secretagogue receptors on acinar cells include those binding cholecystokinin and acetylcholine, whereas secretin receptors regulate duct cells. Two physical models of the cholecystokinin receptor and ligand binding have been proposed through extensive structure-activity studies. Receptor oligomerization has been described for both cholecystokinin and secretin receptors. Ca plays a central role in the control of digestive enzyme secretion and is largely mobilized from intracellular stores. Inositol trisphosphate has been joined by two other Ca-releasing messengers, cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate, in initiating and coordinating Ca signaling. Progress has also been made in determining the roles of specific organelles in Ca release. Ca triggers secretion, and knowledge of the function and regulation of the proteins involved in exocytosis is accumulating. Continuing advances have also been made in understanding the signaling pathways regulating protein synthesis and growth in adult pancreas. The protein kinase mammalian target of rapamycin and its downstream targets play a central role in protein synthesis, whereas the protein phosphatase calcineurin was recently reported to regulate pancreatic growth. Other signaling molecules include the MAP kinases, PKCs, cytoplasmic tyrosine kinases, and nitric oxide.

SUMMARY

The current findings reviewed here are illuminating the structure and function of receptors on pancreatic acinar and duct cells and the multiple intracellular signaling pathways that they initiate. Understanding of these mechanisms is contributing to knowledge of normal pancreatic functions and alterations in disease such as pancreatitis and pancreatic cancer.

Exocrine pancreatic secretion of phospholipid, menaquinone-4, and caveolin-1 in vivo

The exocrine pancreas releases secretory products essential for nutrient assimilation. In addition to digestive enzymes, the release of lipoprotein-like particles containing the membrane trafficking protein caveolin-1 from isolated pancreatic explants has been reported. The present study examined: (1) if gastrointestinal hormones induce the apical secretion of phospholipid in vivo and (2) a potential association of caveolin-1 and the lipid-soluble vitamin K analog menaquinone-4 (MK-4) with these structures. Analysis of isolated acinar cells, purified zymogen granules, and pancreatic juice collected in vivo indicated the presence a caveolin-1 immunoreactive protein that was acutely released in response hormone stimulation. Chloroform-extracted fractions of pancreatic juice also contained high concentrations of MK-4 that was secreted in parallel to protein and phospholipid. The presence of caveolin-1 and MK-4 in the phospholipid fraction of pancreatic juice places these molecules in the secretory pathway of exocrine cells and suggests a physiological role in digestive enzyme synthesis and/or processing.

Secretagogue-induced translocation of CRHSP-28 within an early apical endosomal compartment in acinar cells

Ca(2+)-regulated heat-stable protein (CRHSP-28) is a member of the TPD52 protein family that has been shown to regulate Ca(2+)-dependent secretory activity in pancreatic acinar cells. Immunofluorescence microscopy of isolated lobules demonstrated that CRHSP-28 is localized to a supranuclear apical compartment in acini and accumulates immediately below the apical membrane within 2 min of CCK octapeptide (CCK-8) stimulation. Dual-immunofluorescence microscopy demonstrated an endosomal localization of CRHSP-28 that strongly overlapped with early endosomal antigen-1 (EEA-1) on vesicular structures throughout the apical cytoplasm but showed only minimal overlap with the transferrin receptor, which is present in basolaterally derived endosomes. Significant overlapping of CRHSP-28 with the trans-Golgi network marker-38 was also noted in supranuclear regions of acini. Interestingly, treatment of lobules with brefeldin A reversibly disrupted the vesicular localization of CRHSP-28 and EEA-1 within the apical cytoplasm. The CCK-8-induced accumulation of CRHSP-28 in subapical regions of acini was not altered by inhibition of apical endocytosis with the actin filament-disrupting agent latrunculin B. Immunoelectron microscopy confirmed that CRHSP-28 is associated with the limiting membrane of irregularly shaped vesicular structures of low electron density in the apical cytoplasm that are positive for EEA-1 staining. Sparse, but significant, CRHSP-28 immunoreactivity was also observed along the limiting membrane of zymogen granules. Consistent with immunofluorescence data, CRHSP-28 was found to accumulate in clusters on endosomes and positioned between zymogen granules below the cell apex on CCK-8 stimulation. These data indicate that CRHSP-28 is present within endocytic and exocytic compartments of acinar cells and is acutely regulated by secretagogue stimulation.

PrLZ, a novel prostate-specific and androgen-responsive gene of the TPD52 family, amplified in chromosome 8q21.1 and overexpressed in human prostate cancer

We report a previously unrecognized prostate-specific protein, PrLZ (prostate leucine zipper), a new member of the Tumor Protein D52 (TPD52) family. The gene for PrLZ was localized at chromosome 8q21.1, a locus most frequently amplified in human prostate cancer. Multiple tissue analyses demonstrated PrLZ predominantly in the prostate gland. Although its expression was enhanced by androgens in androgen receptor-expressing cells, PrLZ was detected in all of the human prostate cancer cell lines, regardless of androgen receptor status. Monoclonal anti-PrLZ antibodies were produced and intense immunohistochemical staining of PrLZ was observed in prostate epithelial cells in intraepithelial neoplasia and prostate cancer, whereas lower-level staining was detected in normal and benign epithelial components of the prostate gland. As the only prostate-specific gene identified in the most frequently amplified genomic region in prostate cancer, PrLZ may be the link between chromosome 8q amplification and malignant transformation of the prostate epithelia.

Positive regulation of apoptosis signal-regulating kinase 1 by hD53L1

Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein kinase kinase kinase family member that plays a central role in cytokine- and stress-induced apoptosis by activating c-Jun N-terminal kinase and p38 signaling cascades. ASK1-induced apoptotic activity is up-regulated by two cellular factors, Daxx and TRAF2, through direct protein-protein interactions. Daxx and TRAF2 are death receptor-associated proteins in Fas and tumor necrosis factor-alpha pathways, respectively. Recent studies suggest that calcium signaling may regulate ASK1 pathway. Here we report that human D53L1, a member of the tumor protein D52 family involved in cell proliferation and calcium signaling, up-regulates the ASK1-induced apoptosis. The human D53L1 physically interacts with the C-terminal regulatory domain of ASK1 and promotes ASK1-induced apoptotic activity by activating caspase signaling in mammalian cells. In luciferase reporter assays, hD53L1 activates c-Jun N-terminal kinase-mediated transactivation in the presence of ASK1. Expression of hD53L1 enhances autophosphorylation and kinase activity of ASK1 but has no effect on ASK1 oligomerization that is necessary for kinase activity and on binding of ASK1 to MKK6, a downstream factor of ASK1. Taken together, these results suggest that activation of ASK1 by hD53L1 may provide a novel mechanism for ASK1 regulation.

CaM kinase II regulation of CRHSP-28 phosphorylation in cultured mucosal T84 cells

Ca(2+)-regulated heat-stable protein of 28 kDa (CRHSP-28; a member of the tumor protein D52 family) is highly expressed in exocrine glands and was shown to regulate digestive enzyme secretion from pancreatic acinar cells. We found CRHSP-28 highly expressed in cultured mucosal secretory T84 cells, consistent with an important regulatory role in apical membrane trafficking. Stimulation of cells with carbachol (CCh) induced rapid, concentration-dependent phosphorylation of CRHSP-28 on at least two serine residues. Isoelectric focusing and immunoblotting were used to characterize cellular mechanisms governing CRHSP-28 phosphorylation. Phosphorylation depends on elevated cellular Ca2+, being maximally induced by ionomycin and thapsigargin and fully inhibited by BAPTAAM. In vitro phosphorylation of recombinant CRHSP-28 was 10-fold greater by casein kinase II (CKII) than Ca2+/calmodulin-dependent protein kinase II (CaMKII). However, phosphopeptide mapping studies demonstrated that CaMKII induced an identical phosphopeptide profile to endogenous CRHSP-28 immunoprecipitated from T84 cells. Although calmodulin antagonists had no effect on CCh-stimulated phosphorylation, disruption of actin filaments by cytochalasin D inhibited phosphorylation by 50%. Confocal microscopy indicated that CRHSP-28 is expressed in perinuclear regions of cells and accumulates immediately below the apical membrane of polarized monolayers following CCh stimulation. CaMKII was also localized to the subapical cytoplasm and was clearly displaced following actin filament disruption. These data suggest that CRHSP-28 phosphorylation is regulated by a CaMKII-like enzyme and likely involves a translocation of the protein within the apical cytoplasm of epithelial cells.

Dietary and hormonal stimulation of rat exocrine pancreatic function regulates CRHSP-28 phosphorylation in vivo

Dietary regulation of digestive enzyme secretion from the pancreas is essential for the breakdown of macronutrients in the gastrointestinal tract. Ca(2+)-responsive heat stable protein (CRHSP)-28 is a regulatory protein that modulates the exocytosis of digestive enzymes from pancreatic acinar cells. In the present study, isoelectric focusing and immunoblotting were used to characterize CRHSP-28 phosphorylation in isolated rat acinar cells and also after hormonal and dietary stimulation of rat pancreas in vivo. CRHSP-28 was highly phosphorylated in isolated acini after stimulation with a physiologic range of concentrations of cholecystokinin-octapeptide (CCK-8). Activation of the high affinity state of the CCK-A receptor with the synthetic peptide JMV-180 confirmed the physiologic relevance of the response. CRHSP-28 phosphorylation was contingent on elevated cellular Ca2+ because it was maximally stimulated by Ca2+ ionophore, but unchanged after protein kinase C, cAMP or cyclic guanosine monophosphate activation. Intravenous infusion of rats with a secretory concentration of the CCK analog, caerulein, stimulated CRHSP-28 phosphorylation by 100% over control (P < 0.01) within 15 min of dosing. Moreover, CRHSP-28 phosphorylation was stimulated by 150% over control (P < 0.05) immediately after consumption of a semipurified AIN-93 diet. These data demonstrate that CRHSP-28 phosphorylation occurs in vivo and can be used as a functional indicator of nutrient-driven acinar cell activation.

Alternative splicing as a mechanism for regulating 14-3-3 binding: interactions between hD53 (TPD52L1) and 14-3-3 proteins

D52 (TPD52)-like proteins are coiled-coil motif-bearing proteins first identified through their expression in human breast carcinoma, which have been proposed to represent signalling intermediates and regulators of vesicle trafficking. D52-like gene transcripts are subject to alternative splicing, with sequences encoding a region termed insert 3 being affected in all three D52-like genes. We have now identified a 14-3-3 binding motif within one of two alternatively spliced exons encoding insert 3. As predicted from the distribution of 14-3-3 binding motifs in four hD52-like bait proteins tested, only a hD53 isoform encoding a 14-3-3 binding motif bound both 14-3-3beta and 14-3-3zeta preys in the yeast two-hybrid system. Since D53 proteins carrying 14-3-3 binding motifs are predicted to be widely expressed, polyclonal antisera were derived to specifically detect these isoforms. Using soluble protein extracts from breast carcinoma cell lines, pull-down assays replicated interactions between recombinant 14-3-3beta and 14-3-3zeta isoforms and exogenously expressed hD53, and co-immunoprecipitation analyses demonstrated interactions between endogenous 14-3-3 and both endogenously and exogenously-expressed hD53 protein. Co-expressed hD53 and 14-3-3 proteins were similarly demonstrated to co-localise within the cytoplasm of breast carcinoma cell lines. These results identify 14-3-3 proteins as partners for hD53, and alternative splicing as a mechanism for regulating 14-3-3 binding.