Non-endocrine Cell Line Research Articles

Engineered IRES-mediated promoter-free insulin-producing cells reverse hyperglycemia.

Endogenous insulin supplementation is essential for individuals with type 1 diabetes (T1D). However, current treatments, including pancreas transplantation, insulin injections, and oral medications, have significant limitations. The development of engineered cells that can secrete endogenous insulin offers a promising new therapeutic strategy for type 1 diabetes (T1D). This approach could potentially circumvent autoimmune responses associated with the transplantation of differentiated β-cells or systemic delivery of viral vectors. We utilized CRISPR/Cas9 gene editing coupled with homology-directed repair (HDR) to precisely integrate a promoter-free EMCVIRES-insulin cassette into the 3' untranslated region (UTR) of the GAPDH gene in human HEK-293T cells. Subsequently quantified insulin expression levels in these engineered cells, the viability and functionality of the engineered cells when seeded on different cell vectors (GelMA and Cytopore I) were also assessed. Finally, we investigated the therapeutic potential of EMCVIRES-based insulin secretion circuits in reversing Hyperglycaemia in T1D mice. Our results demonstrate that HDR-mediated gene editing successfully integrated the IRES-insulin loop into the genome of HEK-293T cells, a non-endocrine cell line, enabling the expression of human-derived insulin. Furthermore, Cytopore I microcarriers facilitated cell attachment and proliferation during in vitro culture and enhanced cell survival post-transplantation. Transplantation of these cell-laden microcarriers into mice led to the development of a stable, fat-encapsulated structure. This structure exhibited the expression of the platelet-endothelial cell adhesion molecule CD31, and no significant immune rejection was observed throughout the experiment. Diabetic mice that received the cell carriers reversed hyperglycemia, and blood glucose fluctuations under simulated feeding stimuli were very similar to those of healthy mice. In summary, our study demonstrates that Cytopore I microcarriers are biocompatible and promote long-term cell survival in vivo. The promoter-free EMCVIRES-insulin loop enables non-endocrine cells to secrete mature insulin, leading to a rapid reduction in glucose levels. We have presented a novel promoter-free genetic engineering strategy for insulin secretion and proposed an efficient cell transplantation method. Our findings suggest the potential to expand the range of cell sources available for the treatment of diabetes, offering new avenues for therapeutic interventions.

The cellular regulators PTEN and BMI1 help mediate NEUROGENIN-3–induced cell cycle arrest

Neurogenin-3 (NEUROG3) is a helix-loop-helix (HLH) transcription factor involved in the production of endocrine cells in the intestine and pancreas of humans and mice. However, the human NEUROG3 loss-of-function phenotype differs subtly from that in mice, but the reason for this difference remains poorly understood. Because NEUROG3 expression precedes exit of the cell cycle and the expression of endocrine cell markers during differentiation, we investigated the effect of lentivirus-mediated overexpression of the human NEUROG3 gene on the cell cycle of BON4 cells and various human nonendocrine cell lines. NEUROG3 overexpression induced a reversible cell cycle exit, whereas expression of a neuronal lineage homolog, NEUROG1, had no such effect. In endocrine lineage cells, the cellular quiescence induced by short-term NEUROG3 expression required cyclin-dependent kinase inhibitor 1A (CDKN1A)/p21CIP1 expression. Expression of endocrine differentiation markers required sustained NEUROG3 expression in the quiescent, but not in the senescent, state. Inhibition of the phosphatase and tensin homolog (PTEN) pathway reversed quiescence by inducing cyclin-dependent kinase 2 (CDK2) and reducing p21CIP1 and NEUROG3 protein levels in BON4 cells and human enteroids. We discovered that NEUROG3 expression stimulates expression of CDKN2a/p16INK4a and BMI1 proto-oncogene polycomb ring finger (BMI1), with the latter limiting expression of the former, delaying the onset of CDKN2a/p16INK4a -driven cellular senescence. Furthermore, NEUROG3 bound to the promoters of both CDKN1a/p21CIP1 and BMI1 genes, and BMI1 attenuated NEUROG3 binding to the CDKN1a/p21CIP1 promoter. Our findings reveal how human NEUROG3 integrates inputs from multiple signaling pathways and thereby mediates cell cycle exit at the onset of differentiation.

Clump formation in mouse pituitary-derived non-endocrine cell line Tpit/F1 promotes differentiation into growth-hormone-producing cells.

The adenohypophysis comprises six types of endocrine cells, including PIT1-lineage cells such as growth hormone (GH)-producing cells and heterogeneous non-endocrine cells, such as pituitary stem/progenitor cells as a source of endocrine cells. We determine the expression of characteristic stem cell marker genes, including sex-determining region Y-box 2 (Sox2), in mouse pituitary-derived non-endocrine cell lines Tpit/E, Tpit/F1 and TtT/GF. We observed high expression of fibroblast growth factor (FGF) receptors in Tpit/F1 cells, which we characterised by cultivation in medium containing a basic FGF and B27 supplement as used for neural stem-cell differentiation. A 4-day cultivation of Tpit/F1 produced floating embryonic stem-cell-like clumps accompanied by a three-fold increase in Sox2 expression. Passages in these clumps maintained the proliferative activity and Sox2 expression levels. After 10 days of cultivation, Tpit/F1 cell clumps were immuno-positive for SOX2 and Ki67 (proliferation marker) and loosely attached to the well bottom. An additional 10 days of cultivation induced the emergence of GH-positive/pituitary-specific transcription factor (PIT1)-negative cells showing migration from the clumps. Pit1 overexpression in attached cells could not induce GH production. Finally, we confirmed the presence of PIT1-negative GH-producing cells (3.2-7.7 % of all GH-positive cells) in rat pituitary. Thus, we demonstrate that Tpit/F1 has the plasticity to differentiate into one type of hormone-producing cell.

Enteroendocrine cell expression of a cholecystokinin gene construct in transgenic mice and cultured cells.

CCK is predominantly expressed in subsets of endocrine cells in the intestine and neurons in the brain. We evaluated the expression of a CCK gene construct in transgenic mice and cultured cells to identify a genomic region that directs correct tissue- and cell-specific expression in enteroendocrine cells. The CCKL1 transgene contained 6.4 kb of mouse Cck fused to lacZ. Expression was evaluated in three transgenic lines (J11, J12, J14) by measurement of beta-galactosidase in tissue homogenates and frozen sections. Correct tissue-specific expression was observed, with beta-galactosidase activity detected in intestine and brain. However, there were differences seen in cell-specific expression in the intestine. Line J14 exhibited expression in CCK-endocrine cells, with expressing cells arising at the normal time during fetal development. However, transgene expression in line J12 intestine was limited to neurons of the enteric nervous system, which reflect an early fetal expression pattern for CCK. Analysis of an additional 15 transgenic founder mice demonstrated intestinal expression in 40% of transgenics, with expressing mice following either an endocrine cell pattern or a neuronal pattern in approximately equal numbers. CCKL1 transfection analysis in cultured cells also demonstrated enteroendocrine cell expression, with 100-fold enhanced activity in the enteroendocrine cell line STC-1 compared with nonendocrine cell lines. The results suggest that the minimal cis-regulatory DNA elements necessary for appropriate CCK expression in enteroendocrine cells reside within the 6.4-kb mouse genomic fragment.

Functional Characterization of a Promoter Region in the Human MEN1 Tumor Suppressor Gene

Our previous studies on the human MEN1 (multiple endocrine neoplasia type 1) gene revealed heterogeneity of MEN1 2.8 kb transcripts related to variation in their 5′ UTR only. Six distinct exons 1 (e1A–e1F) were isolated that suggested the existence of multiple but not already identified transcriptional start sites (TSS) and of a complex transcriptional control. Identification of a minimal promoter region and its adjacent regulatory regions appears an inescapable step to the understanding of MEN1 gene transcriptional regulation in normal and pathological situations. For this purpose, we subcloned the ∼2000 bp region situated directly upstream of the exon 2 in front of a luciferase reporter gene, and we analyzed functional consequences of 5′ and 3′ serial deletions, comparatively in a series of endocrine versus non-endocrine cell lines. Primer extension and RPA experiments demonstrate that in HEK293 cells transcription initiated simultaneously at several points in endogenous MEN1 promoter as well as in transfected promoter fragments in reporter plasmids, mainly in Inr elements that are efficiently employed to synthetize previously described exons e1A–e1D. Functional consequences of TSS deletion are directly related to cellular context. The minimal promoter region is localized between −135 and −36. Five large adjacent cis-regulatory regions (UR1–UR5) exist upstream of this minimal promoter region, whose activity depend not only on the cellular context but also on the presence of a downstream sequence DR1. Five small cis-regulatory elements (C1–C5) are localized between −325 and −107. Overexpression of exogenous menin, the MEN1 gene's product, in mouse embryonic fibroblasts from Men1 −/− knock-out mice dose-dependently decreases MEN1 promoter activity, through sequences surrounding the minimal promoter. Our data highlight the existence of a complex transcriptional regulation of the MEN1 gene, whose activity is clearly modulated depending not only on the cellular context but also on menin intracellular levels. They are the molecular bases required for a future understanding of a potential specific transcription control in endocrine cells.

Tetracycline-regulated secretion of human insulin in a transfected non-endocrine cell line.

Long-term constitutive secretion of insulin by implantation of ex vivo transfected cells such as fibroblasts or myoblasts or in situ by intramuscular injection of naked plasmid DNA provides a potential approach to gene therapy for diabetes mellitus. A mechanism for regulating insulin secretion will be necessary to realize the therapeutic potential of this approach. A second obstacle is the inability of non-endocrine host cells to fully process proinsulin. Therefore, alteration of the wild-type cDNA will be necessary to achieve processing of proinsulin by endogenous endoproteases within these cells. The cDNAs for beta-galactosidase (beta), human wild-type proinsulin (hppI1) and a mutated construct (hppI4), in which the dibasic PC2 and PC3 cleavage sites had been altered to form furin cleavage sites, were sub-cloned into four vectors (pCR3, pVR1012, pIRES, pTRE), including a tetracycline responsive plasmid (pTRE) that requires co-transfection with another plasmid encoding a transactivator (pTet-off) for transgene expression. Transient transfection of the COS-7 fibroblast cell line with these constructs was performed using DEAE-dextran and liposomes. Analysis of vector efficiencies revealed that pTRE/pTet-off>pIRES>pCR3>pVR1012. Further analysis demonstrated total pro/insulin secretion of 2.33 ng/10(6) cells/24 h with > or =25% processed to insulin in hppI-1.pTRE/pTet-off-transfected cells compared with 0.39 ng/10(6) cells/24 h and >70% processing in hppI-4.pTRE/pTet-off-transfected cells. In co-transfection studies with pTRE-hppI1/pTet-off and pTRE-hppI4/pTet-off constructs, pro/insulin secretion was inhibited to 65-66% and 36-38% of control (100%) in the presence of 0.01 and 0.1 microg/ml tetracycline respectively over a 24-h incubation period. Furthermore, reversal of tetracycline inhibition was demonstrated for pTRE-hppI1/pTet-off- and pTRE-hppI4/pTet-off-transfected cells. After a 48-h incubation with 1.0 microg/ml tetracycline, total pro/insulin levels were 10 and 14% compared with untreated cells respectively. On tetracycline removal, total proinsulin levels increased and were equivalent to untreated groups 72 h later. In conclusion, regulation of fully processed human insulin secretion has been achieved in a transiently transfected non-endocrine cell line.

Anomalous Overexpression of p27Kip1 in Sporadic Pancreatic Endocrine Tumors

Background. Little is known about the cellular defects and molecular mechanisms leading to pancreatic endocrine tumors (PETs). p27Kip1 is a universal cyclin-dependent kinase inhibitor (CDKI), which acts as a tumor suppressor and a negative regulator of cell cycle. From previous reports, quiescent cells show high levels of p27Kip1 expression while neoplastic and proliferating cells show no detectable p27Kip1 expression. We hypothesize that in malignant sporadic PETs, p27Kip1 expression would be decreased compared with benign PETs and normal pancreatic tissue.Methods. Western analysis was performed on 28 PETs (7 malignant, 21 benign), 2 nonendocrine cell lines, and 5 endocrine cell lines. Signal intensities were quantitated using densitometry and standardized to normal pancreas.Results. Unexpectedly, increased p27Kip1 expression as compared with control was seen in both benign and malignant tumors, as well as in all four pancreatic islet tumor cell lines, but not fibroblast or pituitary cell lines, evaluated. There was no difference in p27Kip1 level between benign and malignant tumors.Conclusion. This represents the first report of anomalous p27Kip1 overexpression in sporadic PETs, and is part of a growing literature describing the paradoxical overexpression of p27Kip1 in human tumors that includes other endocrine tumors. These studies suggest a unique molecular pathway leading to endocrine tumorigenesis.

Expression of SNARE proteins in enteroendocrine cell lines and functional role of tetanus toxin-sensitive proteins in cholecystokinin release

In neurons, synaptic vesicle exocytosis involves the formation of a core complex particle including syntaxin-1, synaptosomal-associated protein of 25 kDa (SNAP-25) and vesicle-associated membrane protein (VAMP)-2/synaptobrevin. The expression of these proteins was investigated in a panel of cell lines, including lines of endocrine and intestinal origin, by Western blotting and/or immunocytochemistry. The three core complex proteins were detected in the enteroendocrine, cholecystokinin (CCK)-secreting, cell lines STC-1 and GLUTag, and in the endocrine non-intestinal cell lines CA-77 and HIT-T15. In contrast, SNAP-25 and syntaxin-1 were undetected in the intestinal non-endocrine cell lines IEC-6, HT-29 and Caco-2, whereas a slight expression of VAMP-2 was documented in IEC-6 and HT-29 cells. Co-immunoprecipitation experiments indicated that syntaxin-1, SNAP-25 and VAMP-2 were present in a complex similar to that identified in brain. In the STC-1 cell line, treatment of streptolysin-O-permeabilized cells with tetanus toxin (Tetx) selectively cleaved VAMP-2 and VAMP-3/cellubrevin, and simultaneously abolished Ca 2+-induced CCK secretion (IC 50∼12 nM). These results show that endocrine cell lines of intestinal origin express syntaxin-1, SNAP-25 and VAMP-2, and suggest a key role for a Tetx-sensitive protein (for example VAMP-2 and/or VAMP-3) in the CCK secretion by STC-1 cells.

Analysis of molecular mechanisms controlling neuroendocrine cell specific transcription of the chromogranin A gene.

Chromogranin A (CgA), a member of the granin/secretogranin family of acidic glycoproteins that play multiple roles in the process of regulated secretion of peptide hormones and neurotransmitters, is specifically expressed in endocrine and neuroendocrine cells. We previously cloned and characterized the human (h) CgA gene and showed that nucleotides -55 to +32 relative to the transcriptional start site that contain a consensus cAMP element (CRE) and TATA-box motif were sufficient for neuroendocrine cell-specific expression. Here, we examined the role of the well conserved CRE in basal and cAMP-stimulated transcription in neuroendocrine cells. Transient transfection studies with hCgA gene promoter/chloroamphenical acetyl transferase (CAT) reporter constructs were conducted in a panel of neuroendocrine cell lines as well as in nonendocrine cell lines. Deletion or mutation of the CRE resulted in loss of neuroendocrine cell specific transcriptional activity. Mutation of a well conserved region (the TG-box) located between the CRE and the TATA box had no effect or resulted in only a modest decrease in activity. Mutation of the CRE in 5'-extended (-2300 to +32 and -700 to +32) constructs resulted in a 50-75% decrease in basal activity in neuroendocrine cells. This emphasized the importance of the CRE in basal transcription and also suggested that other elements between -700 and -55 may act independently of the CRE to contribute to full basal activity in some neuroendocrine cells. Dibutyryl cAMP stimulated transcriptional activity in neuroendocrine cells, and this was abolished by mutation of the CRE. In the presence of a PKA inhibitor, dibutyryl cAMP-induced activity was completely abolished and basal activity was decreased by up to 85%. Similar protein-DNA complexes were formed in gel retardation assays with a CgA-CRE oligonucleotide and nuclear extracts from both neuroendocrine and nonendocrine cells. A predominant complex that was supershifted by addition of a CREB antibody was identical in all cell types. By immunoblot analysis, levels of total CREB protein and phosphorylated (Ser 133) CREB did not differ between neuroendocrine and nonendocrine cells. Phosphorylated CREB was increased by forskolin treatment, an effect that was blocked by a PKA-inhibitor. Expression of the transcriptional cointegrator, CREB-binding protein (CBP), assessed by both RT-PCR and Western blot analysis, did not differ between neuroendocrine and nonendocrine cells. In summary, the CRE in the hCgA gene proximal promoter is critical for both basal and cAMP-induced expression in neuroendocrine cells via a PKA-mediated pathway. However, the neuroendocrine specificity of hCgA gene transcription mediated by the CRE is not a function of levels of total CREB or phosphorylated CREB or its cointegrator CBP. Specificity may be achieved by a PKA-responsive CRE-binding protein other than CREB expressed specifically in neuroendocrine cells, expression of a repressor molecule that binds CREB in nonendocrine cells, or may lie downstream of a CRE-binding protein, e.g. in the activity or amount of cointegrators other than CBP, which are required to couple transactivators to the basal transcriptional machinery.

The basic helix-loop-helix transcription factor BETA2/NeuroD is expressed in mammalian enteroendocrine cells and activates secretin gene expression.

The gene encoding the hormone secretin is expressed only in enteroendocrine S cells and insulin-producing pancreatic beta cells during development. A 120-bp enhancer directs cell-specific expression of the rat secretin gene in secretin-expressing cells. The enhancer includes an E-box sequence, CAGCTG, which is important for transcriptional activity. To further characterize the role of the E box, a consensus binding site for basic helix-loop-helix (bHLH) proteins, we have examined factors that interact with this element in the secretin gene. The results suggest that transcription is activated by a recently isolated tissue-specific bHLH protein, BETA2, heterodimerized to the ubiquitously expressed bHLH proteins, Pan 1 and Pan 2, the rodent homologues of E47 and E12. The importance of BETA2 for transcriptional activation of secretin is further illustrated by antisense experiments inhibiting BETA2 expression in secretin-producing cell lines, which resulted in the inhibition of most E box-dependent transcription. Expression of BETA2 in a nonendocrine cell line conferred the ability to express secretin-reporter genes that are transcribed at minimal levels in the absence of BETA2. Secretin-producing enteroendocrine cells in the murine small intestine showed specific immunostaining with BETA2 antibodies, corroborating observations in cell lines. Thus BETA2 is to our knowledge the first transcription factor identified that specifically activates cell type-specific expression of an intestinal hormone gene.

Production of Bioactive Salmon Calcitonin from the Nonendocrine Cell Lines COS-7 and CHO

Takahashi, K.-I., Y.-C. Liu, N. Hayashi, F. Goto, M. Kato, H. Kawashima and T. Takeuchi. Production of bioactive salmon calcitonin from the nonendocrine cell lines COS-7 and CHO. Peptides 18(3) 439–444, 1997.—To produce bioactive salmon calcitonin from the conventional nonendocrine cell lines, COS-7 and CHO, we devised a salmon calcitonin expression vector by combining the amino-terminus of human calcitonin precursor with a salmon calcitonin sequence, inserting the efficient furin-cleavable processing sequence Arg-X-Arg-X-Lys-Arg before salmon calcitonin, and deleting the carboxyl-terminal extension peptide. This chimeric calcitonin precursor terminates at glycine to easily receive an amidation reaction. COS-7 and CHO produced a high level of bioactive calcitonin by the resorption pit formation assay. Although amidating activity is highly expressed in CHO, but only a little in COS-7 cells, both cells produced a similar level of bioactive calcitonin. Thus, the engineered salmon calcitonin expression vector enables nonendocrine cells even with low amidation activity to produce bioactive calcitonin.

Processing of mutated human proinsulin to mature insulin in the non-endocrine cell line, CHO.

Heterologous genes encoding proproteins, including proinsulin, generally produce mature protein when expressed in endocrine cells while unprocessed or partially processed protein is produced in non-endocrine cells. Proproteins, which are normally processed in the regulated pathway restricted to endocrine cells, do not always contain the recognition sequence for cleavage by furin, the endoprotease specific to the constitutive pathway, the principal protein processing pathway in non-endocrine cells. Human proinsulin consists of B-Chain-C-peptide-A-Chain and cleavage at the B/C and C/A junctions is required for processing. The B/C, but not the C/A junction, is recognised and cleaved in the constitute pathway. We expressed a human proinsulin and a mutated proinsulin gene with an engineered furin recognition sequence at the C/A junction and compared the processing efficiency of the mutant and native proinsulin in Chinese Hamster Ovary cells. The processing efficiency of the mutant proinsulin was 56% relative to 0.7% for native proinsulin. However, despite similar levels of mRNA being expressed in both cell lines, the absolute levels of immunoreactive insulin, normalized against mRNA levels, were 18-fold lower in the mutant proinsulin-expressing cells. As a result, there was only a marginal increase in absolute levels of insulin produced by these cells. This unexpected finding may result from preferential degradation of insulin in non-endocrine cells which lack the protection offered by the secretory granules found in endocrine cells.

Production of bioactive enkephalin from the nonendocrine cell lines COS-7, NIH3T3, Ltk-, and C2C12.

Enkephalin is synthesized from proenkephalin in neuroendocrine cells. For the attempt to induce nonneuroendocrine origin cells to produce enkephalin, we used a mammalian expression vector for fusion peptides, pMEproCT beta, in which a fused peptide is designed to be cleaved by a yeast Kex2-like endoprotease furin. Met-Enkephalin was expressed in four nonneuroendocrine cell lines: COS-7, C2C12, Ltk-, and NIH3T3. The four cell lines produced a marked amount of Met-enkephalin, which appeared as a single peak on reverse-phase HPLC. Because transplantation of adrenal medullary cells to the subarachnoid space has been used to alleviate terminal cancer pain, and enkephalin appears to play a central role in relieving pain, this enkephalin expression vector may be useful for direct enkephalin expression in pericancerous tissues.

The neuroendocrine precursor 7B2 is a sulfated protein proteolytically processed by a ubiquitous furin-like convertase.

The neuroendocrine granule-associated protein 7B2, unlike many other neuroendocrine precursor proteins stored in secretory granules, carries in its primary structure the Arg-Xaa-Arg/Lys-Arg processing site usually found in constitutively secreted precursor proteins and recognized by the ubiquitously expressed convertase, furin. pro7B2 (30 kDa), when expressed in endocrine (AtT-20, PC12, and GH4C1) or non-endocrine (Ltk-) cell lines using recombinant vaccinia viruses, was converted to a 23-kDa form. Mutation of the P4 Arg to Gly completely prevented this conversion. When excess pro7B2 was coexpressed with the pro-protein convertases PC1, PC2, or furin, only furin could induce complete processing. In addition, coexpression of pro7B2 in LoVo cells, which are devoid of endogenous furin activity, with each one of the three convertases, showed that only furin was able to induce processing of this precursor. pro7B2 processing in AtT-20 was completely abolished when protein transport into Golgi compartments was blocked by cell incubation at either 15 or 37 degrees C in the presence of monensin or brefeldin A. Furthermore, pulse-chase experiments in the presence of Na2[35S]SO4 showed that pro7B2 is Tyr-sulfated in the trans-Golgi network before it is processed. These results demonstrate that pro7B2 is first processed by a furin-like enzyme within the trans-Golgi network into a 23-kDa form that is then sequestered into secretory granules.

Expression and characterization of VIP and two VIP mutants in NIH 3T3 cells

Prepro-vasoactive intestinal peptide (preproVIP) was expressed in NIH 3T3 cells, and the preproVIP-derived peptides produced by the cells were analyzed by chromatography combined with sequence-specific radio-immunoanalysis. In accordance with what has previously been reported on processing in non-endocrine cell lines, the VIP precursor was processed poorly in these non-endocrine cells. Mainly an extended form of VIP could be detected in the media from the cells, and no immunoreactivity specific for amidated VIP was found. However, by changing the dibasic cleavage site positioned N-terminal to the VIP sequence in the precursor into the consensus sequence (Arg,X,Lys/Arg,Arg) for the ubiquitous processing enzyme furin, thought to process, e.g. insulin receptors, factor VII, and by deleting residues 156–170 in the VIP precursor, expression of amidated VIP was obtained in this fibroblast cell line. Peptides from the wild-type VIP precursor as well as peptides from the mutated VIP precursor were found to be able to stimulate the adenylate cyclase in cells expressing the VIP receptor.

Production of bioactive gastrin from the non-endocrine cell lines CHO and COS-7

We made a mutated progastrin cDNA construct that contains a cleavage site (-Arg −4-Arg −3-Lys −2 -Arg −1) specific for the Kex2-like endoprotease furin, located ahead of the bioactive gastrin. For expressing the mutated progastrin cDNA, we used two non-endocrine cell lines, CHO and COS-7. CHO cells exhibit amidating enzyme activity and levels of amidation enzyme mRNA as high as those in the pituitary-derived endocrine cell line GH 3, whereas COS-7 cells have far less amidating activity and lower amounts of mRNA. Mutant progastrin-expressing CHO cells produced mostly amidated gastrin. Gel filtration showed the size of this gastrin corresponded to that of the synthetic human gastrin-17. In contrast, COS-7 cells produced glycine-extended gastrin and only a small amount of amidated gastrin. The difference in the amount of amidated gastrin products produced by the two non-endocrine cell lines is due to differing amounts of the amidation enzyme contained in each cell line.

Processing of mutated proinsulin with tetrabasic cleavage sites to mature insulin reflects the expression of furin in nonendocrine cell lines

Furin is a mammalian propeptide-processing endoprotease in nonendocrine cells and has been demonstrated to be present in virtually all nonendocrine cells, including fibroblasts, epithelial cells, and hepatocytes. Furin cleaves the concensus processing site -Arg-4-X-3-Lys/Arg-2-Arg-1 decreases X+1-. Some subunit-containing precursor proteins, including an insulin receptor precursor, possess an additional basic residue at position -3, thus forming a tetrabasic processing site. This implies that a tetrabasic processing site must be easily cleavable in nonendocrine cells. We created a mutant proinsulin DNA with a peptide structure comprised of B- and A-chains linked to the C-peptide by a pair of tetrabasic residues, in the following order: B-chain-Arg-Arg-Lys-Arg-C peptide-Arg-Arg-Lys-Arg-A-chain. The native proinsulin structure was B-chain-Arg-Arg-C-peptide-Lys-Arg-A-chain. Both the native and mutant proinsulins were expressed in the following four cell lines: a monkey kidney-derived cell line (COS-7), a Chinese hamster ovary-derived cell line (CHO), a human liver cancer-derived cell line (HepG2), and a mouse fibroblast-like cell line (NIH3T3). We used these cell lines because they contain different quantities of furin mRNA, ranking as follows: NIH3T3 > HepG2 > COS > CHO. When mutant insulin was expressed in these cells, the conversion of proinsulin to mature insulin was approximately 85% in NIH3T3, 70% in HepG2, 60% in COS, and 50% in CHO. The conversion correlated well with the furin expression in each cell line as measured by the density of its Northern blot band. Moreover, in CHO, the cell line with the lowest furin expression, coexpression of mutant proinsulin with furin resulted in complete conversion of proinsulin to mature insulin.

Processing of mutated proinsulin with tetrabasic cleavage sites to mature insulin reflects the expression of furin in nonendocrine cell lines.

Furin is a mammalian propeptide-processing endoprotease in nonendocrine cells and has been demonstrated to be present in virtually all nonendocrine cells, including fibroblasts, epithelial cells, and hepatocytes. Furin cleaves the concensus processing site -Arg-4-X-3-Lys/Arg-2-Arg-1 decreases X+1-. Some subunit-containing precursor proteins, including an insulin receptor precursor, possess an additional basic residue at position -3, thus forming a tetrabasic processing site. This implies that a tetrabasic processing site must be easily cleavable in nonendocrine cells. We created a mutant proinsulin DNA with a peptide structure comprised of B- and A-chains linked to the C-peptide by a pair of tetrabasic residues, in the following order: B-chain-Arg-Arg-Lys-Arg-C peptide-Arg-Arg-Lys-Arg-A-chain. The native proinsulin structure was B-chain-Arg-Arg-C-peptide-Lys-Arg-A-chain. Both the native and mutant proinsulins were expressed in the following four cell lines: a monkey kidney-derived cell line (COS-7), a Chinese hamster ovary-derived cell line (CHO), a human liver cancer-derived cell line (HepG2), and a mouse fibroblast-like cell line (NIH3T3). We used these cell lines because they contain different quantities of furin mRNA, ranking as follows: NIH3T3 > HepG2 > COS > CHO. When mutant insulin was expressed in these cells, the conversion of proinsulin to mature insulin was approximately 85% in NIH3T3, 70% in HepG2, 60% in COS, and 50% in CHO. The conversion correlated well with the furin expression in each cell line as measured by the density of its Northern blot band. Moreover, in CHO, the cell line with the lowest furin expression, coexpression of mutant proinsulin with furin resulted in complete conversion of proinsulin to mature insulin.

Expression and processing of prohormones in nonendocrine cells

Pancreatic polypeptide (PP) and gastrin are initially synthesized as larger precursors that require posttranslational processing to produce the biologically active peptides. These steps include tryptic cleavage at paired basic residues, their subsequent removal by carboxypeptidase H, and formation of a carboxy-terminal amide moiety via the action of peptidyl-glycine alpha-amidating monooxygenase (PAM). Although these posttranslational processing reactions are presumed to occur primarily in the secretory granule of endocrine cells, nonendocrine cells that do not possess these structures nevertheless are able to posttranslationally process a wide variety of proteins destined for export. In these studies we sought to determine whether the mechanisms for prohormone processing are present in nonendocrine cell lines. We examined two fibroblast cell lines (psi-2, BHK) a hepatocyte cell line (Hepa), and an exocrine pancreatic cell line (AR42J). We used the pZIPneo(SVX) retroviral vector to express cDNA clones encoding human PP and gastrin in the nonendocrine cells. Transfected psi-2, BHK, and Hepa cells produced a precursor of PP that appeared to be secreted constitutively, with little remaining in intracellular stores. Almost no posttranslational processing of the PP precursor was evident in these cells. By contrast, AR42J cells were capable of expressing and storing fully processed and carboxy-terminally amidated PP and gastrin. These data support the notion that the sorting mechanisms in endocrine and exocrine cells are similar and that the posttranslational processing of peptide hormone precursors requires storage in secretory granules.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficient amidation of C-peptide deleted NPY precursors by non-endocrine cells is affected by the presence of Lys-Arg at the C-terminus

Post-translational processing of peptide precursors producing amidated, biologically active peptides generally occurs in specially differentiated endocrine or neural cells. However, we have previously shown that a C-peptide-deleted precursor of neuropeptide Y (NPY 1–39) in which the precursor terminates in the sequence Gly-Lys-Arg was partially amidated by the non-endocrine cell line, CHO. In the present study we show that two other non-endocrine cell lines, NIH 3T3 and BHK, also possess amidating activities and that the NPY 1–39 precursor was completely converted to NPY 1–36 amide by the NIH 3T3 cell line. The role of the two basic residues (Lys-Arg) in the C-terminus was studied by transfection of a construct encoding a NPY precursor terminating with glycine alone. Both the CHO and NIH 3T3 cell lines, transfected with this construct, secreted a significantly smaller fraction of NPY reactive material as amidated NPY compared to the fraction of amidated NPY secreted by the cells transfected with the NPY 1–39 precursor. It is concluded that the capacity to perform C-terminal amidation appears to be a universal feature of eukaryotic cells and that the carboxypeptidase E-like enzyme influences the amidation process, beyond its known ability to remove the C-terminal basic residues.