Negative regulators of cell proliferation

Roles and mechanisms of optineurin in bone metabolism

Optineurin (OPTN) is a widely expressed multifunctional articulatory protein that participates in cellular or mitochondrial autophagy, vesicular transport, and endoplasmic reticulum (ER) stress via interactions with various proteins. Skeletal development is a complex biological process that requires the participation of various osteoblasts, such as bone marrow mesenchymal stem cells (BMSCs), and osteogenic, osteoclastic, and chondrogenic cells. OPTN was recently found to be involved in the regulation of osteoblast activity, which affects bone metabolism. OPTN inhibits osteoclastogenesis via signaling pathways, including NF-κB, IFN-β, and NRF2. OPTN can promote the differentiation of BMSCs toward osteogenesis and inhibit lipogenic differentiation by delaying BMSC senescence and autophagy. These effects are closely related to the development of bone metabolism disorders, such as Paget's disease of bone, rheumatoid arthritis, and osteoporosis. Therefore, this review aims to explore the role and mechanism of OPTN in the regulation of bone metabolism and related bone metabolic diseases. Our findings will provide new targets and strategies for the prevention and treatment of bone metabolic diseases.

The human FOXM1 homolog promotes basal progenitor cell proliferation and cortical folding in mouse

Cortical expansion and folding are key processes in human brain development and evolution and are considered to be principal elements of intellectual ability. How cortical folding has evolved and is induced during embryo development is not well understood. Here, we show that the expression of human FOXM1 promotes basal progenitor cell proliferation and induces cortical thickening and folding in mice. Human-specific protein sequences further promote the generation of basal progenitor cells. Human FOXM1 increases the proliferation of neural progenitors by binding to the Lin28a promoter and increasing Lin28a expression. Furthermore, overexpression of LIN28A rescues the proliferation of human FOXM1 knockout neural progenitor cells. Together, our findings demonstrate that a human gene can increase the number of basal progenitor cells in mice, leading to brain size increase and gyrification, and may thus contribute to evolutionary brain development and cortical expansion.

Short and sweet: evolution of a small glycopeptide from a bladder disorder to an anticancer lead

Glycopeptides are a class of molecules that comprise two distinct families of biologically important scaffolds, peptides and oligosaccharides, each playing important roles in cellular communication and signaling. Rarely are small, endogenous secreted glycopeptides found that have significant impact on the progression of a specific disease state, but such is the case for the antiproliferative factor (APF) found in the urine and tissue of patients with the poorly understood bladder diseases collectively referred to as interstitial cystitis (IC). APF is a 9-mer peptide containing a sialylated O-linked trisaccharide glycan attached to the N-terminal threonine. APF dramatically inhibits normal bladder cell proliferation and is thought to cause some of the characteristic pathological changes in the bladder of IC patients. Importantly, APF also potently inhibits the growth of certain tumor cells. The details of the cellular receptors to which APF interacts, and the structural features that are critical for its potency are now beginning to unfold. This interesting molecule is a powerful model for the design of new treatments and diagnostic tests for IC, as well as an unprecedented lead agent for novel anticancer drug design.

Tight junction biology and kidney dysfunction

The epithelial tight junction (TJ) has three major functions. As a "gate," it serves as a regulatory barrier separating and maintaining biological fluid compartments of different composition. As a "fence," it generates and maintains the apicobasal polarity of cells that form the confluent epithelium. Finally, the TJ proteins form a trafficking and signaling platform that regulates cell growth, proliferation, differentiation, and dedifferentiation. Six examples are selected that illustrate the emerging link between TJ dysfunction and kidney disease. First, the glomerular slit diaphragm (GSD) is evolved, in part, from the TJ and, on maturation, exhibits all three functions of the TJ. GSD dysfunction leads to proteinuria and, in some instances, podocyte dedifferentiation and proliferation. Second, accumulating evidence supports epithelial-mesenchymal transformation (EMT) as a major player in renal fibrosis, the final common pathway that leads to end-stage renal failure. EMT is characterized by a loss of cell-cell contact and apicobasal polarity, which are hallmarks of TJ dysfunction. Third, in autosomal dominant polycystic kidney disease, mutations of the polycystins may disrupt their known interactions with the apical junction complex, of which the TJ is a major component. This can lead to disturbances in epithelial polarity regulation with consequent abnormal tubulogenesis and cyst formation. Fourth, evidence for epithelial barrier and polarity dysregulation in the pathogenesis of ischemic acute renal failure will be summarized. Fifth, the association between mutations of paracellin-1, the first TJ channel identified, and clinical disorders of magnesium and calcium wasting and bovine renal fibrosis will be used to highlight an integral TJ protein that can serve multiple TJ functions. Finally, the role of WNK4 protein kinase in shunting chloride across the TJ of the distal nephron will be addressed.

A cell regulatory agent, CeReS-18, inhibits mouse 3T6 cell proliferation but not polyomavirus replication

We have purified a cell regulatory sialoglycopeptide, CeReS-18, from intact bovine cerebral cortex cells. This is an 18-kDa molecule that reversibly inhibits cellular DNA synthesis and the proliferation of a wide array of target cells. In the present study, the effect of CeReS-18 on mouse 3T6 host cell proliferation and polyomavirus replication was investigated. The results showed that CeReS-18 was able to inhibit 3T6 cell cycling in a concentration-dependent, calcium-sensitive, and reversible manner. Despite the inhibition of cell proliferation, CeReS-18 did not influence polyomavirus infection of 3T6 cells. Indirect immunofluorescent assays revealed that CeReS-18-treated, and cell cycle-arrested, 3T6 cells remained permissive to polyomavirus replication. Electron microscopy and immunogold labeling showed that new viral particles were assembled inside the nuclei of infected cells in the presence of CeReS-18 and during cell cycle arrest. The cellular requirements for the replication of polyomavirus DNA and the synthesis of viral proteins, as well as for the assembly of viral particles, therefore, remained available in CeReS-18-inhibited 3T6 cells. In addition, although polyomavirus infection can be mitogenic, infection of CeReS-18-treated 3T6 cells did not reverse the cell cycle arrest mediated by this cell cycle inhibitor.

An endogenous melanocyte-inhibiting tripeptide pyroGlu-Phe-GlyNH2 delays in vivo growth of monoclonal experimental melanoma

The melanocyte-inhibiting tripeptide (MTP) pyroGlu-Phe-GlyNH2 is present in tissue cultures of non-transformed melanocytes and melanoma cells and influences melanocyte growth in vitro. The objective of the present study was to investigate a possible effect of MTP on the in vivo growth of B16A2, a monoclonal experimental melanoma. The B16A2 clone was established by the limited dilution technique. It has a reduced DNA content and displays slower growth both in vivo and in vitro compared to the parent cell line (B16). B16A2 cells were injected subcutaneously into hairless mice at four sites (300 000 cells in 0.25 ml buffer/site). MTP was given by i.p. injection 3 times a week at two concentrations (1 pmol and 1 nmol/animal). The control animals received the equal volume of solvent. The animals were sacrificed 1 and 2 weeks after tumour transplantation, and all tumours were weighed. One week after transplantation, the animals who received 1 pmol MTP had fewer tumours and a reduced tumour load. Two weeks after the transplantation, the differences between control and treated animals were no longer observed. The results indicate that MTP temporarily delays in vivo tumour growth.

CeReS-18, a cell regulatory sialoglycopeptide, inhibits proliferation and migration of rat vascular smooth muscle cells

CeReS-18, a cell regulatory sialoglycopeptide, has been shown to inhibit proliferation of a wide array of target cells. In the present study, the effect of CeReS-18 on vascular smooth muscle cell (SMC) proliferation was characterized in cultured rat aorta SMCs (A7r5). More extensively, the effect of CeReS-18 on platelet-derived growth factor (PDGF)-induced SMC migration was examined using a modified Boyden's chamber assay. CeReS-18 inhibits both SMC proliferation and migration in a concentration-dependent, calcium-sensitive, and reversible manner. Furthermore, cells preincubated with the inhibitor had an increased sensitivity to CeReS-18-mediated inhibition of SMC migration. Immunoprecipitation and in vitro phosphorylation assays demonstrated that MAP kinase activity was inhibited in the CeReS-18-treated cells and pretreatment with CeReS-18 suppressed the activation of MAP kinase stimulated by PDGF. However, it is not likely that the suppression of the MAP kinase pathway was directly responsible for the ability of CeReS-18 to inhibit migration of the rat aorta smooth muscle cells since a MEK-specific inhibitor, PD98059, did not influence A7r5 cell migration.

Effects of tachyplesin on the morphology and ultrastructure of human gastric carcinoma cell line BGC-823

AIM: To investigate the morphological and ultrastructural changes in the human gastric carcinoma cell line BGC-823 after being treated with tachyplesin.

METHODS: Tachyplesin was isolated from acid extracts of Chinese horseshoe crab (Tachypleus tridentatus) hemocytes. BGC-823 cells and the cells treated with 2.0 mg/L tachyplesin were examined respectively under light microscope, scanning and transmission electron microscope.

RESULTS: BGC-823 cells had undergone the restorational alteration in morphology and ultrastructure after tachyplesin treatment. The changes were as follows: the shape of cells was unanimous, the volume enlarged and cells turned to be flat and spread, the nucleo-cytoplasmic ratio lessened and nuclear shape became rather regular, the number of nucleolus reduced and its volume lessened, heter-chromatin decreased while euchromatin increased in nucleus. In the cytoplasm, mitochondria grew in number with consistent structure relatively, Golgi complex turned to be typical and well-developed, rough endoplasmic reticulum increased and polyribosome decreased. The microvilli at cellular surface were rare and the filopodia reduced while lamellipodia increased at the cell edge.

CONCLUSION: Tachyplesin could alter the malignant morphological and ultrastructural characteristics of human gastric carcinoma cells effectively and have a certain inducing differen-tiation effect on human gastric carcinoma cells.

A lymphoma growth inhibitor blocks some but not all prolactin-stimulated signaling pathways

Cytokines and hormones activate a network of intracellular signaling pathways to regulate cell division, survival and differentiation. In parallel, a series of growth inhibitory mechanisms critically restrict cell population sizes. For example, mitogens can be opposed in crowded cell cultures through contact-inhibition or by autocrine release of antiproliferative substances. Here, we characterize a small, heat-stable growth inhibitor secreted by a rat T lymphoma line when cultured at high cell density. Short term incubation (<60 min) of prolactin-responsive Nb2 lymphoma cells at high density selectively blocked prolactin stimulation of p42/p44 mitogen-activated protein kinases and transcription factors Stat1 and Stat3 but not prolactin activation of Stat5 or the tyrosine kinase Jak2. The selective effects of cell density on prolactin signaling were reversible. Furthermore, exposure of cells at low density to conditioned media from cells incubated at high density had the same inhibitory effects on prolactin signaling. This selective inhibition of discrete prolactin signals was mimicked by short term preincubation of cells at low density with staurosporine or genistein but not with bis-indoleyl maleimide, cyclic nucleotide analogs, calcium ionophore A23187, or phorbol 12-myristate 13-acetate. A heat-stable, proteinase K-resistant, low molecular weight factor with these characteristics was recovered from high density culture medium. The partially purified inhibitor suppressed Nb2 cell growth with a sigmoidal concentration response consistent with a saturable, receptor-mediated process.

CeReS-18 inhibits growth and induces apoptosis in human prostatic cancer cells

BACKGROUND

Polypeptide growth factors are positive and negative regulators of prostatic growth and function, and many positive regulators of growth in the prostate have been extensively studied. However, very few inhibitors of prostate cell proliferation have been identified. We have isolated a unique 18-kDa sialoglycopeptide (CeReS-18) which inhibits cell proliferation of three separate lines of human prostate cancer cells, as well as inducing cellular cytotoxicity via an apoptotic pathway unrelated to the Bcl-2 family of proteins.

METHODS

Cell cycle inhibition was analyzed by direct cell counts with a Coulter (Miami, FL) cell counter. Apoptotic cells were analyzed by electron microscopy, annexin V-fluorescein isothiocyanate (FITC) staining, fluorescence microscopy, and propidium iodide uptake measured with a fluorescence-activated cell sorter. Expression of the proteins of the Bcl-2 family was detected by Western blot analysis.

RESULTS

We found that CeReS-18 inhibits cell proliferation of androgen-responsive, LNCaP.FGC human prostate cancer cells, as well as of androgen-nonresponsive DU-145 and PC3 human prostate cancer cells. Furthermore a, fivefold increase over the inhibitory concentration of CeReS-18 elicited a cytotoxic response by all three cell lines. We thus characterized the cytotoxic mechanism as apoptotic in nature, and we measured the expression of several members of the Bcl-2 family in PC3 cells upon treatment with CeReS-18.

CONCLUSIONS

The data indicate that CeReS-18 is a potent inhibitor of cellular progression through the cell cycle by both androgen-responsive and androgen-nonresponsive human prostate cancer cells. In addition, treatment of both types of cells with increased concentrations of CeReS-18 induces cellular cytotoxicity, characterized as apoptosis.

Isolation of a proliferation inhibitor factor from uterine myomatosis fibroblasts

In this work, we report the isolation of a factor from the culture supernatant of confluent fibroblasts from human cervix with the diagnosis of uterine myomatosis. This factor possesses the capacity to inhibit the proliferation of normal fibroblasts. The proliferation inhibitor factor (PIF) was purified from the culture supernatant by precipitation with 80% ammonium sulfate, and by molecular sieve chromatography. Our results indicate that PIF is a protein of 23 kDa, which is highly sensitive to trypsin treatment, and is thermolabile, since temperatures equal to, or above, 60 degrees C eliminate the protein activity in 15 to 20 min. Western blot analyses identified no cross reactions of the purified PIF with TGF-alpha, TNFalpha, IFNgamma, or IL-1beta, suggesting that PIF is a new protein belonging to the group of factors secreted by fibroblasts able to inhibit cellular proliferation.

Response of the human peritoneal mesothelial cell to injury: an in vitro model of peritoneal wound healing

BACKGROUND

The denudation of the peritoneal mesothelium and damage to the underlying interstitium is a frequent finding in patients receiving continuous ambulatory peritoneal dialysis as a treatment for end-stage renal failure. The response of the mesothelium to injury from repeated episodes of infection or from exposure to dialysis fluids has not been extensively studied. The present study describes a simple and reproducible method with which to investigate the response of human mesothelial cells to injury.

METHODS

The model of peritoneal injury consists of mechanically wounding a monolayer of human peritoneal mesothelial cells with a glass probe and following the repopulation of the denuded area by time-lapse photomicroscopy. In addition immunohistochemistry was used to follow the response of marker proteins for stress fibers and focal adhesions as well as macromolecules associated with the extracellular matrix.

RESULTS

Under serum-free conditions the wound (0.58 +/- 0.094 mm; mean +/- SD; N = 20) closed within 72 +/- 5 hours (N = 8). This rate of healing was enhanced by fetal calf serum, by human serum (10%) and by undiluted spent non-infected dialysate. The repair process over the first 48 hours was the result of cell migration, was independent of cell proliferation and involved the de novo synthesis of several different extracellular matrix components. An early event in the healing process was the rapid reorganization of intracellular stress fibers together with the formation of associated focal adhesions in cells at the wound edge.

CONCLUSION

This in vitro model should prove invaluable in characterizing the process of wound healing within the peritoneal cavity, thus allowing a better understanding of the response to infection as well as any effect of dialysis fluids in this pattern of cell behavior.

Inhibition of cyclin D-cdk activity in cell cycle arrest of Swiss 3T3 cells by CeReS-18, a novel cell regulatory sialoglycopeptide

CeReS-18 is a unique negative regulator of cell proliferation with a wide array of target cells. To elucidate the mechanism by which CeReS-18 mediates cell growth inhibition, the possibility that CeReS-18 alters the function of G1 cyclins and their respective cyclin-dependent kinases (cdks) has been examined in mouse fibroblasts (Swiss 3T3) synchronized by CeReS-18. We show here that cyclin D-associated cdk activity is significantly inhibited in the CeReS-18-treated cells. Corresponding to the inhibited cdk function, we demonstrate a low expression of cyclin D in mid G1 determined by Western blot analysis, and cyclin D was greatly reduced in the immunocomplex recovered with antibody to cdk4 and cdk6. Previously, we have shown that the retinoblastoma susceptibility gene product (pRb), a key substrate of cyclin D-cdk complex, was maintained in the hypophosphorylated state in the CeReS-18-inhibited cells. We conclude here that cyclin D/cdk4,6/pRb is the major pathway by which CeReS-18 mediates cell cycle arrest.

Effect of dibutyryl cAMP and several reagents on apoptosis in PC12 cells induced by a sialoglycopeptide from bovine brain

The sialoglycopeptide (SGP) prepared from bovine brain induces apoptosis in rat pheochromocytoma (PC12) cells. This apoptosis accompanies a decrease in cell growth, DNA fragmentation to oligonucleotide repeats, and morphological changes involving cell shrinkage. Although the growth of PC12 cells was maintained by nerve growth factor (NGF) or basic fibroblast growth factor (bFGF), the apoptosis induced by SGP occurred in the presence of these reagents. The addition of macromolecule synthesis inhibitors or depolarization of membrane potential by extracellular K+ did not prevent apoptosis. Apoptosis was prevented only by a cAMP analog, dibutyryl cAMP, or high concentrations of serum.

Transformation by ras suppresses expression of the neurotrophic growth factor pleiotrophin

An 18-kDa protein (p18) was detected in lysates and conditioned medium from contact-arrested NIH 3T3 fibroblasts, but was not detected when the cells were transformed by the oncogene ras. Analysis of transformation-defective cell clones generated after mutagenesis of the ras-retroviral vector used to transduce the ras gene showed an inverse correlation between p18 expression and the degree of transformation. p18 expression was high in non-transformed clones, intermediate in a partially transformed clone, undetectable in fully transformed clones, and detectable only at the non-permissive temperature in a clone which was cold-sensitive for ras transformation. In non-transformed cells, p18 expression varied with the degree of confluence. It was almost undetectable in medium from sparse, proliferating cells, but increased as the cells approached confluence and peaked 2-4 days after confluence. Microsequencing of partially purified p18 identified it as the developmentally regulated neurotrophic factor pleiotrophin. In further experiments, pleiotrophin was undetectable or almost undetectable in medium from fully transformed cells expressing the oncogenes v-src, truncated c-raf, activated c-fms, or polyomavirus middle tumor antigen; it was low but easily detectable in medium from SV40 large tumor antigen-expressing cells, which form soft agar colonies but not foci. Thus, pleiotrophin expression in NIH 3T3 cells is associated with quiescence, and suppression of pleiotrophin is related to oncogenic transformation.

Cell association increases RPE outgrowth from primary explant

PURPOSE

Most studies of cell growth of the RPE employ cultures which have been previously passaged, but here we investigated freshly-explanted RPE cells, which may have been growth-quiescent for years, within the eye to determine whether co-culture affects initial outgrowth in primary culture.

METHODS

Bovine or human RPE were co-cultured in primary culture with several cell types to test the effects of homologous or heterologous cell association. For bovine RPE, cell number was measured over 14 days in cultures of RPE alone, or RPE in co-culture with irradiated living cells, with fixed-killed cells, with cells separated from the RPE by a semipermeable membrane, or in medium conditioned by the cell types used for co-culture. For human RPE, isolates from all donors were randomized, over a 13-month period, to co-culture or to culture alone. The number of cultures attaining a cell number at one month that was sufficient for further propagation was compared.

RESULTS

Co-culture with irradiated living cells increased the growth of primary cultures of both bovine and human RPE. Living cells were required; fixed-killed cells were ineffective. The outgrowth-promoting activity was not tissue or species specific, and it appeared to require close cell association between the RPE and the co-culture cell population. Conditioned media were ineffective and rather were slightly growth inhibitory. Primary RPE cells showed an earlier expression of vimentin (a marker of Gzero-G1 transition), more rapid cell spreading, and a greater increase in cell number between 7 and 14 days after explant when grown in co-culture than when cultured alone.

CONCLUSIONS

The results indicate that cell association between non-mitotic RPE cells and previously cultured cells of many types increases the outgrowth of the RPE by accelerating the early stages of growth activation in vitro. Co-culture methods offer a practical means for increasing the likelihood of producing cultures from small RPE isolates. Further, should cells involved in proliferative pathologies in situ associate with non-mitotic RPE within the monolayer, the latter cells may also be activated, leading to an augmentation of the pathology.

Inhibition of hormone and growth factor responsive and resistant human breast cancer cells by CeReS-18, a cell regulatory sialoglycopeptide

We have previously documented that CeReS-18, a cell regulatory sialoglycopeptide, inhibits the cellular proliferation of normal and transformed cell types from a diverse range of species. Most cell types studies exhibit a similar sensitivity to the reversible but growth inhibitory effects of CeReS-18 at 7 x 10(8) M concentration, while at higher concentrations CeReS-18 can elicit cytotoxicity. The present study was conducted to examine the effect of CeReS-18 on the proliferation of human mammary epithelial carcinoma cells. MCF-7 cells, which are estrogen receptor positive (ER+), and BT-20 cells, which are estrogen receptor negative (ER+), were utilized. Both cell lines show equal sensitivity to growth inhibition elicited by CeReS-18. Complete cessation of cell cycling was achieved with 7 x 10(-8) M CeReS-18, and the arrest was shown to be completely reversible. Flow cytometric analysis, performed on CeReS-18 treated cells from both cell types, revealed that the majority of these cells were arrested in the G1 phase of the cell cycle. When cells were treated simultaneously with inhibitor and stimulatory concentrations of mitogens such as epidermal growth factor (EGF), basic fibroblast growth factor (b-FGF), estrogen, insulin-like growth factors I and II (IGFI and IGFII), no alteration of the inhibitory activity of CeReS-18 was observed. CeReS-18 clearly abrogated the mitogenic activity that these growth factors elicited with human mammary carcinoma cells.

CeReS-18, a novel cell surface sialoglycopeptide, induces cell cycle arrest and apoptosis in a calcium-sensitive manner

Very few growth inhibitors have been identified which can inhibit the proliferation of a broad spectrum of human breast cancer cell lines. CeReS-18, a novel cell surface sialoglycopeptide growth inhibitor, can reversibly inhibit the proliferation of both estrogen receptor positive (MCF-7) and negative (BT-20) human breast cancer cell lines. In addition, at concentrations above those required for the reversible inhibition of cell proliferation, CeReS-18 can also induce cell death in MCF-7 cells. Changes in nuclear and cytoplasmic morphology, characteristic of apoptosis, were detected in MCF-7 cells treated with a cytotoxic concentration of CeReS-18, and internucleosomal DNA cleavage was also observed. The sensitivity of MCF-7 and BT-20 cells to the biological properties of CeReS-18 could be influenced by altering the calcium concentration in the extracellular growth medium, such that when the calcium concentration in the environment was decreased, and increased sensitivity to CeReS-18-induced growth inhibition and cytotoxicity were observed. The addition of the calcium chelating agent EGTA to MCF-7 cells, cultured in a normal calcium environment, could mimic the increased sensitivity to the biological effects of CeReS-18 observed under reduced calcium conditions.

Eicosanoid inhibitors enhance synergistically the effect of transforming growth factor beta 1 on CCL 64 cell proliferation

The interactions between drugs suppressing the production of arachidonic acid metabolites-eicosanoids and transforming growth factor beta 1 (TGF-beta 1) were investigated using CCL64 cells. These experiments, designed as complete factorial combination of treatments, demonstrated that both esculetin and eicosatetraynoic acid significantly potentiated the inhibitory effect of TGF-beta 1 on [3H]thymidine incorporation. The expression of overadditive effects depended both on the type and concentration of combined factors. These results corresponded with cell cycle analysis data (increased cell number in G1 and decreased cell number in S and G2/M phases) and with the results monitoring cell number following treatment with eicosatetraynoic acid, esculetin, 3-[1-(4-chlorobenzyl)-3-t-butyl-thio-5-isopropylindol-2-yl]-2,2-di methyl propanoic acid (MK-886) and indomethacin. Summarizing, the degree of significance of combined effects supports the hypothesis of synergistic potentiation of TGF-beta 1 effects caused by eicosanoid inhibitors. The results indicate that either the lack of some eicosanoids or a certain type of misbalance in the metabolism of arachidonic acid leading to its abundance might modulate TGF-beta 1 effects on the cell cycle and proliferation in CCL64 cells.

Purification and stability characterization of a cell regulatory sialoglycopeptide inhibitor

Previous attempts to physically separate the cell cycle inhibitory and protease activities in preparations of a purified cell regulatory sialoglycopeptide (CeReS) inhibitor were largely unsuccessful. Gradient elution of the inhibitor preparation from a DEAE HPLC column separated the cell growth inhibitor from the protease, and the two activities have been shown to be distinct and non-overlapping. The additional purification increased the specific biological activity of the CeReS preparation by approximately two-fold. The major inhibitory fraction that eluted from the DEAE column was further analyzed by tricine-SDS-PAGE and microbore reverse phase HPLC and shown to be homogeneous in nature. Two other fractions separated by DEAE HPLC, also devoid of protease activity, were shown to be inhibitory to cell proliferation and most likely represented modified relatives of the CeReS inhibitor. The highly purified CeReS was chemically characterized for amino acid and carbohydrate composition and the role of the carbohydrate in cell proliferation inhibition, stability, and protease resistance was assessed.

Role of calcium in growth inhibition induced by a novel cell surface sialoglycopeptide

Our laboratory has purified an 18 kDa cell surface sialoglycopeptide growth inhibitor (CeReS-18) from intact bovine cerebral cortex cells. Evidence presented here demonstrates that sensitivity to CeReS-18-induced growth inhibition in BALB-c 3T3 cells is influenced by calcium, such that a decrease in the calcium concentration in the growth medium results in an increase in sensitivity to CeReS-18. Calcium did not alter CeReS-18 binding to its cell surface receptor and CeReS-18 does not bind calcium directly. Addition of calcium, but not magnesium, to CeReS-18-inhibited 3T3 cells results in reentry into the cell cycle. A greater than 3-hour exposure to increased calcium is required for escape from CeReS-18-induced growth inhibition. The calcium ionophore ionomycin could partially mimic the effect of increasing extracellular calcium, but thapsigargin was ineffective in inducing escape from growth inhibition. Increasing extracellular calcium 10-fold resulted in an approximately 7-fold increase in total cell-associated 45Ca+2, while free intracellular calcium only increased approximately 30%. However, addition of CeReS-18 did not affect total cell-associated calcium or the increase in total cell-associated calcium observed with an increase in extracellular calcium. Serum addition induced mobilization of intracellular calcium and influx across the plasma membrane in 3T3 cells, and pretreatment of 3T3 cells with CeReS-18 appeared to inhibit these calcium mobilization events. These results suggest that a calcium-sensitive step exists in the recovery from CeReS-18-induced growth inhibition. CeReS-18 may inhibit cell proliferation through a novel mechanism involving altering the intracellular calcium mobilization/regulation necessary for cell cycle progression.

Calcium influences sensitivity to growth inhibition induced by a cell surface sialoglycopeptide

While studies concerning mitogenic factors have been an important area of research for many years, much less is understood about the mechanisms of action of cell surface growth inhibitors. We have purified an 18 kDa cell surface sialoglycopeptide growth inhibitor (CeReS-18) which can reversibly inhibit the proliferation of diverse cell types. The studies discussed in this article show that three mouse keratinocyte cell lines exhibit sixty-fold greater sensitivity than other fibroblasts and epithelial-like cells to CeReS-18-induced growth inhibition. Growth inhibition induced by CeReS-18 treatment is a reversible process, and the three mouse keratinocyte cell lines exhibited either single or multiple cell cycle arrest points, although a predominantly G0/G1 cell cycle arrest point was exhibited in Swiss 3T3 fibroblasts. The sensitivity of the mouse keratinocyte cell lines to CeReS-18-induced growth inhibition was not affected by the degree of tumorigenic progression in the cell lines and was not due to differences in CeReS-18 binding affinity or number of cell surface receptors per cell. However, the sensitivity of both murine fibroblasts and keratinocytes could be altered by changing the extracellular calcium concentration, such that increased extracellular calcium concentrations resulted in decreased sensitivity to CeReS-18-induced proliferation inhibition. Thus the increased sensitivity of the murine keratinocyte cell lines to CeReS-18 could be ascribed to the low calcium concentration used in their propagation. Studies are currently under way investigating the role of calcium in CeReS-18-induced growth arrest. The CeReS-18 may serve as a very useful tool to study negative growth control and the signal transduction events associated with cell cycling.