Structural determinants of the capacity of heparin to inhibit the proliferation of vascular smooth muscle cells. II. Evidence for a pentasaccharide sequence that contains a 3-O-sulfate group

Earlier work from our laboratory demonstrated that heparin inhibited the proliferation of vascular smooth muscle cells in vivo and in vitro. Both anticoagulant and non-anticoagulant heparin species were equally effective as antiproliferative agents. Previous structure-function studies indicated that hexasaccharide and larger fragments retained antiproliferative activity, whereas tetra- and disaccharides were inactive. These experiments also suggested that both N- and O-sulfates of heparin were necessary for growth inhibitory capacity. In this paper, we have further analyzed the structural determinants of the antiproliferative activity of heparin. These experiments were done using synthetically prepared and therefore chemically defined heparin oligosaccharides. We present evidence that a pentasaccharide fragment retains antiproliferative activity, and that the 3-O-sulfate on the internal glucosamine residue is critical for growth inhibitory capacity of the pentasaccharide. We also show that heparins obtained from different manufacturers differ significantly in their ability to suppress smooth muscle cell proliferation.

Inhibition of rat cervical epithelial cell growth by heparin and its reversal by EGF

The effects of heparin on the in vitro growth of rat cervical epithelial cells were examined. Heparin was found to inhibit in a dose dependent fashion the log-phase growth of rat cervical epithelial cells (RCEC) grown in the absence of medium supplements. An inhibition of growth is observed at concentrations as low as 500 ng/ml and 50% inhibition of growth occurs at a concentration of 5 micrograms/ml. The growth inhibitory activity of heparin is independent of anticoagulant activity since three separate non-anticoagulant preparations of heparin all inhibit growth. Other glycosaminoglycans including chondroitin 4-sulfate, chondroitin 6-sulfate, dermatan sulfate, hyaluronic acid, and keratin sulfate do not inhibit the growth of rat cervical epithelial cells. The ability of heparin to inhibit the log-phase growth of rat cervical epithelial cells is dependent on the composition of the medium in which the cells are grown. The addition of greater than or equal to 7.5 ng/ml epidermal growth factor to epithelial cultures blocks the growth inhibitory activity of heparin. These results suggest that components of the extracellular matrix modulate the growth responses of epithelial cells and may be important in regulating cellular proliferation in normal and pathological states.

Heparin and glomerular epithelial cell-secreted heparin-like species inhibit mesangial-cell proliferation

The regulation of cell growth in the kidney glomerulus plays a key role in many physiologic and pathologic processes. In this communication, the authors examine the possible role of heparin-like species as inhibitors of mesangial-cell proliferation. Heparin profoundly inhibited the growth of cultured mesangial cells in a dose-dependent manner, with an ED50 = 5-10 micrograms/ml. The antiproliferative activity of heparin was reversible and specific for mesangial cells as the target cell in the glomerulus. Heparin was much more effective than other glycosaminoglycans. Cultured glomerular epithelial cells were found to secrete both stimulators and inhibitors of mesangial-cell growth. Approximately half of the inhibitory activity was destroyed by a highly purified heparinase; the other half was sensitive to trypsin. Approximately 80% of the mitogenic activity was protease-sensitive. These results suggest that heparin and glomerular epithelial cells may participate in mesangial-cell growth regulation.

Binding and internalization of heparin by vascular smooth muscle cells

Previous work from our laboratory has demonstrated that heparin specifically inhibits the proliferation of vascular smooth muscle cells in vivo and in vitro. In this paper, we examine the binding and mode of internalization of heparin by smooth muscle cells. For these studies, radiolabeled and fluoresceinated (FITC) heparin probes were synthesized that retained their antiproliferative capacity. Binding of 3H-heparin to these cells occurs via specific, high-affinity binding sites (Kd = 10(-9) M, 100,000 binding sites per cell). Approximately 80% of the heparin bound to the cell surface was shed into the culture medium within 2 hr. The heparin that was left on the cell surface was internalized with biphasic kinetics. Approximately 50% of the bound material was internalized within 2 hr. After this initial rapid uptake, the rate slowed substantially, with the remaining heparin requiring 1-2 days to be internalized. Binding and uptake of FITC heparin was monitored using video image intensification fluorescence microscopy. When smooth muscle cells were exposed to FITC heparin at 4 degrees C, a diffuse surface staining pattern was observed. After warming the cells to 37 degrees C, intensely fluorescent vesicles were seen superimposed over the diffuse surface staining within 2 min. After 15 min at 37 degrees C, numerous large punctate vesicles were seen inside the cell. After 2 hr these vesicles had concentrated in the perinuclear region. This pattern of uptake, when considered along with the presence of specific, high-affinity binding sites and the initial rapid uptake of 3H-heparin, suggests that heparin enters smooth muscle cells by both receptor-mediated and other endocytic pathways.

Effect of heparin on vascular smooth muscle cells. I. Cell metabolism

Previous work from our laboratory has shown that heparin inhibits the proliferation of vascular smooth muscle cells in vivo and in vitro. The mechanism of action of this glycosaminoglycan is unknown. In this communication, we have examined the antiproliferative effect of heparin on smooth muscle and other cell types, and have investigated several aspects of heparin on smooth muscle cell metabolism. Smooth muscle and closely related cell types from several species, including human, were much more sensitive to heparin than any other cell type tested, including primary and established cell lines, normal and transformed cell pairs, fibroblasts, epithelial, and endothelial cells. Flow microfluorimetric analysis of cell cycle distribution indicated that heparin blocked either the G0----S transition or a very early S-phase event in smooth muscle cells. Heparin rapidly inhibited DNA and RNA synthesis, but did not affect the rate of protein synthesis. The decrease in nucleic acid synthesis could be accounted for by an inhibition of thymidine and uridine uptake. Interestingly, heparin did not block amino acid or glucose transport. Although no change in the overall rate of protein synthesis was observed in the presence of heparin, we noted at least two changes in the synthesis of specific proteins by smooth muscle cells: two 35,000-dalton proteins which appeared in the culture medium of heparin-treated cells, and the transient disappearance of a 48,000-dalton protein in the substrate attached material of smooth muscle cells exposed to heparin. The role of the observed changes in smooth muscle cell metabolism is yet to be determined, but they may provide valuable clues to the molecular mechanisms controlling the antiproliferative activity of heparin.

Effect of heparin on vascular smooth muscle cells. II. Specific protein synthesis

Heparin suppresses the proliferation of vascular smooth muscle cells both in vivo and in vitro. The mechanism of action of the antiproliferative activity of heparin is not known. We have detected differences in the synthesis of specific proteins when vascular smooth muscle cells are exposed to heparin and report here that many characteristics of these protein alterations parallel the properties of the antiproliferative activity. The induction into the culture medium of a pair of proteins of approximately 35,000 dalton mw in heparin-treated smooth muscle cell cultures and the antiproliferative effect of heparin share the following characteristics: 1) the effect is reversible, 2) the effect is specific for smooth muscle cells, 3) anticoagulant and non-anticoagulant heparin are equally effective, 4) the effect is lost with time in culture and, 5) heparin is the most potent glycosaminoglycan in producing the effect. Furthermore, heparin causes a transient suppression of a 48,000 dalton substrate-attached protein, whereas chondroitin sulfate A and C and dermatan sulfate had much less effect. Dextran sulfate was almost as effective as heparin in suppressing the synthesis of the substrate-attached protein. These proteins appear to be noncollagenous and the induced synthesis of the 35,000 dalton proteins is inhibited by actinomycin D. Although a direct relationship between these specific protein changes and the antiproliferative effect of heparin has not been proven, these protein alterations may play a crucial role in the effect of heparin on smooth muscle cell growth.

Structural determinants of the capacity of heparin to inhibit the proliferation of vascular smooth muscle cells

Previous work from our laboratory has demonstrated that both anticoagulant and nonanticoagulant heparin species can inhibit the proliferation of vascular smooth muscle cells in vivo and in vitro. In this communication, we report studies on the structure-function relationships of heparin to its antiproliferative effect on vascular smooth muscle cells. These structure-function studies were carried out by preparing discrete sizes of heparin fragments and by chemically modifying heparin. The compounds were tested for their ability to inhibit rat and calf aortic smooth muscle cell growth. The minimum fragment size which retains some growth inhibitory activity is a hexasaccharide; maximal antiproliferative activity was obtained with dodecasaccharide and larger fragments. Both O-sulfation and N-substitution were found to be important for the growth inhibitory effect. Comparison of the antiproliferative and anticoagulant activities of the different heparin species has allowed us to identify several heparin molecules which have lost their anticoagulant properties, but retain antiproliferative activity.

Inhibition of vascular smooth muscle cell growth by endothelial cell-derived heparin. Possible role of a platelet endoglycosidase

Bovine aortic endothelial cells release a heparin-like substance in the presence of 0.4% fetal calf serum. This substance inhibited the growth of smooth muscle cells in vitro by about 70%. Substitution of platelet-poor plasma for serum resulted in minimal liberation of inhibitory activity from the cells unless at least 10-fold higher concentrations of platelet-poor plasma were utilized. This suggested that a platelet product was involved in the release process. Therefore, we examined the ability of the platelet heparitinase described in the preceding communication to release heparin-like species from cultured endothelial cells. Our results show that when endothelial cells were exposed to serum-free medium containing 1 ng/ml of the purified platelet endoglycosidase, at least as much inhibitory activity was released as was obtained with 0.4% serum. Dose response experiments indicated that only 10 pg/ml of the enzyme were necessary to liberate 50% of the inhibitory activity from endothelial cells. The heparin-like nature of the inhibitory substance was demonstrated by its sensitivity to Flavobacterium heparinase. Utilizing appropriate controls, the release of heparin-like material by the endoglycosidase was shown to be enzyme-specific and was not due to artifacts of experimental manipulations. In addition, this enzyme did not convert prereleased material to an active component, but directly liberated the active heparin-like species from endothelial cells. A simple model describing the possible role of heparin-like components and the endoglycosidase in the normal and injured wall is presented.

Differentiation-dependent stimulation of neovascularization and endothelial cell chemotaxis by 3T3 adipocytes

3T3 cells that have undergone adipose differentiation in vitro secrete factor(s) that stimulate angiogenesis (neovascularization) in vivo. When medium containing 0.5% fetal calf serum was conditioned by 3T3-F442A adipocytes, it stimulated angiogenesis when placed on the chicken chorioallantoic membrane. Control medium or medium conditioned by preadipocytes did not stimulate angiogenesis, even at much higher doses. Thus, the production of the angiogenic activity is strongly dependent upon differentiation of the adipocytes. The degree of the angiogenic response to adipocyte-conditioned medium was potentiated by heparin; heparin added to unconditioned medium or to preadipocyte-conditioned medium was not angiogenic. The adipocyte-conditioned medium also strongly stimulated, in a differentiation-dependent fashion, the motility of aortic and capillary endothelial cells in a modified Boyden chamber assay. Checkerboard analysis cells indicated that 75% of the motility-stimulating activity was chemotactic in nature. The chemotactic activity has an apparent specificity for endothelial cells, in that chemotaxis of smooth muscle cells and fibroblasts was stimulated to a much lesser extent. These results, in conjunction with our previous demonstration of an endothelial cell mitogen produced by 3T3 adipocytes [Castellot, J. J., Jr., Karnovsky, M. J. & Spiegelman, B. M. (1980) Proc. Natl. Acad. Sci. USA 77, 6007-6011] indicate that the differentiation of these cells is closely linked to the production of factors that stimulate angiogenesis in vivo and growth and chemotaxis of endothelial cells in vitro.

Cultured endothelial cells produce a heparinlike inhibitor of smooth muscle cell growth

Using cultured cells from bovine and rat aortas, we have examined the possibility that endothelial cells might regulate the growth of vascular smooth muscle cells. Conditioned medium from confluent bovine aortic endothelial cells inhibited the proliferation of growth-arrested smooth muscle cells. Conditioned medium from exponential endothelial cells, and from exponential or confluent smooth muscle cells and fibroblasts, did not inhibit smooth muscle cell growth. Conditioned medium from confluent endothelial cells did not inhibit the growth of endothelial cells or fibroblasts. In addition to the apparent specificity of both the producer and target cell, the inhibitory activity was heat stable and not affected by proteases. It was sensitive flavobacterium heparinase but not to hyaluronidase or chondroitin sulfate ABC lyase. It thus appears to be a heparinlike substance. Two other lines of evidence support this conclusion. First, a crude isolate of glycosaminoglycans (TCA-soluble, ethanol-precipitable material) from endothelial cell-conditioned medium reconstituted in 20 percent serum inhibited smooth muscle cell growth; glycosaminoglycans isolated from unconditioned medium (i.e., 0.4 percent serum) had no effect on smooth muscle cell growth. No inhibition was seen if the glycosaminoglycan preparation was treated with heparinase. Second, exogenous heparin, heparin sulfate, chondroitin sulfate B (dermatan sulfate), chondroitin sulfate ABC, and hyaluronic acid were added to 20 percent serum and tested for their ability to inhibit smooth muscle cell growth. Heparin inhibited growth at concentrations as low as 10 ng/ml. Other glycosaminoglycans had no effect at doses up to 10 mug/ml. Anticoagulant and non- anticoagulant heparin were equally effective at inhibiting smooth muscle cell growth, as they were in vivo following endothelial injury (Clowes and Karnovsk. Nature (Lond.). 265:625-626, 1977; Guyton et al. Circ. Res. 46:625-634, 1980), and in vitro following exposure of smooth muscle cells to platelet extract (Hoover et al. Circ. Res. 47:578-583, 1980). We suggest that vascular endothelial cells may secrete a heparinlike substance in vivo which may regulate the growth of underlying smooth muscle cells.

Potent stimulation of vascular endothelial cell growth by differentiated 3T3 adipocytes

3T3 cells that have undergone adipose differentiation in vitro secrete into the culture medium a potent growth stimulatory activity for bovine aortic endothelial cells. When medium containing 2% fetal calf serum, which does not support significant endothelial cell growth, is conditioned by 3T3-F442A adipocytes, the endothelial cells grow rapidly (doubling time, 24 hr) at a rate equal to the growth rate in 20% fetal calf serum. The potency of the conditioned medium is further shown by the fact that it can be diluted 1:5 with little apparent loss of activity and shows a half-maximal stimulation at 10 microliter/ml. Serum is not required for either the secretion of this mitogen by the adipocytes or its action on the endothelial cells, as shown by the fact that the latter are stimulated to divide in serum-free medium conditioned by the adipocytes. The growth stimulatory activity appears to be specific for vascular endothelial cells in that no other cell type examined, including vascular smooth muscle cells and pericytes, are significantly stimulated by medium conditioned by 3T3-F442A cells. Similarly, medium conditioned by no other cell type examined has more than 10% of the activity of medium conditioned by the adipocytes. The specificity and potency of the adipocyte-derived factor suggest that it may play a role in the vascularization of this tissue during development. Preliminary biochemical analysis indicates that the adipocyte factor is nondialyzable and is not inactivated by heat or proteases. The protease insensitivity distinguishes the adipocyte growth stimulatory activity from the low levels of activity secreted by fibroblasts and preadipocytes, suggesting that the adipocyte mitogen is a product specifically related to the differentiation process.

Animal cells reversibly permeable to small molecules

A cell preparation, useful for studying the regulation of metabolism, was developed by making monolayer baby hamster kidney cells permeable. Hypertonically treated cells were permeable to nucleotides, but retained their gross cellular morphology, intact organelles, 100% of their DNA, and 91% of their total protein. The permeable cell synthesized DNA, RNA, and protein rapidly when supplied with the appropriate substrates and cofactors. They either could remain permeable or were able to "reseal" when replaced in complete medium under appropriate conditions. Optimal conditions for DNA synthesis were established for permeable cells, giving rates equal to those of intact cells. Replication rather than repair was shown by the cell-cycle dependence of DNA synthesis and its discontinuous nature. Ribonucleotide reductase was active in permeable cells, permitting equal rates of DNA synthesis when ribonucleotide diphosphates or deoxyribonucleotide triphosphates were provided. Hydroxyurea did not inhibit DNA synthesis in permeable cells supplied with deoxyribonucleotide di- or triphosphates, but drastically inhibited DNA synthesis when ribonucleotide diphosphates were supplied. Hydroxyurea is therefore primarily an inhibitor of ribonucleotide reductase. Permeability was reversed, exposing permeable cells to [(3)H]thymidine triphosphate, which was incorporated, which labeled nuclei of cells that went on to mitosis. The reversible permeability procedure should prove especially useful in studying the functions of poorly penetrating compounds, such as drugs. Intact cells were unaffected by cytosine arabinoside triphosphate, while cells that had been made permeable and resealed were killed.