Activation of mammalian procollagenase and basement membrane-degrading enzymes by a low molecular weight angiogenesis factor

Factors involved in capillary growth in the heart

Growth of capillaries in the heart occurs under physiological circumstances during endurance exercise training, exposure to high altitude and/or cold, and changes in cardiac metabolism or heart rate elicited by modification of thyroid hormone levels. Capillary growth in all these conditions can be linked with increased coronary blood flow, decreased heart rate, or both. This paper brings evidence that, although increased blood flow due to long-term administration of coronary vasodilators results in capillary growth, a long-term decrease in heart rate induced by electrical bradycardial pacing in rabbits and pigs, or by chronic administration of a bradycardic drug, alinidine, in rats, stimulates capillary growth with little or no change in coronary blood flow. Decreased heart rate results in increased capillary wall tension, increased end-diastolic volume and increased force of contraction, and thus stretch of the capillary wall. This could lead to release of various growth factors possibly stored in the capillary basement membrane. Correlation was found between capillary density (CD) and the levels of low molecular endothelial cell stimulating angiogenic factor (ESAF) both in rabbit and pig hearts with CD increased by pacing. There was no relation between expression of mRNA for basic fibroblast growth factor and CD in sham-operated and paced rabbit hearts. In contrast, mRNA for TGF beta was increased in paced hearts, and the possible role of this factor in the regulation of capillary growth induced by bradycardia is discussed.

Low molecular weight angiogenesis factors

A number of substances have been proposed for the role of angiogenesis factors. Many of these are of protein origin and are therefore amenable to the tools of the molecular biologist. However a number of low molecular weight angiogenesis factors are emerging as important initiators and/or cofactors of neovascularization. Of these a number are known to stimulate angiogenesis indirectly, possibly through an inflammatory response. Some putative angiogenic factors stimulate microvessel endothelial cells nonspecifically, also causing migration and proliferation of large vessel cells. Others are specific for microvessel cells either for stimulating migration, proliferation or both. The nature and action of the low molecular weight factors in vivo and in vitro are reviewed.

Induction of endothelial cell migration by proline analogs and its relevance to angiogenesis

The proline analogs cis-4-hydroxy-L-proline, 3,4-dehydro-L-proline, and 2-azetidinecarboxylic acid induced increases in the migration rate of retinal capillary endothelial cells, aortal endothelial cells, corneal endothelial cells, aortal smooth muscle cells, and retinal pericytes. cis-4-hydroxy-D-proline did not. The optimal concentration for migration induction by any of the active agents was approximately 10(-5) M. At higher concentrations (5 x 10(-4) M) migration was not induced or was inhibited. When tested by subcutaneous implant assays in rabbits, cis-4-hydroxy-L-proline and 2-azetidinecarboxylic acid consistently elicited a marked angiogenic response. Whereas these compounds are known to modulate collagen synthesis and secretion, the concentrations at which they are effective inducers of migration suggest that they may have a more specific target than general collagen synthesis.

Matrix control of tumor angiogenesis

Endothelial cell migration is a key feature of angiogenesis. Epidermal Growth Factor (EGF) or Tumor Angiogenesis Factor (TAF) induce cell migration and angiogenesis. When the matrix components, collagen or fibronectin, were used as a substratum in the phagokinesis assays, EGF- or TAF-induced cell migration was inhibited. It has been proposed that TAF activates cellular protease causing the matrix degradation that is evident during neovascularization in vitro. If such degradation leads to cell migration and angiogenesis, then other agents that interfere with the synthesis or assembly of matrix components should stimulate cell migration and angiogenesis. The proline analogues cis hydroxyproline, azetidine and dehydroproline are known modulators of cellular collagen synthesis. At optimal concentration (10(-5)M) these analogues caused 3-fold increases in endothelial cell migration rates in vivo as tested by a subcutaneous implant assay. We conclude from these studies that: (i) matrix components control cellular migration rates; high concentration of collagen or fibronectin inhibit angiogenically active inducers of endothelial cell migration. (ii) Intracellular modulation of synthesis of collagens leads to angiogenesis by stimulating cell migration. These findings relate to tumor angiogenesis and that TAF might trigger angiogenesis either by activation of latent proteases or by some modification of matrix assembly during synthesis that affects cell adhesion and migration.

A synergistic effect on microvessel cell proliferation between basic fibroblast growth factor (FGFb) and endothelial cell stimulating angiogenesis factor (ESAF)

Stimulation of microvessel endothelial cells grown on collagen gels by endothelial cell stimulating angiogenesis factor was confirmed (1). The potent endothelial cell growth stimulating activity of basic fibroblast growth factor has also been demonstrated for cells grown on collagen gels. However, the growth factor activity of basic fibroblast growth factor towards microvessel endothelial cells was almost eliminated when experiments were carried out in the presence of diafiltered foetal calf serum, but was partially restored by the addition of endothelial cell stimulating angiogenesis factor to medium containing the diafiltered serum. In contrast to these observations, foetal skin fibroblasts were stimulated by basic fibroblast growth factor when grown in medium with diafiltered serum, and this stimulation was not modified by the presence of endothelial cell stimulating angiogenesis factor.