Glucose inhibits replication of cultured human endothelial cells

Cell biology as an approach to the study of the vascular complications of diabetes

Diabetic retinopathy and macroangiopathy share the common histologic features of cellular injury and proliferation. We have studied factors in the diabetic milieu that may affect the metabolism and growth of vascular cells derived from retinal capillaries and arteries. Using cultured endothelial cells from retinal capillaries and human aorta and vascular supporting cells (pericytes and aortic smooth muscle cells), we found that the metabolism and proliferation of endothelial cells from capillaries are responsive to insulin and insulin like growth factors (IGFs). At concentrations ranging from below physiologic levels to physiologic, insulin and IGFs were additive in their effect on growth of endothelial cells from retinal capillaries. Endothelial cells from arteries were much less responsive to insulin and IGFs, although like capillary endothelium they also have high affinity receptors for these hormones. No differences in response to insulin and IGFs were observed in the vascular supporting cells. Both pericytes and arterial smooth muscle cells responded to insulin and IGFs. The effect of insulin and IGFs was also additive with other growth factors tested, including platelet-derived growth factor. Hyperglycemia inhibited the growth of capillary pericytes but had no effect on other vascular cells. The results of these studies provide us with a potential explanation for the different histologic features observed in diabetic microangiopathy and macroangiopathy.

Binding, degradation, and biological activity of insulin in vascular smooth muscle cells

The interaction of insulin with the vascular smooth muscle was studied using cultures derived from the bovine aortic arch. The cultured cells exhibited specific binding of 125I-insulin that was reversible and dependent on pH. Both insulin and insulinlike growth factor (IGF) I competed for 125I-insulin binding; IGF I, however, was less effective than insulin by at least an order of magnitude. Insulin binding was accompanied by internalization and degradation of the hormone in a temperature- and time-dependent manner. Chloroquine and other lysosomotropic agents elevated the internalized insulin and reduced its degradation. Pre-exposure of cell cultures to insulin resulted in downregulation of cell surface receptors. Insulin stimulated alpha-aminoisobutyric acid transport in confluent smooth muscle cells. The maximal response was observed at 100 ng/ml insulin with a half-maximal effect at 10 ng/ml. Sparse, serum-starved smooth muscle cells responded to insulin with a dose-dependent increase in [3H]-thymidine incorporation into DNA. Although the effect was already apparent at 1 ng/ml insulin, it reached near maximal level only at 10,000 ng/ml. IGF I also stimulated DNA synthesis in smooth muscle cells; however, at low concentrations insulin was more efficient in this respect. Human growth hormone was inactive. The data indicate the presence of specific receptors for insulin in bovine aortic smooth muscle cells. These receptors appear to mediate the metabolic activity as well as part of the mitogenic effect of insulin in these cells.