Purpose: To explore whether, following direct contact, there is mutual influence between pericytes (PC) and endothelial cells (EC), and to establish whether protein kinase C (PKC) activation, a condition associated with hyperglycemia, can affect, via the mRNA-binding Hu-antigen R (HuR)/ELAV protein, the expression of vascular endothelial growth factor (VEGF).
Methods: PC and EC were cultured separately or in direct contact (1:1 ratio), and exposed or not to phorbol esters, a PKC activator (100 nM for 15 min). Barrier integrity was evaluated by measuring endothelial electrical resistance and permeability to sodium fluorescein. Immunocytochemistry was performed to visualize EC and PC in coculture, and to evaluate phorbol 12-myristate-13-acetate (PMA)-induced HuR translocation. PKCβI/βII, HuR, and VEGF protein content was measured with western blotting, VEGF secretion in cell culture medium was evaluated with enzyme-linked immunosorbent assay (ELISA), and quantification of VEGF mRNA was performed with real-time quantitative PCR.
Results: In monocultures, VEGF mRNA/protein basal levels were more elevated in PC than in EC. However, the basal expression of VEGF protein, but not mRNA, in PC and EC was affected by culture conditions. In fact, physical contact with PC upregulated VEGF protein levels in the EC, while VEGF was downregulated in PC cocultured with EC. In this last condition, PKCβII and HuR protein basal levels were also decreased in monocultured PC. Moreover, in basal conditions, the amount of VEGF released from the coculture was higher than from the monocultures. Direct activation of PKCβ induced HuR translocation from the nuclear area to the cytoplasm, and increased the protein levels of the kinase itself, HuR, and VEGF in PC and EC in both culture conditions. Concerning VEGF mRNA, PKC activation induced an increase in PKC levels only in monocultured EC and, conversely, a significant decrease in the same transcript amount in cocultured PC. PMA stimulus also led to a significant increase in VEGF secretion in coculture.
Conclusions: When cocultured with PC, EC form a significantly tighter barrier than the endothelial monolayer. The physical contact leads to opposite changes in VEGF protein levels in PC and EC. In particular, in basal conditions, cocultured PC seemed to downregulate their own expression of this proproliferating factor, as well as that of PKCβII and HuR, likely to maintain the 1:1 ratio with the cohabiting EC. In mono- and cocultured PC/EC, PKC direct activation led to a similar increase in PKCβI/βII, HuR, and VEGF protein levels, changes that may also occur at early stages of diabetic retinopathy. The release of VEGF in the medium was favored by physical contact between PC and EC and was further increased by PMA exposure. In contrast with the effects on VEGF protein, PKCβ activation induced modifications in VEGF mRNA content that are different in function of the cell type and the culture conditions. These findings suggest that the changes in the VEGF protein and transcript observed in PC/EC can be ascribed to distinct and concomitant pathways. Further studies on this in vitro coculture model would be useful to better understand the PC/EC interaction in physiologic and pathological conditions.