Binding of recombinant variants of human tissue-type plasminogen activator (t-PA) to human umbilical vein endothelial cells

Binding of mutant tissue-type plasminogen activators to human endothelial cells and their extracellular matrix

We previously demonstrated that tissue-type plasminogen activator (t-PA) specifically bound to its receptor (t-PAR) on human umbilical vein endothelial cells (HUVEC). In addition to analyses of t-PA binding to plasminogen activator inhibitor-1 (PAI-1) in the extracellular matrix (ECM) and to the t-PAR, we further evaluated the binding of three t-PA mutants, deltaFE1X t-PA lacking finger (F), epidermal growth factor-like (E) domains and one sugar chain at Asn177 thus comprising two kringles (K1 and K2) and protease (P) domains, deltaFE3X t-PA with three glycosylation sites deleted at Asn117, 184, and 448, and deltaFEK1 t-PA comprising K2 and P domains without glycosylation. Wild-type t-PA bound to ECM with high affinity, which was completely blocked by anti-PAI-1 IgG. Wild-type t-PA, deltaFE1X t-PA and deltaFEK1 t-PA bound to two classes of binding sites with high and low affinities on monolayer HUVEC. However, all t-PAs bound to a single class of binding site in the presence of anti-PAI-1 IgG. DeltaFEK1 t-PA bound t-PAR maximally among these t-PAs. These results suggested that the high affinity binding of t-PA mainly occurred with PAI-1 on ECM while the low affinity binding was with t-PAR. The deletion of F, E domains and sugar chains had no effect on binding with t-PAR. However, since only K1-missing t-PA (deltaFEK1) exhibited significantly increased binding sites among these t-PAs, it was suggested that the binding to t-PAR was mediated mainly by K2 domain and that the increase of binding was due to direct exposure of K2 domain.

Regulation of tissue plasminogen activator activity by cells. Domains responsible for binding and mechanism of stimulation

A number of cell types have previously been shown to bind tissue plasminogen activator (tPA), which in some cases can remain active on the cell surface resulting in enhanced plasminogen activation kinetics. We have investigated several cultured cell lines, U937, THP1, K562, Molt4, and Nalm6 and shown that they bind both tPA and plasminogen and are able to act as promoters of plasminogen activation in kinetic assays. To understand what structural features of tPA are involved in cell surface interactions, we performed kinetic assays with a range of tPA domain deletion mutants consisting of full-length glycosylated and nonglycosylated tPA (F-G-K1-K2-P), DeltaFtPA (G-K1-K2-P), K2-P tPA (BM 06.022 or Reteplase), and protease domain (P). Deletion variants were made in Escherichia coli and were nonglycosylated. Plasminogen activation rates were compared with and without cells, over a range of cell densities at physiological tPA concentrations, and produced maximum levels of stimulation up to 80-fold with full-length, glycosylated tPA. Stimulation for nonglycosylated full-length tPA dropped to 45-60% of this value. Loss of N-terminal domains as in DeltaFtPA and K2P resulted in a further loss of stimulation to 15-30% of the full-length glycosylated value. The protease domain alone was stimulated at very low levels of up to 2-fold. Thus, a number of different sites are involved in cell interactions especially within finger and kringle domains, which is similar to the regulation of tPA activity by fibrin. A model was developed to explain the mechanism of stimulation and compared with actual data collected with varying cell, plasminogen, or tPA concentrations and different tPA variants. Experimental data and model predictions were generally in good agreement and suggest that stimulation is well explained by the concentration of reactants by cells.

Release and preparation of intact and unreduced N-linked oligosaccharides from Sf-9 insect cells

Glycosylation, the addition of carbohydrates to a peptide backbone, is the most extensive cotranslational and posttranslational modification made to proteins by eukaryotic cells. The glycosylation profile of a recombinant glycoprotein can significantly affect its biological activity, which is particularly important when being used in human therapeutic applications. Therefore, defining glycan structures to ensure consistency of recombinant glycoproteins among different batches is critical. In this study we describe a method to prepare N-linked glycans derived from insect cell glycoproteins for structural analysis by capillary electrophoresis. Briefly, glycoproteins obtained from uninfected Spodoptera frugiperda Sf-9 insect cells were precipitated with ammonium sulfate and the glycans were chemically cleaved by hydrazinolysis. Following the regeneration of the glycan reducing terminal residue and the removal of contaminating proteins and peptides, the glycans were fluorescently labeled by reductive amination. Fluorescent labeling greatly enhanced the detection limit of the glycan structures determined by capillary electrophoresis. Five major glycan structures were found that migrated between tetra-mannosylated hexasaccharide and nonamannosylated undecasaccharide standards. Upon alpha-mannosidase digestion the number of glycan structures was reduced to two major structures with shorter migration times than the undigested glycans. None of the glycans were susceptible to hexosaminidase or galactosidase treatment. These results are consistent with the majority of previous results demonstrating hypermannosylated glycan structures in Sf-9 insect cells.