Endocytosis of a chimera between human pro‐urokinase and the plant toxin saporin: an unusual internalization mechanism

Abstract

A fluorescent derivative of a chimeric toxin between human pro-urokinase and the plant ribosome-inactivating protein saporin (p-uPA-Sap(TRITC)), has been prepared in order to study the endocytosis of this potentially antimetastatic conjugate in the murine model cell line LB6 clone19 (Cl19) transfected with the human urokinase receptor gene. The physiological internalization of urokinase-inhibitor complexes is triggered by the interaction of plasminogen inhibitors (PAIs) with receptors belonging to the low density lipoprotein-related receptor protein (LRP) family, and involves a macro-quaternary structure including uPAR, LRP, and PAIs. However, in contrast to this mechanism, we observed a two-step process: first, the urokinase receptor (uPAR) acts as the anchoring factor on the plasma membrane; subsequently, LRP acts as the endocytic trigger. Once the chimera is bound to the plasma membrane by interaction with uPAR, we suggest that a possible exchange may occur to transfer the toxin to LRP via the saporin moiety and begin the internalization. So an unusual endocytic process is described, where the toxin enters the cell via a receptor different from that used to bind the plasma membrane.

Saponins modulate the intracellular trafficking of protein toxins

“…Type I ribosome inactivating proteins such as saporin consist of only the A-domain. The low toxicity of saporin is believed to be a result of an insufficient endocytosis of saporin [23]. However as seen in Fig.…”

Targeting of Tumor Cells by Cell Surface Urokinase Plasminogen Activator-dependent Anthrax Toxin

“…However, these approaches would only be expected to slow the progression of tumors without having a direct cytotoxic action that could eradicate the malignant cells. Several studies have targeted protein toxins to uPAR, in several cases by fusion of toxin catalytic domains to ATF (73)(74)(75). However, the present study is the first attempt of which we are aware to exploit the substrate specific protease activity of the plasminogen activators to target cytotoxic bacterial toxin fusion proteins to tumor FIG.…”

Metabolic zonation of the liver: The oxygen gradient revisited

“…Although glutamine synthetase was long considered to be an example of static zonation, recent findings showing that its expression area can extend in response to thyroid hormones [15], [16] and rspondins (RSPO) [17] changed this view. Accordingly, it can now be also considered to be dynamic, though the signals changing its expression may be scarce.…”