Effect of von Willebrand Factor on the Pharmacokinetics of Recombinant Human Platelet Glycoprotein Ibα-Immunoglobulin G1 Chimeric Proteins

Identification of peptide antagonists to glycoprotein Ibalpha that selectively inhibit von Willebrand factor dependent platelet aggregation

GPIbalpha is an integral membrane protein of the GPIb-IX-V complex found on the platelet surface that interacts with the A1 domain of von Willebrand factor (vWF-A1). The interaction of GPIbalpha with vWF-A1 under conditions of high shear stress is the first step in platelet-driven thrombus formation. Phage display was used to identify peptide antagonists of the GPIbalpha-vWF-A1 interaction. Two nine amino acid cysteine-constrained phage display libraries were screened against GPIbalpha revealing peptides that formed a consensus sequence. A peptide with sequence most representative of the consensus, designated PS-4, was used as the basis for an optimized library. The optimized selection identified additional GPIbalpha binding peptides with sequences nearly identical to the parent peptide. Surface plasmon resonance of the PS-4 parent and two optimized synthetic peptides, OS-1 and OS-2, determined their equilibrium dissociation GPIbalpha binding constants ( K Ds) of 64, 0.74, and 31 nM, respectively. Isothermal calorimetry corroborated the K D of peptide PS-4 with a resulting affinity value of 68 nM. An ELISA demonstrated that peptides PS-4, OS-1, and OS-2 competitively inhibited the interaction between the vWF-A1 domain and GPIbalpha-Fc in a concentration-dependent manner. All three peptides inhibited GPIbalpha-vWF-mediated platelet aggregation induced under high shear conditions using the platelet function analyzer (PFA-100) with full blockade observed at 150 nM for OS-1. In addition, OS-1 blocked ristocetin-induced platelet agglutination of human platelets in plasma with no influence on platelet aggregation induced by several agonists of alternative platelet aggregation pathways, demonstrating that this peptide specifically disrupted the GPIbalpha-vWF-A1 interaction.

Endothelial cells of human colorectal cancer and healthy colon reveal phenotypic differences in culture

Antiangiogenic drugs have been used successfully for the treatment of colorectal cancer (CRC). Viable tumor endothelial cells (TEC) and normal endothelial cells (NEC) of uninvolved colon tissue of the same patient have not been available to optimize treatment strategies in vitro. Therefore, our target was to establish a protocol for the isolation of TEC and NEC. These cells were isolated with very high purity via magnetic cell sorting of tissue samples obtained from CRC and healthy colon of eight patients. TEC and NEC expressed CD31, CD105, VE-cadherin, VCAM-1, ICAM-1 and E-selectin, formed capillaries in basal membrane extract and were able to take up acetylated low-density lipoprotein. They were negative for podoplanin, CD45, CD68 and cytokeratin-20 indicating blood vessel endothelial lineage. Intense staining of von Willebrand factor (vWF) was observed in five of eight NEC cultures, whereas vWF was absent or only slightly expressed in all TEC cultures in vitro. Low intracellular concentration of vWF was also detected in TEC and NEC at the tissue level. This demonstrated that differences exhibited by TEC and NEC in vivo are stably perpetuated in culture. The isolated cultures may provide a useful in vitro model to elucidate epigenetic effects on angiogenesis in cancer and to optimize antiangiogenic therapy.

Platelet-VWF complexes are preferred substrates of ADAMTS13 under fluid shear stress

Endothelial cells secrete prothrombotic ultralarge von Willebrand factor (VWF) multimers, and the metalloprotease ADAMTS13 cleaves them into smaller, less dangerous multimers. This reaction is stimulated by tensile force applied to the VWF substrate, which may occur on cell surfaces or in the circulating blood. The cleavage of soluble VWF by ADAMTS13 was accelerated dramatically by a combination of platelets and fluid shear stress applied in a cone-plate viscometer. Platelet-dependent cleavage of VWF was blocked by an anti-GPIbalpha monoclonal antibody or by a recombinant soluble fragment of GPIbalpha that prevents platelet-VWF binding. Multimeric gel analysis showed that shear and platelet-dependent cleavage consumed large VWF multimers. Therefore, ADAMTS13 preferentially acts on platelet-VWF complexes under fluid shear stress. This reaction is likely to account for a majority of VWF proteolysis after secretion and to determine the steady-state size distribution of circulating VWF multimers in vivo.