Radiation-Induced von Willebrand Factor Release

To the Editor:

von Willebrand factor (vWF ) is a large multimeric adhesive glycoprotein with a molecular weight ranging from 0.4 × 10⁶ to over 20 × 10⁶. It is synthesized by megakaryocytes and endothelial cells, and serves a major role in the initial step of platelet adhesion at sites of vessel injury by forming a bridge between the platelet and constituents of the subendothelium. Ionizing radiation has been shown to increase vWF release from cultured human endothelial cells within 24 hours after exposure to doses above 10 Gy.1,2 However, the mechanism of this radiation-induced vWF release remains unclear. In a recent report, Jahroudi et al3 addressed this issue and demonstrated that radiation induced an increase in vWF mRNA accumulation due to enhanced transcriptional activity. Furthermore, the radiation inducible promoter region of the vWF gene was identified. These observations provide important insights into the molecular mechanisms that control endothelial vWF release upon irradiation.

There are two distinct pathways of endothelial vWF release. The majority of the newly synthesized protein is constitutively secreted, while a small portion is first stored in specific organelles, the Weibel-Palade bodies, and rapidly released upon stimulation (the regulated pathway).4 VWF secreted by the constitutive pathway consists of predominantly small multimers, while the vWF stored in Weibel-Palade bodies contains the largest and biologically most active multimers.5,6 Although it is generally accepted that vWF is secreted to both the luminal and abluminal side of the cell, the polarity of the two secretory pathways remains a matter of debate. The findings by Jahroudi et al suggest an important role for the constitutive secretory pathway in this process.

We have pursued a different approach to investigate the secretory mechanism of radiation-induced vWF release. By making use of a three-compartment culture model with a loose collagen gel as artificial subendothelial support, we were able to quantify vWF and analyze the multimeric composition of the protein in both the luminal, abluminal, and cellular compartment. We have previously shown in this model that the basal, constitutive release of vWF occurs predominantly at the abluminal side, whereas regulated vWF release is polarized toward the luminal compartment.7,8 After irradiation, most of the vWF was released into the abluminal compartment, with no significant changes in cellular vWF content.2 Moreover, immunofluorescence studies showed no discernible depletion of the Weibel-Palade bodies after irradiation, in contrast to what was observed after stimulation with phorbol myristate acetate (PMA). These observations favor a role for the constitutive pathway in radiation-induced vWF release. Recent unpublished observations from our laboratory on the multimeric composition of vWF provide additional evidence to support this notion. Samples from the luminal, cell-associated, and abluminal compartments of cultured human umbilical vein endothelial cells were analyzed for the multimeric composition by Western blotting (Fig 1). At 48 hours after 20 Gy single-dose irradiation, vWF multimers of predominantly low molecular weight were found in both the luminal and abluminal compartment. No changes in the multimeric pattern of intracellular vWF were observed. A similar distribution of vWF multimers was found in the respective compartments of unstimulated endothelial cells. In contrast, after stimulation with PMA, which causes the release of vWF from the Weibel-Palade bodies, a significant increase in the amount of high-molecular-weight multimers was observed in both the luminal and the abluminal compartment. This was accompanied by a selective decrease of the large multimers in the cell-associated fraction.

Both the data on secretion polarity and multimeric composition indicate that ionizing radiation enhances vWF release mainly through the constitutive pathway of secretion, and are in agreement with the data of Jahroudi et al.