Heat shock of vascular endothelial cells induces an up-regulatory transcriptional response of the thrombomodulin gene that is delayed in onset and does not attenuate.

Thrombomodulin is a vascular endothelial cell transmembrane protein that forms a 1:1 complex with thrombin, this interaction product forming the basis of a physiologically important natural anticoagulant system. Transcriptional down-regulation of thrombomodulin occurs following exposure of cultured endothelial cells to cytokines, while up-regulation is induced by retinoic acid and dibutyryl cyclic AMP. Thrombomodulin is also regulated developmentally, appearing in the parietal endoderm of 7.5-day-old mouse embryos. We determined that cell surface functional thrombomodulin in cultured human umbilical vein endothelial cells (HUVEC) and A549 cells increased 3.2- and 6.7-fold, respectively, in response to 24 h of continuous 42 degrees C heat shock stress. Northern analyses of thrombomodulin mRNA accumulation also showed a delayed response that was characterized by an augmentation in mRNA levels that started 12-18 h after the initiation of the stress, and continued to rise, without attenuation, during 48 h of continuous heat shock. Nuclear run-on studies confirmed that the predominant mechanism of augmentation was transcriptional. Furthermore, the heat shock-induced up-regulation of thrombomodulin in HUVEC abrogated the suppressive effect of tumor necrosis factor. Analysis of the 5' region of the thrombomodulin gene revealed six highly conserved tandem copies of the five base pair recognition unit that is the consensus sequence for a heat shock element. We hypothesize that the stress-induced augmentation in thrombomodulin gene transcription is mediated via heat shock factors binding to the heat shock element and that the stress response of thrombomodulin may have a biological role to protect the vascular endothelium during a variety of stresses, including inflammation, infection, and/or development.