Platelet surface membranes are highly mitogenic for coronary artery smooth muscle cells--A novel mechanism for sustained proliferation after vessel injury?

The effects of isolated platelet surface membranes on DNA synthesis and proliferation of bovine coronary artery smooth muscle cells (SMC) were studied. Platelet membranes were very potent mitogens for SMC. The potency was about 10-fold higher than the maximum effects of platelet-derived growth factor-BB (PDGF). Platelet membrane-induced mitogenesis was inhibited by rapamycin, wortmannin or heating for 15 min at 70 degrees C but not by the PDGF receptor antagonist SCH 13.929 or by neutralizing PDGF antibodies. Only a partial (30%) inhibition was seen with PD 98059. In contrast, PDGF-induced SMC mitogenesis was heat-stable but sensitive to SCH 13. 929, PDGF antibodies, and PD 98059. These findings provide evidence for a novel mechanism for platelet-induced SMC proliferation that is independent of PDGF secretion. Platelet membranes, attached to or incorporated into the vessel wall, could maintain sustained SMC proliferation following injury.

Patient-controlled versus staff-controlled analgesia with pethidine after allogeneic bone marrow transplantation.

Patients treated by allogeneic bone marrow transplantation (aBMT) suffer prolonged oropharyngeal mucositis pain. The aim of this study was to prospectively compare patient-controlled analgesia (PCA) with an established regimen of staff-controlled analgesia using pethidine (meperidine). Twenty patients undergoing aBMT for haematologic neoplasias or malignant lymphomas randomly received pethidine intravenously either continuously plus supplemental bolus doses on request through the transplant unit staff or by PCA. Pain intensity was assessed by patient self report using a visual analogue scale (VAS) and daily pethidine intake was documented. In addition, the pethidine consumption of 20 aBMT-patients receiving staff-controlled analgesia prior to initiation of the study, but not reporting pain, was compared retrospectively with that of patients receiving the same analgesia regimen under study conditions. PCA significantly diminished both pethidine consumption and pain intensity compared with staff-controlled analgesia. The maximum pethidine intake was 440.1 +/- 111.8 mg/24 h in the patient-controlled and 640.9 +/- 128.9 mg/24 h in the staff-controlled analgesia group (mean +/- 95% CI). Mean pain scores remained under 50% but reached 70% in the staff-controlled analgesia group. Pethidine dosage by staff-controlled analgesia increased under study conditions, suggesting that mere pain-assessment and a 'competing' analgesic method motivated the BMT-unit staff to administer higher pethidine doses. This observation is discussed as a possible Hawthorne effect. Previous studies using morphine demonstrated that PCA diminishes opioid requirement compared to continuous or staff-controlled application in bone marrow recipients. In contrast to these studies, PCA additionally improved pain relief in the present investigation.

Antimitogenic effects of vasodilatory prostaglandins in coronary artery smooth muscle cells.

Vasodilatory prostaglandins (PGI2, PGE2, PGE1) are known inhibitors of proliferation of vascular smooth muscle cells (SMC) after stimulation with mitogenic factors. However, endogenous prostaglandins do not prevent SMC proliferation subsequent to vessel injury in vivo. Since vascular cells produce large amounts of antiproliferative prostaglandins, especially subsequent to COX-2 expression, insufficient vascular PGI2 formation is not likely to explain the failure of endogenous prostaglandins to prevent excessive SMC growth. In this paper we demonstrate a rapid development of tolerance to PGI2 in SMC, resulting in diminished antiproliferative activity. These findings may not only be relevant for the control of SMC growth by endogenously synthesized prostaglandins but also for clinical use of PGI2 mimetics.

Thromboxane A2 induces cell signaling but requires platelet-derived growth factor to act as a mitogen.

This study investigates thromboxane A2-induced cell signaling and mitogenesis of bovine coronary artery smooth muscle cells. The thromboxane mimetic U 46619 [(15S)-hydroxy-11,9-(epoxymethano) prosta-5Z,13E-dienoic acid] (10 microM) stimulated [Ca2+]i signals, phosphorylation of MAP kinase (mitogen-activated protein kinase), and expression of c-fos mRNA in smooth muscle cells. In contrast, no stimulation of DNA synthesis or cell proliferation by U 46619 was observed. However, platelet-derived growth factor-BB (20 ng/ml)-induced mitogenesis was potentiated by U 46619. Similar results were obtained with I-BOP [1S-(1 alpha,2 beta(5Z),3 alpha(1E,3R*), 4 alpha)]-7-[3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7-oxabicyclo [2.2.1] heptan-2-yl]-5-heptenoic acid]. These potentiating effects were abrogated by a specific thromboxane receptor antagonist, suggesting that the potentiation of platelet-derived growth factor-BB-induced smooth muscle cell mitogenesis by U 46619 and I-BOP was mediated by thromboxane receptors. It is concluded that thromboxane A2 generated by blood platelets at the site of vessel injury induces cell signaling in smooth muscle cells but acts as a mitogen only in the presence of growth factor(s).

Androstenedione increases thromboxane A2 receptors in human erythroleukemia cells.

Previous studies have demonstrated an increased thromboxane A2 (TXA2) receptor expression in human erythroleukemia (HEL) cells and rat aortic smooth muscle (RASM) cells in response to testosterone treatment. HEL cells have served as a model for megakaryocytes, the progenitor cell for platelets. Platelets have previously been shown to convert androstenedione to testosterone. This study investigated the effects of androstenedione on the TXA2 receptor density in HEL and cultured RASM cells. Both cell lines were incubated with vehicle, 150 nM testosterone or 250, 500 or 750nM androstenedione for 48 hours. Co-incubation with testosterone or androstenedione significantly (p<0.05) increased the maximum number of TXA2 binding sites (Bmax) in HEL cells compared to controls. There was no significant change in Kd values. In a separate series of experiments, HEL cells were incubated with the androgen receptor antagonist hydroxyflutamide (2.5mM). Treatment with androstenedione (500nM) significantly (p<0.05) increased the Bmax value by 35% compared to control and hydroxyflutamide completely antagonized this effect of androstenedione. Incubation with hydroxyflutamide alone had no effect on the Bmax values compared to control. RASM cells also showed an increase in Bmax values by 25% and 23% over control (95+/-6.6, 118+/-7.2 and 117+/-5.1 fmoles/mg protein, control, testosterone and androstenedione, n=3). Both cell lines converted androstenedione to testosterone. The results raise the possibility that the adrenal androgen, androstenedione can regulate the expression of TXA2 receptors either on its own or via conversion to testosterone and through an androgen receptor.