Janet L Funk

Parathyroid hormone-related protein (PTHrP) is a multifunctional peptide that enhances blood flow in non-central nervous system (CNS) vascular beds by causing vasodilation. PTHrP expression is induced in non-CNS organs in response to ischemia. Experiments were therefore undertaken to determine whether PTHrP can be induced in brain in response to ischemic injury and whether PTHrP can act locally as a vasodilator in the cerebral vasculature, an effect that could be neuroprotective in the setting of stroke. PTHrP expression was examined by Northern analysis and immunohistochemical staining in male Sprague-Dawley rats subjected to permanent middle cerebral artery occlusion (MCAO). Vasodilatory effects of superfused PTHrP(1-34) on pial arterioles were determined by intravital fluorescence microscopy. Effects of PTHrP(1-34) peptide administration on MCAO infarction size reduction were assessed. PTHrP expression was induced in the ischemic hemisphere as early as 4 h after MCAO and remained elevated for up to 24 h. Increased immunoreactive PTHrP at sites of ischemic tissue injury was located in the cerebral microvessels. Superfusion with PTHrP(1-34) peptide for up to 25 min increased pial arteriolar diameter by 30% in normal animals. In animals with permanent MCAO, PTHrP(1-34) peptide treatment significantly decreased cortical infarct size (-47%). In summary, PTHrP expression increases at sites of ischemic brain injury in the cerebrovasculature. This local increase in PTHrP could be an adaptive response that enhances blood flow to the ischemic brain, thus limiting cell injury.

Parathyroid hormone-related protein (PTHrP) and the PTH/PTHrP receptor are expressed in most normal tissues, including brain, where PTHrP is though to act locally in an autocrine or paracrine fashion. Previous in situ localization studies in adult rodents have documented CNS PTHrP expression in neurons but not in glial cells. However, a recent report describing immunoreactive PTHrP in human astrocytomas suggests that PTHrP expression may be a marker of dedifferentiation and/or malignant transformation in glial cells. To begin to test this hypothesis, constitutive and regulated PTHrP expression were examined in cultured fetal and transformed (U-373 MG) human astrocytes. PTHrP was expressed in untreated fetal astrocytes and U-373 MG cells, as determined by Northern analysis, immunocytochemical staining, and detection of PTHrP(1-84) protein in conditioned media. Epidermal growth factor and tumor necrosis factor, important growth factors in astrocyte development and malignant transformation, stimulated PTHrP expression in both cell types. Treatment of U-373 MG cells or fetal astrocytes with PTHrP(1-34) consistently inhibited cellular proliferation, as measured by [(3)H]-thymidine incorporation. These findings suggest that PTHrP, a peptide whose expression is induced by mitogens in both immature and transformed human astrocytes, may feedback inhibit cellular proliferation, an effect that may be of importance during malignant transformation as well as CNS development. Furthermore, when combined with previous evidence of PTHrP expression by PTH/PTHrP receptor-positive neurons, our demonstration of regulated PTHrP expression by receptor-positive astrocytes identifies PTHrP as a potential peptide mediator of cross-talk between glial cells and neurons.