Defects in muscarinic receptor-coupled signal transduction in isolated parotid gland cells after in vivo irradiation: evidence for a non-DNA target of radiation

Abstract

Radiation-induced dysfunction of normal tissue, an unwanted side effect of radiotherapeutic treatment of cancer, is usually considered to be caused by impaired loss of cell renewal due to sterilisation of stem cells. This implies that the onset of normal tissue damage is usually determined by tissue turnover rate. Salivary glands are a clear exception to this rule: they have slow turnover rates (460 days), yet develop radiation-induced dysfunction within hours to days. We showed that this could not be explained by a hypersensitivity to radiation-induced apoptosis or necrosis of the differentiated cells. In fact, salivary cells are still capable of amylase secretion shortly after irradiation while at the same time water secretion seems specifically and severely impaired. Here, we demonstrate that salivary gland cells isolated after in vivo irradiation are impaired in their ability to mobilise calcium from intracellular stores (Ca 2 þ i ), the driving force for water secretion, after exposure to muscarinic acetylcholine receptor agonists. Using radioligandreceptor-binding assays it is shown that radiation caused no changes in receptor density, receptor affinity nor in receptor-G-protein coupling. However, muscarinic acetylcholine agonist-induced activation of protein kinase C alpha (PKCa), measured as translocation to the plasma membrane, was severely affected in irradiated cells. Also, the phorbol ester PMA could no longer induce PKCa translocation in irradiated cells. Our data hence indicate that irradiation specifically interferes with PKCa association with membranes, leading to impairment of intracellular signalling. To the best of our knowledge, these data for the first time suggest that, the cells' capacity to respond to a receptor agonist is impaired after irradiation.

Radioprotective Effect of Lidocaine on Neurotransmitter Agonist-Induced Secretion in Irradiated Salivary Glands

“…Secretory activity of salivary glands decreased to 50% of normal levels whereas less than 3% of apoptosis activity was observed 3 days after radiation [5] and that no significant cell loss was observed within 10 days after radiation [7]. This rapid radiation induced changes is not compatible with mitotic or apoptotic cell death and that an alternative hypothesis of early-stage radiation damage to the salivary glands is needed [4], [7], [8].…”

“…The exact mechanism of radiation-induced salivary gland injury per se remains elusive. Although radiation induced sterilization of progenitor cells which prevents the replenishment of saliva-producing cells is considered as the main cause for late-stage effects [2], [3], the mechanism of early-stage functional damage is different [2], [4]. Salivary acinar cells are well differentiated and have a low mitotic index [2].…”

“…Recently, researchers demonstrated that receptor-effector signal transduction of water secretion in salivary glands was impaired after irradiation [4], [7], [9]. Coppes et al [4] suggested that radiation impairs the muscarinic acetylcholine agonist-induced activation of protein kinase C-alpha, measured as its translocation to the plasma membrane, leading to damage in the ability to mobilize calcium from intracellular stores, which is the driving force for water secretion.…”

Effect of Cevimeline on Radiation-Induced Salivary Gland Dysfunction and AQP5 in Submandibular Gland in Mice

“…The translocation of AQP5 by cevimeline is mediated by elevation of intracellular Ca ‫ם2‬ concentrations 9) . Radiation impairs mobilization of Ca ‫ם2‬ from intracellular Ca ‫ם2‬ stores (such as endoplasmic reticulum 4) ). It is possible that radiation-induced disorders in mobilization of Ca ‫ם2‬ might impair transport of AQP5.…”

Sensitivity of Salivary Glands to Radiation: from Animal Models to Therapies

“…Restoration of AQP5 levels in mouse submandibular acinar cells, with the oral administration of the muscarinic agonist cevimeline, prior to a single dose of 15 Gy resulted in partial preservation of salivary flow rates up to day 28 in vivo (Takakura et al , 2007). Reports on the impairment of calcium signaling following irradiation have been inconsistent; therefore, it is unclear whether it has a role in chronic loss of function (O’Connell et al , 1998, 1999b; Coppes et al , 2005). Expression of PKCδ has been shown to regulate apoptosis in salivary acinar cells (reviewed in Reyland, 2007), and PKCδ-deficient mice exhibited significantly lower levels of radiation-induced apoptosis (1 and 5 Gy) (Humphries et al , 2006).…”