¹¹C-Hydroxytryptophan Uptake and Metabolism in Endocrine and Exocrine Pancreas

Abstract

Determination of the residual b-cell mass using noninvasive tools might help to follow up the efficacy of new treatments in both type 1 (insulin-dependent) and type 2 (non-insulin-dependent) diabetes mellitus, including islet transplantation. b-cells are neuroendocrine cells that can take up and metabolize the serotonin precursor 5-hydroxytryptophan. The serotonin pathway may therefore be an attractive target for the development of an imaging tracer for residual b-cell mass. The aim of this study was to evaluate the uptake mechanism and the retention of the PET tracer 11 C-hydroxytryptophan in endocrine and exocrine pancreas in vitro and in vivo. Methods: The exocrine human pancreas carcinoma cell line (PANC-1) and the endocrine human insulinoma cell line (CM) were applied for in vitro 11 Chydroxytryptophan accumulation/efflux experiments and blocking studies using inhibitors of key enzymes and transporters involved in the serotonin pathway. Animal experiments were performed on normal Wistar rats and on rats pretreated with the monoamine oxidase (MAO) inhibitor clorgyline. After intravenous injection of 11 C-hydroxytryptophan, a 60-min dynamic PET scan was acquired followed by an ex vivo biodistribution study. Autoradiography and hematoxylin-eosin staining were performed on the dissected pancreas to localize the radioactivity within the pancreatic tissue. Results: 11 C-hydroxytryptophan accumulated rapidly in both endocrine CM cells and exocrine PANC-1 cells. In the exocrine cells, a rapid efflux of radioactivity was observed, whereas most radioactivity remained trapped in the endocrine cells. PET images showed clear accumulation of 11 C-hydroxytryptophan in the pancreas in both animal groups, but with a significant 3-fold higher retention of the radiopharmaceutical in clorgyline-treated animals. Ex vivo biodistribution studies confirmed the results obtained by PET. Autoradiographs did not discriminate between the exocrine and endocrine pancreas in control animals, whereas autoradiographs showed intense radioactive spots colocalized with the islets of Langerhans in clorgyline-treated animals. The islets of Langerhans represent only 2%-3% of the total pancreas and consist of about 80% b-cells. In both type 1 and type 2 diabetes, dysregulation of glucose homeostasis and b-cell function and loss of b-cells are responsible of hyperglycemia (1). b-cells are neuroendocrine cells arising from precursor cells of the embryologic neural crest (2,3). Pancreatic b-cells regulate a large variety of body functions through paracrine and endocrine actions via dedicated amines and peptides, including the neurotransmitter serotonin. As a consequence, b-cells have an efficient mechanism for the uptake and metabolism of the amine precursors L-3,4-dihydroxyphenylalanine and 5-hydroxytryptophan through specific receptors (4,5).5-hydroxytryptophan and its precursor tryptophan are taken up in b-cells by the L-type amino acid transporter (LAT). Tryptophan is converted to 5-hydroxytryptophan by tryptophan hydroxylase (TPH). Subsequently,...

Quantitative Imaging of Serotonergic Biosynthesis and Degradation in the Endocrine Pancreas

“…Thus, the findings by Di Gialleonardo et al (10) in the PANC1 cell line may not be representative for higher species such as nonhuman primates or humans.…”

“…Furthermore, the uptake was similar in magnitude in a and b cells, but the retention in b cells was higher, resulting in an increased relative accumulation in b cells 60 min after intravenous administration with 75% of the tracer found within the b cells (9). Di Gialleonardo et al (10) performed in vitro uptake studies using the ductal cell line PANC1 as a model for exocrine tissue. They showed that accumulation in PANC1 cells was comparable to that of INS-1 cells but not mediated by normal serotonergic metabolism because the accumulated tracer metabolite was resistant to degradation by MAO-A.…”

“…5,1,3,5,10,15,20,30,45,60, and 90 min after each injection to measure the radioactivity concentration in whole blood and in plasma. For 4 animals (examined as baseline [n 5 4] or after preadministration of carbidopa [n 5 1] or clorgyline [n 5 3]), we performed analysis of tracer metabolites to investigate effect on 11 C-5-HTP metabolism by inhibition of DDC and MAO-A.…”

“…Ekholm et al investigated the fate of tritiated 5-HTP in the rodent pancreas and at 60 min after administration found preferential accumulation in islets of Langerhans and, in particular, in the granules of the b cells (9). Recently, Di Gialleonardo et al studied the mechanisms of 11 C-5-HTP uptake in cell lines derived from rodent insulinoma (INS-1) and human ductal cells (PANC1) and reported DDC-and MAO-Aspecific uptake in INS-1 but not in PANC1 cells (10). However, the total uptake and retention in PANC1 was in the same range as for INS-1, potentially causing a significant background signal from nonendocrine tissue in vivo.…”

Positron Emission Tomography Ligand [11C]5-Hydroxy-Tryptophan Can Be Used as a Surrogate Marker for the Human Endocrine Pancreas

“…Radiolabeled 5-HTP therefore accumulates in the form of serotonin in islets of Langerhans as opposed to the exocrine pancreatic parenchyma (9,22). In vitro studies of [ 11 C]5-HTP show high selective retention (8-55 times) in human islets compared with exocrine cells (18).…”