Intricacies of the Molecular Machinery of Catecholamine Biosynthesis and Secretion by Chromaffin Cells of the Normal Adrenal Medulla and in Pheochromocytoma and Paraganglioma

Abstract

The adrenal medulla is composed predominantly of chromaffin cells producing and secreting the catecholamines dopamine, norepinephrine, and epinephrine. Catecholamine biosynthesis and secretion is a complex and tightly controlled physiologic process. The pathways involved have been extensively studied, and various elements of the underlying molecular machinery have been identified. In this review, we provide a detailed description of the route from stimulus to secretion of catecholamines by the normal adrenal chromaffin cell compared to chromaffin tumor cells in pheochromocytomas. Pheochromocytomas are adrenomedullary tumors that are characterized by uncontrolled synthesis and secretion of catecholamines. This uncontrolled secretion can be partly explained by perturbations of the molecular catecholamine secretory machinery in pheochromocytoma cells. Chromaffin cell tumors also include sympathetic paragangliomas originating in sympathetic ganglia. Pheochromocytomas and paragangliomas are usually locally confined tumors, but about 15% do metastasize to distant locations. Histopathological examination currently poorly predicts future biologic behavior, thus long term postoperative follow-up is required. Therefore, there is an unmet need for prognostic biomarkers. Clearer understanding of the cellular mechanisms involved in the secretory characteristics of pheochromocytomas and sympathetic paragangliomas may offer one approach for the discovery of novel prognostic biomarkers for improved therapeutic targeting and monitoring of treatment or disease progression.

Keywords: PPGL; adrenomedullary function; catecholamines.

Figure 1

The catecholamine biosynthetic pathway in an adrenomedullary chromaffin cell or a pheochromocytoma cell. Norepinephrine and epinephrine are stored in separate chromaffin storage vesicles. Abbreviations: LAT: L-type amino acid transporter; TH: tyrosine hydroxylase; L-DOPA: L-3,4-dihydroxyphenylalanine; AADC: aromatic L-amino acid decarboxylase; DBH: dopamine β-hydroxylase; PNMT: phenylethanolamine-N-methyltransferase; BH4: tetrahydrobiopterin; 02: molecular oxygen; VitB6: pyridoxalphosphate; VitC: ascorbate; VMAT: vesicular monoamine transporters; GR: glucocorticoid receptor.

Figure 2

Schematic overview of the stimulation–secretion coupling in the adrenomedullary chromaffin cell with the multiple functionally definable stages and the different secretory pathways. Abbreviations: ER: endoplasmic reticulum; Ach: acetylcholine; VAMP: vesicle-associated membrane protein; SNAP: synaptosomal-associated protein; NSF: N-ethylmaleimide Soluble Factor proteins; CADPS: Ca²⁺ dependent secretion activator; CALM: calmodulin; PACAP: pituitary adenylate cyclase-activating polypeptide; PAC1 receptor: PACAP-preferring receptor; GR: glucocorticoid receptor.

Figure 3

The Pseudohypoxia group (cluster I) divided into two subgroups: tricarboxylic acid (TCA) cycle related, containing germline pathogenic variants in succinate dehydrogenase subunits SDHA, SDHB, SDHC, and SDHD as well as SDHAF2 (SDHx), assembly factor for the succinate dehydrogenase complex, and FH, a second enzyme in the tricarboxylic acid (TCA) cycle. The second subgroup: VHL/EPAS1—related with somatic and germline pathogenic variants. Pathogenic variants in three additional genes encoding for malate dehydrogenase 2 (MDH2), prolyl hydroxylase 1 (PHD1, also known as egl nine homolog 2; EGLN2), and iron regulatory protein 1 (IRP1) were not included previously in the molecular classification by TCGA but were recently discovered. Based on their signaling pathways, it is believed that these new genes should be included as part of the cluster I pseudohypoxia group because MDH2 is part of to the TCA cycle and both PDH1 and IRP1 belong to the VHL/EPAS1 related subgroup. Cluster I is characterized by the expression of genes involved in the “hypoxic response”, resulting in a “pseudo-hypoxic” phenotype with uncontrolled expression of HIF1α regulated genes such as VEGF. HIF1α regulates the transcription of genes associated with tumorigenesis and angiogenesis. Wnt altered signaling group (cluster III) consists of newly recognized somatic mutations in CSDE1 as well as somatic gene fusions affecting MAML3. This group exclusively consists of somatic mutations that activate the Wnt pathway, which is not activated under normal conditions. Wnt signaling and therefore increased expression of β-catenin is associated with a poorer prognosis and a higher metastatic potential of tumors. There is still much unknown about this group. Kinase signaling group (Cluster II) consists of germline or somatic pathogenic variants in the driver genes RET, NF1, TMEM127, MAX, and HRAS. This cluster is characterized by an increased activation of the MAP kinase and the P13K/AKT pathways, which results in an increased expression of genes involved in protein synthesis, kinase signaling, endocytosis, and preservation of differentiated/mature chromaffin cell catecholamine biosynthetic machinery. MAX mutated tumors are an exception, since they show an intermediate catecholamine biochemical phenotype with detectable expression of PNMT and some production of epinephrine. MAX is a distinct sub-cluster of the kinase signaling group and was recently proposed to be possibly redivided in a new group, the cortical admixture group [187,188,189,193].