Neuroendocrine tumors constitute a heterogeneous group of tumors arising from hormone-secreting cells and are generally accompanied by dysregulated secretion. Pheochromocytoma belongs to this family: it develops from the chromaffin cells of the adrenal medulla and causes excessive catecholamine release, giving rise to severe clinical symptoms such as arterial hypertension, an increased risk of stroke, and various cardiovascular complications. While this hypersecretion is well known to pathologists and clinicians, it had never before been finely explored at the cellular and molecular scales. This is precisely the gap that the present study seeks to fill, in a context where identifying the mechanisms that trigger the hypersecretory activity of neuroendocrine tumors remains an unresolved issue, potentially affecting prognosis.
To address this, the authors combined two complementary approaches. On the one hand, catecholamine secretion was measured by carbon-fiber amperometry, directly on human tumor cells cultured from freshly resected pheochromocytomas: each amperometric spike corresponds to the release of a single secretory granule, and its size reflects the amount of catecholamines it contains. On the other hand, differential analysis by mass spectrometry compared the expression of exocytosis proteins between tumor tissue and the matched adjacent healthy tissue of each patient.
The analysis, based on 7,344 individual amperometric spikes, reveals that, in most patients, tumor cells display a significantly increased number of exocytotic events per cell, associated with faster kinetics. In contrast, the mean charge per spike does not increase—and even decreases slightly in nine tumors—suggesting that each granule does not contain more catecholamines than in non-tumor cells. The hypothesis favored by the authors is therefore a global increase in the number of secretory granules, consistent with the observed overexpression of vesicular monoamine transporters, chromogranins, and enzymes involved in granule biogenesis. The proteomic analysis further highlights the overexpression of several key players in calcium-regulated exocytosis, including SNARE proteins and their regulators; the calcium sensor synaptotagmin-1 is among the most strongly overexpressed proteins, which could enhance calcium sensitivity and release probability.
This work thus establishes that dysregulation of calcium-regulated exocytosis, at the level of the individual tumor cell, is a cause of the catecholamine hypersecretion associated with pheochromocytoma—tumor proliferation alone being unable to account for it. The authors emphasize that a clinical need persists: to identify molecules capable of specifically correcting this hypersecretion by targeting the alterations of the secretory machinery brought to light here.