Increasing Catecholamine Secretion Through NPY in Pheochromocytomas With False-Negative ¹²³I-MIBG Scintigraphy

Introduction 

¹²³I-MIBG has been well established as a functional imaging tool, and ¹³¹I-MIBG therapy is being considered for catecholamine-secreting tumors. Tumors with the characteristics of a noradrenergic biochemical phenotype, small, malignant, metastatic, extra-adrenal, bilateral, and hereditary, especially SDHx-related tumors, are reported to correlate with reduced MIBG uptake. However, the potential molecular mechanisms influencing MIBG uptake have been poorly studied.

Patients and Methods 

To identify critical genes that may enhance MIBG accumulation in pheochromocytomas (PCCs), we performed RNA-seq analyses for 16 operated patients with PCCs (6 MIBG-negative and 10 MIBG-positive) combined with RT-qPCR for 27 PCCs (5 MIBG-negative and 22 MIBG-positive) and examined primary cultures of the surgical tissues.

Results 

In the present study, 6 adrenal nodules of 66 nodules surgically removed from 63 patients with PCCs (9%) were MIBG negative. MIBG, a guanethidine analog of norepinephrine, can enter chromaffin cells through active uptake via the cellular membrane, be deposited in chromaffin granules, and be released via Ca²⁺-triggered exocytosis from adrenal chromaffin cells. When we compared expression of several catecholamine biosynthesis and secretion-associated genes between MIBG-negative and MIBG-positive tumors using transcriptome analyses, we found that neuropeptide Y, which is contained in chromaffin granules, was significantly increased in MIBG-negative tumors. NPY stimulated norepinephrine secretion dose-dependently in primary cell culture derived from MIBG-positive PCC. In our study, MIBG-negative PCCs were all norepinephrine-hypersecreting tumors.

Conclusions 

These data indicate that NPY upregulation in PCCs may stimulate chromaffin granule catecholamine secretion, which is associated with false-negative ¹²³I-MIBG scintigraphy.