HIF and MYC signaling in adrenal neoplasms of the neural crest: implications for pediatrics

Abstract

Pediatric neural crest-derived adrenal neoplasms include neuroblastoma and pheochromocytoma. Both entities are associated with a high degree of clinical heterogeneity, varying from spontaneous regression to malignant disease with poor outcome. Increased expression and stabilization of HIF2α appears to contribute to a more aggressive and undifferentiated phenotype in both adrenal neoplasms, whereas MYCN amplification is a valuable prognostic marker in neuroblastoma. The present review focuses on HIF- and MYC signaling in both neoplasms and discusses the interaction of associated pathways during neural crest and adrenal development as well as potential consequences on tumorigenesis. Emerging single-cell methods together with epigenetic and transcriptomic analyses provide further insights into the importance of a tight regulation of HIF and MYC signaling pathways during adrenal development and tumorigenesis. In this context, increased attention to HIF-MYC/MAX interactions may also provide new therapeutic options for these pediatric adrenal neoplasms.

Keywords: MYC; catecholamines; hypoxia; neural crest; neuroblastoma; paraganglioma; pheochromocytoma; sympathoadrenal cell lineage.

Figure 1

Neural crest and adrenal chromaffin cell development. (A) Human adult adrenal zonation. (B) Schematic illustration of human adrenal and sympathetic ganglia development from the neural crest. (C) Chromaffin cell and sympathoblast development from neural crest-derived cells in humans (adapted from (12)). DA, dorsal aorta; N, notochord.

Figure 2

Oxygen-sensing and HIF-MYC signaling. Key molecule of oxygen-sensing system are hypoxia-inducible factors (HIFs), which regulate the transcription of a wide range of oxygen responsive genes. In presence of oxygen, HIF proline hydroxylases (PHDs) hydroxylate proline residues within HIFα subunits, which lead to proteasomal degradation of HIFα by the von Hippel-Lindau (VHL) tumor suppressor. Hypoxic conditions (absence of oxygen) or specific mutations, that affect HIFα degradation, lead to a stabilization of HIFα. Subsequently, HIFα subunits translocated to the nucleus where they form a complex with aryl hydrocarbon receptor nuclear translocator (ARNT, also known as HIFβ) and specific co-factors and bind to hypoxia-responsive elements (HREs) leading to transcription of HIF target genes (classical HIF signaling). MYC proteins encoded by MYC proto-oncogenes (c-MYC, MYCN, l-MYC) are localized in the nucleus and form heterodimers with myc-associated factor X (MAX), which enables recognition by the hexameric DNA sequence CACGTG (E-Box) and subsequent transcriptional activation of MYC target genes (classical MYC signaling). Moreover, other binding partners of MYC and MAX also regulate MYC target gene transcription, including MAX dimerization protein 1 (MAD) and HIFαs. Thereby, opposite effects on MYC target gene expression were described for HIF1α and HIF2α. While HIF1α dimerizes with MAX and thereby suppresses binding to the E-box, HIF2α leads to stabilization of the MYC/MAX complex and thus to activation of MYC target genes.

Figure 3

Expression of MYCN, HIFs and chromaffin cell markers in the developing human adrenal medulla. Expression patterns of genes of interest obtained by single nucleus RNA sequencing data (18) of the developing human adrenal medulla were visualized by https://adrenal.kitz-heidelberg.de/developmental_programs_NB_viz/ (last request: March 2022). (A) UMAP plot of adrenal medullary cells of developing adrenal and of the fetal adrenal eight weeks post conception (8pcw). Different colors highlight different cell populations. Expression pattern of (B) HIF1α, (C) EPAS1, (D) TH, (E) PNMT and (F) MYCN in the developing human adrenal and of the fetal adrenal eight weeks post conception. The color indicates the normalized gene expression (blue low expression; yellow high expression). Data on c-Myc and MAX expression were not available. UMAP: Uniform manifold approximation and projection; SCP, Schwann cell precursors.

Figure 4

Hypothetical model to explain the cellular origin leading to different phenotypic features in pheochromocytomas/paragangliomas (PPGLs) and neuroblastomas in dependence of HIF2α and MYCN. PPGLs and neuroblastomas originate from neural crest cells; while mature neuroblastomas arise from a sympathoblast lineage, mature cluster 2 PPGLs originate form a chromaffin cell lineage. Both lineages arise via Schwann cell precursors that may give rise to more immature cluster 1 PPGLs and mesenchymal-type neuroblastomas. Increased expression and stabilization of HIF2α is associated with an immature phenotype and enhanced aggressiveness in PPGLs, while for HIF2α in neuroblastoma both an oncogenic and a tumor suppressor function are discussed. MYCN amplification correlates with a worse prognosis in neuroblastomas. PNMT, Phenylethanolamine N-methyltransferase; TCA cycle, Tricarboxylic acid cycle.