Identification of DLK1-positive cell clusters in the human adrenal cortex and their potential involvement in adrenocortical carcinoma.

Abstract

The adrenal glands are vital endocrine organs responsible for the synthesis and secretion of multiple steroids and hormones. They are composed of an inner adrenal medulla and an outer adrenal cortex. The human adrenal cortex is further subdivided into three distinct zones that differ both morphologically and functionally: the Zona Glomerulosa (ZG), the Zona Fasciculata (ZF), and the Zona Reticularis (ZR). In rats, another zone has been identified between the ZG and the ZF, termed the undifferentiated zone (ZU). This zone has been shown to consist of adrenocortical progenitor cells, expressing Sonic Hedgehog and Delta like homologue 1 (Dlk1). The presence and function of the ZU in human adrenals is not known. In this project I studied the expression of stem/progenitor and steroidogenic markers in the human adrenal cortex and identified a novel cell population in the subcapsular region, which is hypothesised to be similar to the ZU in rats. This cell population expressed the atypical Notch ligand Delta-like homologue 1 (DLK1) but not steroidogenic markers (similar to the rat model), and we termed this as DLK1-cell clusters (DCCs). Following assessment of DLK1 expression across normal adrenals from foetuses to ageing adults, DCCs appear to be of layered continuous appearance in foetuses and in younger individuals and become clustered later in life. However, these were found to be different entities to aldosterone producing cell clusters (APCCs), which are precursors of aldosterone producing adenomas (APAs). Since DLK1 has shown involvement in carcinogenesis, I assessed whether it is involved in DLK1 in adrenocortical carcinomas (ACCs). DLK1 was significantly upregulated in all ACC samples analysed compared to normal adrenals. Further in vitro experiments using human adrenocortical cell line H295R, showed that DLK1-expressing cells possess cancer stem cell characteristics. Collectively, these results indicate that DLK1 could be a novel marker of cancer stem cells in adrenocortical carcinoma, which could potentially be used as a biomarker for identification and treatment.