Putative role of HIF transcriptional activity in melanocytes and melanoma biology

Abstract

Hypoxia-inducible factor-1α (HIF-1α) is a highly oxygen sensitive bHLH protein that is part of the heterodimeric HIF-1 transcription factor. Under hypoxic stress, HIF-1 activity is induced to control expression of multiple downstream target genes, including vascular endothelial growth factor (VEGF). The normal epidermis exists in a constant mild hypoxic microenvironment and constitutively expresses HIF-1α and HIF-2α. Expression of HIF-1α and/or HIF-2α has been suggested to correlate with the increased malignant potential of melanocytes, therefore, failures of melanoma therapies may be partially linked to high HIF activity. Notably, melanomas that have the V600E BRAF mutation exhibit increased HIF-1α expression. We have utilized a bioinformatics approach to identify putative hypoxia response elements (HREs) in a set of genes known to participate in the process of melanogenesis (includingTRPM1, SLC45A2, HRAS, C-KIT, PMEL and CRH). While some of the mechanistic links between these genes and the HIF pathway have been previously explored, others await further investigation. Although agents targeting HIF activity have been proposed as novel treatment modalities for melanoma, there are currently no clinical trials in progress to test their efficacy in melanoma.

Keywords: HIF-1; keratinocytes; melanocytes; melanoma; oncogenesis; skin.

Figure 1. HIF-1α expression in an invasive malignant melanoma arising in melanocytic nevus. (A) H&E-stained section of the nevus side of the lesion. (B) H&E-stained section of the melanoma side of the same lesion. (C) HIF-1α localization in the section containing the nevus side. (D) HIF-1α localization in the sections containing the melanoma side. Arrows indicate melanoma cells with cytoplasmatic localization of HIF-1α signal. Asterisks indicate keratinocytes with nuclear HIF-1α positivity. Diamonds indicate the benign nevus cells with negligible to negative HIF-1α staining. H&E staining was performed with standard methodology using a Leica tissue stainer. HIF-1α immunostaining was performed using an anti-HIF1A rabbit polyclonal antibody (HPA001275, 1:50, Sigma) and detected using the using iVIEW DAB system with a VENTANA automatic stainer.

Figure 2. Overview of the position and localization of putative hypoxia response elements (HREs) in genes involved in the regulation of melanogenesis. The number ‘0’ refers to the transcriptional start site of the gene. Positive numbers indicate specific base pairs located after the start site, and negative numbers indicate specific base pairs before the start site. The red line refers to the first exon of the gene and the black and gray dashed lines refer to the first intron. White boxes represent the location of the HRE identified in the gene. The forward and reverse arrows refer to the HRE direction, i.e., the forward or reverse strand, respectively. (A) Reviews the HREs identified in genes that produced strong matrix scores during database analysis (p < 0.001). (B) Lists the HREs identified in key genes involved in the regulation of the melanocyte differentiation program.