Reviewing and reconsidering invasion assays in head and neck cancer

Abstract

Head and neck squamous cell carcinomas (HNSCC) are malignant tumors that arise from the surface epithelium of the oral cavity, oropharynx and larynx, primarily due to exposure to chemical carcinogens or the human papilloma virus. Due to their location, dental practitioners are well-positioned to detect the lesions. Deadlier than lymphoma or melanoma, HNSCC is incompletely understood. For these reasons, dental practitioners and researchers are focused on understanding HNSCC and the processes driving it. One of these critical processes is invasion, the degradation of the basement membrane by HNSCC cells with subsequent movement into the underlying connective tissue, blood vessels or nerves. Cancer cells metastasize to distant sites via the blood vessels, lymphatics and nerves. Metastasis is associated with poor survival. Since invasion is essential for development and metastasis of HNSCC, it is essential to understand the mechanism(s) driving this process. Elucidation of the mechanisms involved will facilitate the development of targeted treatment, thereby accelerating development of precision/personalized medicine to treat HNSCC. Robust in vitro and in vivo assays are required to investigate the mechanistic basis of invasion. This review will focus on in vitro and in vivo assays used to study invasion in HNSCC, with special emphasis on some of the latest assays to study HNSCC.

Keywords: HNSCC; In vitro; In vivo; Invasion; Squamous cell carcinoma.

Figure 1. HNSCC invasion

In this image, transformed cells (blue) are invading through the basement membrane (gray). Elongated mesenchymal cells represent the importance of EMT in invasion. Invasive tumor islands are present in the connective tissue. Elements of the tumor microenvironment are depicted, including nerves, vasculature and the inflammatory infiltrate.

Figure 2. Boyden chamber assay

(A) Graphical depiction of an invasion assay. 1. HNSCC cells seeded on BME in the upper chamber. 2. Cells invade toward a chemoattractant. (B) Photographs show HNSCC cells (highlighted by arrows) that have penetrated the BME. The upper image shows less invasion than the lower image.

Figure 3. HNSCC models in mouse

Several HNSCC models are available, including those indicated.

Figure 4. Comparison of the histology of in vitro and in vivo models of invasion and human HNSCC

Histological comparison of the oral cancer equivalent (OCE, in vitro), floor-of-mouth (FOM, in vivo) and chick chorioallantoic membrane (CAM, in vivo) models of HNSCC, and human HNSCC, at low magnification (upper panels, scale bar=200um) and high magnification (lower panels, scale bar=50um). Arrows and arrowheads highlight invasive islands and mesenchymal tissue, respectively.