Investigating cellular senescence in the aging tongue.

Abstract

Background: Cellular senescence is a biological phenomenon that occurs at the molecular level in certain cells, wherein they stop to divide, persist, and damage nearby healthy cells by the production of a senescence-associated secretory phenotype (SASP). With age, various tissues in our body accumulate senescent or ‘zombie’ cells. On one hand, cellular senescence inhibits the growth of cancerous cells, and, on the other hand, SASP can cause tissue disruption and age-related diseases such as cancer. Within the oral cavity, the tongue has a high risk of decline and cancer with age. While multiple studies have explored the aging tongue in the context of cancer, there are limited studies evaluating the healthy, aging tongue. Aim: The aim of this study was to evaluate whether the process of cellular senescence occurs within healthy, aged tongue tissues. Our hypothesis is that the healthy aging tongue will undergo cellular senescence. Methods: Tongue specimens from young female primates (4-6 years, n = 3), old female primates (17–19 years old, n = 3), and old female primates treated with rapamycin (17–19 years old, n = 4) were obtained, and a tissue sample from the posterolateral area was taken. Western blot analysis was done to evaluate p16ink4a (p16) and p21WAF1/CIP1 (p21) expression levels, while qRT-PCR was completed to evaluate SASP markers. Histology slides were prepared for standard hematoxylin/eosin and lipofuscin staining. Statistical analysis was done using a one-way ANOVA test to compare the means of p16, p21, and SASP markers. Results: Aged non-human primate (NHP) tongues showed a significant increase in the cell cycle arrest markers p21 and IL1A. Additionally, rapamycin reduced the elevated levels of IL1A expression. While qualitative tissue morphological changes demonstrated no indication of pathology, older NHP tongues had a thickened epithelial tissue layer and a greater number of rete ridges and lipofuscin granules compared to the younger group. Conclusion: While literature demonstrates that cancerous tongues have an increase in cellular senescence markers, we show here that normal, aged tongues also have an increased cellular senescence, albeit without pathology. Further, the increase in IL1A was blunted by rapamycin treatment. Additional research is needed to understand the significance of IL1A attenuation in the tongue for health and disease. Understanding cellular senescence may provide another way to evaluate the aging tongue beyond the clinical exam. Funding Source: This project was in part supported by the Dr. Douglass L. Morell Dentistry Research Fund and the University of Washington School of Dentistry. NHP samples were provided by Dr. Mary Zelinski and Dr. Steven Kohama at the Oregon National Primate Research Center (P51 OD 011092)