The emerging role of oral microbiota in oral cancer initiation, progression and stemness.

Oral squamous cell carcinoma (OSCC) is the most prevalent malignancy among the Head and Neck cancer. OSCCs are highly inflammatory, immune-suppressive, and aggressive tumors. Recent sequencing based studies demonstrated the involvement of different oral microbiota in oral cavity diseases leading OSCC carcinogenesis, initiation and progression. Researches showed that oral microbiota can activate different inflammatory pathways and cancer stem cells (CSCs) associated stemness pathways for tumor progression. We speculate that CSCs and their niche cells may interact with the microbiotas to promote tumor progression and stemness. Certain oral microbiotas are reported to be involved in dysbiosis, pre-cancerous lesions, and OSCC development. Identification of these specific microbiota including Human papillomavirus (HPV), Porphyromonas gingivalis (PG), and Fusobacterium nucleatum (FN) provides us with a new opportunity to study the bacteria/stem cell, as well as bacteria/OSCC cells interaction that promote OSCC initiation, progression and stemness. Importantly, these evidences enabled us to develop in-vitro and in-vivo models to study microbiota interaction with stem cell niche defense as well as CSC niche defense. Thus in this review, the role of oral microbiota in OSCC has been explored with a special focus on how oral microbiota induces OSCC initiation and stemness by modulating the oral mucosal stem cell and CSC niche defense.

Targeting hypoxia-induced tumor stemness by activating pathogen-induced stem cell niche defense.

Tumor hypoxia and oxidative stress reprograms cancer stem cells (CSCs) to a highly aggressive and inflammatory phenotypic state of tumor stemness. Previously, we characterized tumor stemness phenotype in the ATP Binding Cassette Subfamily G Member 2 (ABCG2)-positive migratory side population (SPm) fraction of CSCs exposed to extreme hypoxia followed by reoxygenation. Here, we report that post-hypoxia/reoxygenation SPm+/ABCG2+ CSCs exerts defense against pathogen invasion that involves bystander apoptosis of non-infected CSCs. In an in vitro assay of cancer cell infection by Bacillus Calmette Guerin (BCG) or mutant Mycobacterium tuberculosis (Mtb) strain 18b (Mtb-m18b), the pathogens preferentially replicated intracellular to SPm+/ABCG2+ CSCs of seven cell lines of diverse cancer types including SCC-25 oral squamous cancer cell line. The conditioned media (CM) of infected CSCs exhibited direct anti-microbial activity against Mtb and BCG, suggesting niche defense against pathogen. Importantly, the CM of infected CSCs exhibited marked in vitro bystander apoptosis toward non-infected CSCs. Moreover, the CM-treated xenograft bearing mice showed 10- to 15-fold reduction (p < 0.001; n = 7) in the number of CSCs residing in the hypoxic niches. Our in vitro studies indicated that BCG-infected SPm+/ABCG2+ equivalent EPCAM+/ABCG2+ CSCs of SCC-25 cells underwent pyroptosis and released a high mobility group box protein 1 (HMGB1)/p53 death signal into the tumor microenvironment (TME). The death signal can induce a Toll-like receptor 2/4-mediated bystander apoptosis in non-infected CSCs by activating p53/MDM2 oscillation and subsequent activation of capase-3-dependent intrinsic apoptosis. Notably, SPm+/ABCG2+ but not SP cells undergoing bystander apoptosis amplified the death signal by further release of HMGB1/p53 complex into the TME. These results suggest that post-hypoxia SPm+/ABCG2+ CSCs serve a functional role as a tumor stemness defense (TSD) phenotype to protect TME against bacterial invasion. Importantly, the CM of TSD phenotype undergoing bystander apoptosis may have therapeutic uses against CSCs residing in the hypoxic niche.