Cyclin D1 as a driver of HNSCC - Head and neck squamous cell carcinomas (HNSCCs) are a diverse collection of cancers originating from distinct anatomical sites within the head and neck region including the oral cavity. Human papilloma virus (HPV), tobacco, and alcohol use are etiologic factors HNSCC pathogenesis. Current first-line modalities for HNSCC have limited efficacy in HPV- patients and such approaches are associated with high morbidity and quality of life issues highlighting an unmet clinical need. Cyclin D1 dysregulation through gene amplification, mutation, or protein stabilization is observed frequently in human cancers including HNSCC implying that the cyclin D1/CDK4 kinase provides cells with a proliferative advantage needed for cancer development. Fbxo4, a component of the E3 ligase, SCFFbxo4 that regulates ubiquitin-dependent degradation of cyclin D1, loss occurs in head and neck, esophageal, and uterine cancers among others as demonstrated by us. We have demonstrated that Fbxo4 knockout mice are susceptible to CDK4/6-dependent tumors and that such tumors are addicted to glutamine. Glutamine-dependence is a feature of many tumor cell types due to glutamine being the major anaplerotic carbon source for rapidly dividing cells. Glutamine deprivation can trigger reduced growth and apoptosis in tumor cells offering a potential point of therapeutic intervention. In the context of Fbxo4 loss, glutamine addiction is partially due to cyclin D1 overexpression, thereby suggesting additional SCFFbxo4 substrates that contribute to Fbxo4- dependent tumor suppression and metabolic regulation. Given the emergence of inhibitors of glutamine biosynthetic pathways, our observation provides a potential novel perspective in HNSCC therapy. For this vision, it is essential that we elucidate how the SCFFbxo4 E3 ligase coordinates metabolic reprogramming during HNSCC tumorigenesis. Through a combination of proteomic and genomic approaches we identified additional substrates. Among these is LKB1 (Serine/Threonine Kinase 11, STK11), an established regulator of cell stress and metabolism and depending upon cell context harbors either tumor suppressive or tumor promoting activities. We hypothesize that SCFFbxo4 is a master regulator of HNSCC, coordinating tumor cell proliferation and metabolic reprogramming via cyclin D1 and LKB1, thereby linking Fbxo4 to novel effectors in HNSCC pathogenesis. Loss of Fbxo4 drives a cyclin D1/CDK4-dependent increase in cell proliferation that requires metabolic reprogramming and activation of cellular adaptive pathways. This hypothesis is tested in the following Aims: Aim 1: Determine the ability of p53R172H to cooperate with D1T286A and drive HNSCC in vivo; Aim 2: Determine the mechanisms of Fbxo4-dependent regulation of LKB1 and its impact on HNSCC; Aim 3: Determine the therapeutic potential of targeting glutamine-metabolism in HNSCC with dysregulated Fbxo4-cyclin D/CDK4.
