Head and neck squamous cell carcinomas (HNSCCs) are prevalent cancers with substantial chance of recurrence, for which irradiation is an essential component of both curative and palliative treatment. A notable dichotomy exists in treatment response between human papillomavirus (HPV)-positive and HPV-negative tumors, with HPV-negative HNSCC generally being less responsive to chemo- irradiation treatment and having a poorer prognosis. Furthermore, it has been widely established that hypoxia strongly inhibits radiotherapy efficacy, which subsequently leads to a poor prognosis even in HPV-positive tumors. These differences in HNSCC types and treatment sensitivities warrant patient- and tumor specific treatment approaches, and understanding them may yield novel targets for therapy.
By investigating the mechanisms causing HPV-positive tumors to be more radiosensitive, we found new targets for radiosensitizing treatment combinations in HPV-negative HNSCC. In HPV-positive HNSCC, the viral oncogene E7 binds to retinoblastoma protein (RB), targeting it for ubiquitination and proteasomal degradation. E7-mediated RB degradation leads to overexpression of p16, a cyclin dependent kinase (CDK)-4 and -6 inhibitor that reportedly is mechanistically involved in radiosensitivity. We indeed found that CDK4/6 inhibition using palbociclib significantly radiosensitized RB-positive, HPV-negative, but not HPV-positive, HNSCC cells. Importantly, this radiosensitizing effect also occurred under hypoxia. We found that palbociclib reduced the expression of BRCA1 and RAD51, which resulted in reduced homologous recombination DNA damage repair capacity, and thus could be applied to radiosensitize HPV-negative HSNCC.
HPV-positive HNSCC tumors are considered candidates for treatment de-escalation because of their excellent treatment response and, consequently, prognosis. In contrast, hypoxic HPV-positive tumors also exhibit radioresistance, and thus may require similar treatment schedules as HPV-negative tumors. Earlier, we found that under hypoxia in vitro, tumor cells activate the pAKT survival pathway. Furthermore, in xenografted human HNSCC tumors in vivo, pAKT expression was specifically expressed in hypoxic cells. Therefore, we hypothesized that inhibition of AKT activation may specifically target hypoxic tumor cells. Indeed, in all tested HNSCC cell lines AKT inhibition using MK2206 was significantly more effective in inhibiting cell growth under hypoxia than during normoxic conditions. Interestingly, HPV-positive cell lines were also highly sensitive to pAKT-inhibition at normoxia. Hypoxia significantly reduced radiosensitivity irrespective of HPV-status, yet specifically in HPV-positive cells this could be efficiently reversed by AKT inhibition.
Thus, in HNSCC treatment sensitivity and prognosis differs widely between subtypes, with impact of both HPV and hypoxia status. However, combining radiotherapy with agents that target either the CDK4/6 pathway (for HPV-negative tumors) or pAKT (for hypoxic or HPV-positive tumors) may yield superior tumor control and subsequent prognosis.
