Can liberating p53 from E6 free patients from HPV-related head and neck tumors?

Keywords: :

• Head and Neck Squamous Cell Carcinoma
• HPV
• E6
• p53
• p21
• cell cycle arrest
• apoptosis
• Bortezomib

This article refers to:

Liberation of functional p53 by proteasome inhibition in human papilloma virus-positive head and neck squamous cell carcinoma cells promotes apoptosis and cell cycle arrest

Over half a million patients are diagnosed with Head and Neck Squamous Cell Carcinoma (HNSCC) each year in the world. Most of these patients present with locally advanced tumors, and less than 50% will live for five years after treatment.¹ Concurrent platinum-based chemoradiation protocols improve loco-regional control, and have become a standard therapeutic strategy. However, they are associated with acute, high-grade toxicity that includes neutropenia, mucositis, xerostomia and swallowing impairment.

Consumption of alcohol and tobacco smoke is the major risk factor for HNSCC. However, compelling evidence has accumulated over the last decade for infection of head and neck epithelium by high-risk human papillomaviruses (HPVs) as an emerging etiology for HNSCC (about 25% of all HNSCC cases). The prevalence of HPV-related HNSCC is increasing rapidly in North America and North European countries. Patients with HPV-positive tumors constitute a distinct clinical HNSCC subpopulation, who respond better to therapy and have improved relapse-free and overall survival.²

HPV-driven carcinogenesis is mainly a consequence of deregulation of the E6 and E7 viral oncoproteins.³ E6 binds to the p53 tumor suppressor protein and the E6AP cellular E3-ubiquitin ligase (Fig. 1A), leading to p53 ubiquitination, proteasomal degradation and impaired function. The functional consequences include loss of p21 mediated control of the G₁/S and G₂/M checkpoints, reduced DNA-damage repair and cell cycle inhibition; and decreased expression of the pro-apoptotic Bax and Puma factors and consequent diminished cell death. E7 binds to the Retinoblastoma protein (pRb) and induces its degradation (Fig. 1B). pRb inhibits cell cycle progression via inhibition of transcription factors of the E2F family. Loss of pRb activates E2F and promotes cell proliferation.

Figure 1. Model representing the role of the E6 and E7 viral oncoproteins in HPV-driven carcinogenesis. (A) The p53 tumor suppressor regulates cell cycle arrest at G₁ and G₂/M and apoptosis induction via the regulation of its target genes, such as p21, and Puma and Bax respectively. HPV E6-dependent proteasomal degradation of p53 is blocked by Bortezomib. (B) pRb controls the G₂/M cell cycle checkpoint via the inhibition of E2F. Binding of pRb to HPV E7 induces its proteasomal degradation, the activation of E2F and stimulation of cell proliferation.

Most HPV-related HNSCC contain wild type TP53,⁴ and their better response to genotoxic therapies could be due to activation of wild-type p53. Interestingly, inhibition of E6 in HPV-related HNSCC cell lines in culture leads to p53 stabilization and increases cell death.^5,6 However, there was no direct evidence that p53 is responsible for this effect. Proof is now provided by Li and Johnson.⁷ They have confirmed that inhibition of E6 and E7 by siRNA in HPV-positive cell lines leads to p53 stabilization and triggers apoptotic cell death. In addition, they elegantly demonstrate the direct implication of p53 in this induced cell death, by additional inhibition of p53. The authors further observe that Bortezomib, a proteasome-inhibitor (Fig. 1A), increases p53 and p21 levels and results in dose-dependent death of HPV-positive cells. This effect is partially impaired by anti-p53 siRNA treatment, showing that p53 is implicated to some extent downstream of Bortezomib. Interestingly, the use of sublethal doses of Bortezomib leads to cell cycle arrest of HPV-positive cell cultures at either G₁ or G₂/S, to various extends depending on the cell line. The authors provide convincing evidence that this effect is also dependent on both p53 and p21.

The impact of HPV on improving the survival of patients with HNSCC is now clearly established, and there is a debate about de-escalating and/or modulating standard therapies in order to both better manage HPV-related HNSCC and spare patients treatment-related toxicities. In this context, the work by Li and Johnson provides evidence for a direct role of p53 in the increased sensitivity of HPV-positive lesions to genotoxic agents. It also suggests that liberation of p53 from HPV E6 oncroprotein mediated degradation, by inhibition of E6 or stabilization with the proteasome inhibitor bortezomib, could be interesting therapeutic options. Further work is required to assess the efficacy of bortezomib on irradiated HPV-positive and negative cell cultures. A recent phase I clinical trial has demonstrated that the use of bortezomid with concurrent cisplatin-based chemoradiotherapy is well tolerated by HNSCC patients.⁸ In the light of these new findings, it will be interesting to evaluate tumor response with respect to HPV-status.