Merkel Cell Polyomavirus ST Antigen and NF-kB Inhibition

Introduction to Merkel Cell Carcinoma

Reference reviewed: Griffiths, D., et al. (2013). Merkel cell polyomavirus small T antigen targets the NEMO adaptor protein to disrupt inflammatory signaling. Journal of Virology, 87(24), 13853–13867.

Merkel cell carcinoma (MCC) is a relatively rare disease in which malignant cells form in the skin, usually in individuals who have a weakened immune system or extensive exposure to the sun. Merkel cells are found in the top layer (epidermis) of the skin, close to the nerve endings that respond to touch. Merkel cell carcinoma is also known as neuroendocrine carcinoma of the skin, and is sometimes called trabecular cancer. It forms when Merkel cells grow rapidly and out of control, typically starting in areas of the skin that receive greater sun exposure, such as the neck, arms, and legs. The cancer tends to grow aggressively and metastasize at a relatively early stage, spreading first to nearby lymph nodes and then through the lymphatic system, where it may subsequently affect the lungs, brain, bone, or other organs.

Experiments Performed

In 2008, researchers identified the Merkel cell polyomavirus (MCPyV) in 80% of Merkel cell carcinomas. This polyomavirus is one of the most recently discovered viruses associated with cancer and shows a strong linkage to tumor production and virulence. Research has demonstrated that a monoclonal viral integration pattern is found in most MCC cases, indicating that the virus uses RNA and other cellular mechanisms to expand into non-infected cell structures (p. 13853). Recent studies have identified a new function for inhibiting viral production by using small T antigen (ST) to limit transcription. This limiting effect involves an interaction between the virus and the NF-κB essential modulator protein (NEMO), which reduces the virus's ability to replicate and ultimately subverts it completely.

Hypothesis

293 cells were used as the experimental base. MCPyV ST was isolated from MCC tumor DNA using a methodology derived from base-pair genetics and then cloned to generate additional cells. The cells were maintained in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum and 1% penicillin-streptomycin. Cells were subsequently lysed in an RIPA buffer supplemented with a protease inhibitor cocktail and then separated. Total RNA was harvested and cellular structures were analyzed. Assays and immunofluorescence studies were performed. To delineate the actions of the protein and its capacity to inhibit cellular growth, SILAC-based immunoprecipitations and NF-κB pathway inductions were carried out, with results then calculated (pp. 13854–13855).

Evidence from this study and related work in the field suggests that MCPyV ST is oncogenic. Yet ST alone appears sufficient to reduce viral growth in certain cancerous rodent cells, which implies the same effect may hold in human cells. The authors assert that "specific depletion of MCPyV ST is sufficient to inhibit MCPyV-positive MCC cell growth" (p. 13853). In addition, the authors identify an essential role for NF-κB activation in both the antiviral and the inflammatory response, making it an ideal target for viral subversion. Because viral proteins antagonize all stages of the NF-κB signaling pathway, the authors posit that an interaction between MCPyV ST and the NF-κB protein could represent a robust new approach to treating MCC by targeting specific cellular mechanisms — one that may produce more positive patient outcomes with fewer side effects than traditional chemotherapy.