Chemical-induced Two-stage Skin Carcinogenesis Model: An Experimental In Vivo Mouse Model of Skin Cancer

- In experimental cancer mouse models, the induced cancer in the animal resembles human cancer in phenotype. They provide an efficient platform to evaluate the efficacy of potential therapies. To induce a two-step skin carcinogenesis model, first, prep an immunocompromised anesthetized mouse in the prone position. Shave its fur to expose the underlying skin. In the first step, apply a potent tumor-initiating agent on the exposed dorsal skin.

Those hair follicles in their resting or telogen phase, harbor inactive sebaceous glands. The resulting lack of sebum flow allows the accumulation of the compound within these hair follicles. The compound diffuses into the surrounding cells and induces target gene mutation, primarily in the hair follicle stem cells or primary keratinocyte stem cells. The altered gene expression causes tumor initiation in these cells.

In the second step, apply a suitable tumor growth-promoting agent on the dorsal skin repeatedly during the desired time intervals. The chemical stimulates the expansion of the initiated cell population. The continued cell proliferation leads to the formation of carcinogenic outgrowths called papillomas.

In the following protocol, we show the generation of a two-stage skin carcinogenesis mouse model using the chemicals DMBA, a mutagen, and TPA, a growth-promoting agent.

- Before beginning the experiment, house any aggressive 7 to 9-week-old mice in separate cages to avoid fighting and skin lesions. For skin papilloma induction, first, shave the back skins of the experimental animals, and weigh the mice individually. Forty-eight hours after shaving, use a pipette to apply 50 micrograms of DMBA in 200 microliters of acetone onto the shaved area of each anesthetized mouse.

For skin papilloma promotion, seven days after DMBA application, treat the skin with 5 micrograms of TPA in 200 microliters of acetone, two times a week through the end of the experiment. Examine the animals for papillomas and photograph and record the size of each individual papilloma on a map, every week.

A palpable mass greater than 1 millimeter in diameter is considered a papilloma if it remains longer than one week.