Skin tumor promotion by phorbol esters is a two-stage process

Area of Science:

• Chemical biology
• Dermatology
• Carcinogenesis

Background:

• Phorbol esters are known tumor promoters, with phorbol 12-myristate 13-acetate (PMA) being a potent example.
• Vitamin A derivatives possess antipromoting properties.
• Semisynthetic compounds can combine properties of known biologically active molecules.

Purpose of the Study:

• To investigate the biological effects of phorbol 12-retinoate 13-acetate (PRA), a compound merging vitamin A's antipromoting principle with a phorbol ester structure.
• To evaluate PRA's potential as a tumor promoter and its role in the two-stage tumor promotion model.
• To compare PRA's efficacy as a second-stage promoter with other known compounds like mezerein.

Main Methods:

• Topical application of PRA and PMA to the skin of NMRI mice.
• Induction of skin inflammation, epidermal proliferation, and hyperplasia.
• Assessment of tumor promotion in chemically initiated mouse skin models.
• Investigation of the role of prostaglandin E synthesis in mitogenic effects.
• Comparison of PRA's promoting activity with mezerein.

Main Results:

• A single topical application of PRA induced skin inflammation, proliferation, and hyperplasia similar to PMA.
• Both PRA and PMA's mitogenic effects were mediated by prostaglandin E synthesis.
• PRA did not promote tumor development in initiated mouse skin, even at high doses.
• PRA acted as a potent second-stage tumor promoter when combined with PMA applications.
• PRA demonstrated greater potency as a second-stage promoter than mezerein in NMRI mice.

Conclusions:

• PRA is a nonpromoting irritant mitogen that effectively induces epidermal hyperplasia.
• PRA strongly supports the two-stage theory of carcinogenesis, where initial promotion is followed by chronic hyperproliferation.
• PRA is a highly effective second-stage tumor promoter in the NMRI mouse model, outperforming mezerein.