Promotion - Stages of carcinogenesis

The second stage or promotion is reversible and does not include changes in structure of DNA but rather in the expression of the genome of the initiated cell. This results from an interaction among the changed DNA structure and some environmental factors termed 'promoting agents'. The stage of promotion needs extended interaction with the promoting agent, if the third stage of progression is to be teached. Figure describes the process of promotion in a simplified way. A single application of an initiator to the skin of a mouse produces initiated cells but no tumor

Figure:  The concept of initiation and promotion 

(a) However, if the initiator is applied to the skin first then subsequent application of the promoter results in tumor

(b) If the promoter is withdrawn the tumor regresses leaving behind initiated cells and continued application of promoter on tumor bearing skin lends to invasive carcinoma.

(c) The application of a promoter alone to the skin also does not produce tumor

(d) If the promoter is applied before the initiator is applied even then no tumor is formed.