As previously described, a tumor arises from uncontrolled cell growth. This uncontrolled cell growth is the result of the "green light for growth" being turned on forever. One classic example of this scenario involves the EGF/EGFR/Ras signaling pathway.
Tumor cells do not receive growth suppression signals from tumor suppressor genes (TSGs). TSGs are defined as genes that protect cells from cancerous development. For this reason, TSGs are also called anti-oncogenes. Mutation in TSGs renders TSGs unable to carry out anti-cancer functions.
The gene products of TSGs are tumor suppressor proteins, which come in two categories: Cellular brakes, such as Rb, and cell cycle checkpoint proteins, such as p53.
As an additional type of TSG, TGFβ is somewhat complicated. As cancer stage progresses, TGFβ expression and activity become increasingly elevated. The result, however, is not growth suppression; on the contrary, TGFβ changes from being tumor-suppressing to tumor-promoting. This will be further discussed in more detail later in the course.
In summary, we have discussed three types of TSGs: TGFβ, p53, and Rb.
