Final answer:
The third class of signal molecules that inhibits apoptosis is known as survival factors. They regulate proteins in the Bcl-2 family, contributing to cell survival. Disturbances in these pathways, such as mutations, receptor loss, or overexpression of growth factor pathways, can prevent apoptosis and potentially lead to cancer.
Step-by-step explanation:
The third class of signal molecules that inhibits apoptosis by regulating members of the Bcl-2 family of proteins is survival factors. These molecules play a critical role by inhibiting the process of programmed cell death (apoptosis), ensuring cell survival under various conditions. Survival factors can counteract the pro-apoptotic molecules within the Bcl-2 family, tipping the balance towards cell survival rather than cell death.
A scientist observing that a cancer cell line does not die upon addition of an inducer of apoptosis could hypothesize that an explanation might be a mutation that prevents the initiation of apoptosis signaling. Another possibility is that the cells have lost expression of the receptor for the apoptosis-inducing ligand or, alternatively, the cells might be overexpressing a growth factor pathway that inhibits apoptosis. Each of these scenarios could independently or in combination prevent apoptosis, potentially leading to uncontrolled cell proliferation and tumor growth.
It is important to note that cell division and growth are influenced by signaling pathways that respond to various factors. A gene that codes for a positive cell cycle regulator is referred to as a proto-oncogene or an oncogene. In contrast, a tumor suppressor gene codes for proteins that suppress cell growth and can signal the cell to undergo apoptosis if there are unrepaired damages. These mechanisms are essential for maintaining cellular homeostasis and preventing conditions like cancer.