The Intrinsic Apoptotic Pathway

The intrinsic apoptotic pathway is activated by intracellular death signals, including oxidative stress and DNA damage. When DNA damage occurs, checkpoint proteins like ATM and Chk2 are activated, which phosphorylate p53 and trigger pro-apoptotic responses. These internal cellular stressors, such as cellular injury or hypoxia, initiate the cascade leading to programmed cell death.

The Bcl-2 family contains both pro-apoptotic and anti-apoptotic members that balance cell survival and death. Pro-apoptotic proteins like Bax, Bak, and BH3-only proteins activate apoptosis, while anti-apoptotic proteins like Bcl-XL and Bcl-2 inhibit them. Death signals activate BH3-only proteins, which either inactivate anti-apoptotic proteins or directly bind effector proteins to initiate the pathway.

When Bax and Bak oligomerize on the mitochondrial membrane, they form pores that release cytochrome c into the cytoplasm. Cytochrome c then binds to Apaf-1, triggering assembly of a heptameric complex called the apoptosome. This structure recruits and activates procaspase-9, which becomes caspase-9 and initiates the executioner caspase cascade.

The apoptosome is a heptameric complex formed when cytochrome c binds to Apaf-1 monomers and they assemble together. This structure serves as a platform that recruits and activates the initiator procaspase-9, converting it to active caspase-9. Caspase-9 then activates executioner caspases that cleave cellular proteins, resulting in apoptosis.

Cancer cells often have mutations that inhibit the intrinsic apoptotic pathway. Overexpression of anti-apoptotic proteins like Bcl-2 prevents cell death, while inhibitor of apoptosis proteins (IAPs) inactivate caspase-9. These alterations allow damaged cells to survive when they should undergo apoptosis, contributing to tumor development and progression.

Several anti-cancer drugs have been developed to reactivate the intrinsic apoptotic pathway in cancer cells. Molecules like Nutlin-2 and MI-219 inhibit MDM2 binding to p53, preventing p53 inactivation and promoting apoptosis. Other drugs such as SH122, JP1201, and YM155 inhibit IAPs and activate caspases, initiating apoptosis in cancer cells.

During DNA damage, checkpoint proteins ATM and Chk2 phosphorylate p53, activating it as a transcription factor. p53 then activates pro-apoptotic proteins such as Bax, Bak, PUMA, and Noxa while inhibiting anti-apoptotic proteins like Bcl-2 and Bcl-XL. This coordinated shift in Bcl-2 family protein balance triggers the mitochondrial pathway, leading to apoptosis of damaged cells.