Resistance to Apoptosis via the Intrinsic Pathway Is a Hallmark of Cancer
Internal cellular damage or stress, including DNA damage, absence of cell anchorage, too little or too much oxygen metabolism, oncogene activation, and radiation damage, can stimulate the intrinsic pathway of apoptosis in normal cells.
Most, and perhaps all, cancer cells are more resistant than normal cells to apoptosis through this pathway. Resistance to apoptosis not only increases the probability that the cell will be able to accumulate further genetic damage, but also reduces the likelihood that cancer cells can be eliminated. This is because the antitumor activity of the immune system, as well as most chemotherapy and radiation treatments, depend on apoptosis. Thus, resistance to apoptosis often means resistance to treatment.The intrinsic pathway is considerably more complex than the extrinsic pathway, and this discussion focuses on three major elements of the pathway involved in activating caspases: p53, the mitochondrion, and the Bcl family of proteins (see Figure 2-11). This family of proteins was Oiiginally discovered in a cancer (“Bel” is from B-cell lymphoma, a type of leukemia in which the first such protein was discovered) and includes both pro-apoptotic and anti-apoptotic members. The balance between pro- and anti-apoptotic members determines whether the cell lives or dies. The resistance of cancer cells to apoptosis arises not only from mutations, such as those already described for p53, but also from underexpression of pro-apoptotic mediators and overexpression of anti-apoptotic proteins.
We begin with the mitochondrion, familiar as the “powerhouse” of the cell responsible for generating ATP, but also the central control point for the intrinsic pathway of apoptosis. Recall that the mitochondrion has both an inner membrane, responsible for electron transport, and an outer membrane, responsible for compartmentation of this organelle.
Pro- apoptotic signals cause the outer membrane of the mitochondria to become leaky, releasing several pro-apoptotic proteins not normally found in the cytoplasm. Among the most important is cytochrome c, an electron transport protein that is only loosely attached to the inner membrane. In the cytoplasm, cytochrome c stimulates the assembly of a multiprotein complex (the apoptosome) that directly stimulates the activity of an activating caspase (caspase-9), ultimately leading to the activation of executioner caspases. What then determines the extent of permeability (leakiness) of the mitochondrial outer membrane?The Bcl family members are major regulators of mitochondrial outer membrane permeability. The pro-apoptotic members of this family, such as Bax, lead to permeabilization by assembling to form channels in the outer membrane through which cytochrome c can pass. Pro-apoptotic members of the family can also cause the channel through which ATP normally passes into the cytoplasm to open wider than usual. The anti-apoptotic members of the family, such as Bcl-2, seem to function by binding to pro-apoptotic members, inhibiting their activity. In a healthy cell, anti-apoptotic Bcl members are at high enough concentration to neutralize pro-apoptotic activity. Damage increases the amount of pro-apoptotic Bcl molecules and leads to membrane permeabilization. Thus the balance between pro- and anti-apoptotic members of the family controls the permeability state of mitochondria and the survival of the cell.
With about 20 different members of the Bcl family, the balance between pro- and anti-apoptotic Bcl molecules has multiple controls, but p53 activity is certainly a major player. Recall that when activated (e.g., by DNA damage), p53 acts as a transcription lactor, and at least three different pro-apoptotic Bcl genes are transcriptionally activated by p53. These include Bax, and also the particularly powerful pro-apoptotic protein, PUMA. Downstream, p53 also activates the transcription of the activating caspase-9 gene, and the gene of a major cytoplasmic component of the apoptosome.
In addition to acting as an activating transcription factor, p53 serves as an inhibitory transcription factor for some genes, including that of the aιιli-apoplolic Bcl-2 protein. Finally and independent of transcription, activated p53 can directly activate Bax, which is required for its ability to assemble into channel structures. With these multiple effects on apoptotic genes and proteins, p53 is regarded as a central apoptotic control point, in addition to its role in cell cycle regulation.As noted earlier in the discussion of p53, the importance of apoptosis to Iumorigenesis is that with normal apoptosis, almost all damaged cells are eliminated. Without apoptosis, damaged cells live to accumulate additional damage, which illustrates why multiple mutations and dysfunctions are required for tumors to reach a clinically significant stage. The resistance of cancer cells to apoptosis arises from many types of mutations and disruptions of normal gene expression. Perhaps most importantly, mutation of the p53 gene eliminates its DNA binding and thus transcriptional activity. Related to p53 activity is a protein engaged in p53∖s normal proteolytic breakdown (see previous discussion). Overexpression of this protein (MDM2) in various cancers of soft tissues inhibits the accumulation of p53 to active levels and therefore inhibits both cell cycle arrest and apoptosis. The anti-apoptotic Bcl-2 protein is overexpressed in a variety of human cancers, including 60% of human follicular lymphomas, but also some lung cancers, melanoma, and prostate cancer. Another common apoptotic lesion seen in cancer cells is overexpression of proteins that bind to and directly inactivate caspases, as well as mutation or loss of expression of the caspases themselves.