How apoptosis is regulated, and what goes wrong in cancer

Programmed cell death (apoptosis) is an evolutionarily conserved pathway needed for embryonic development and tissue homoeostasis.¹ Apoptosis is the normal physiological response to many stimuli, including irreparable DNA damage. Various diseases evolve because of hyperactivation (neurodegenerative diseases, immunodeficiency, ischaemia-reperfusion injury) or suppression of programmed cell death (cancer, autoimmune disorders).²

In cancer, the balance between proliferation and programmed cell death is disturbed, and defects in apoptotic pathways allow cells with genetic abnormalities to survive. Most cytotoxic and hormonal treatments, as well as radiation, ultimately kill cancer cells by causing irreparable cellular damage that triggers apoptosis. Consequently, the efficacy of cancer treatments depends not only on the cellular damage they cause but also on the cell's ability to respond to the damage by inducing apoptotic machinery. Accordingly, mutations in apoptotic pathways may result in resistance to drugs and radiation. Such mutations might serve as predictors of chemoresistance and, most importantly, as new treatment targets.

Mitochondria and cell surface receptors mediate the two main pathways of apoptosis.¹ The mitochondrial pathway is thought to be important in response to cancer treatment and is mediated by bcl-2 family proteins. The final execution of cell death is performed by the caspase cascade, which is triggered by release of cytochrome C from mitochondria.

Apoptotic genes

The most studied genes related to apoptosis are the tumour suppressor gene p53, the anti-apoptotic gene bcl-2, and the pro-apoptotic gene bax. Normal wild type p53 can limit cell proliferation after DNA damage by two mechanisms: arresting the cell cycle or activating apoptosis.³ p53 has a dual and complex role in chemosensitivity; it can either increase apoptosis or arrest growth and thereby increase drug resistance. This may explain why promising preclinical data indicating that presence of wild type p53 would predict chemosensitivity have translated into more conflicting clinical data.⁴,⁵ Moreover, drugs that do not cause DNA damage, for instance taxanes and vinca alkaloids, may induce apoptosis through pathways that are independent of p53. Heterogeneous clinical data may also have resulted from use of different protein and molecular based methods to determine the p53 status. Sequencing gives the most complete picture of the p53 status,⁶ but even functional p53 does not exclude defects somewhere downstream in the apoptotic pathway. The importance of p53 for chemosensitivity, however, is supported by the fact that, currently, the most curable cancers are among the minority of tumours in which p53 is not mutated—that is, some haematopoietic and germ cell tumours.

Overexpression of bcl-2 was first associated with follicular B cell lymphomas. Theoretically, overexpression of bcl-2 could provide a survival advantage for cancer cells, but in vivo, bcl-2 expression has been associated with a more favourable prognosis in many malignant diseases. Indeed, in breast cancer, tumours positive for bcl-2 often have oestrogen receptors and a more favourable prognosis. Oestrogen has been shown to be a positive regulator of bcl-2 gene expression in breast cancer cell lines.⁷

The pro-apoptotic protein bax is the most studied member of the bcl-2 family in cancer. Loss of bax function seems to be important in the pathogenesis of colorectal cancers.⁸ In preclinical studies, induction of bax has been reported to restore sensitivity to drug and radiation induced apoptosis, whereas overexpression of bcl-2 has been shown to suppress apoptosis. However, the few clinical studies on the predictive value of bcl-2 family proteins in treatment of haematological malignancies or solid tumours have produced conflicting results.⁵,⁹

Inducing apoptosis

Several strategies have been tried to induce the apoptotic programme. The first approach was gene directed therapy to restore normal p53. Although the results have been interesting, refinement of the vectors and delivery concepts is needed. The first phase I pharmacokinetic study with bcl-2 antisense oligonucleotide (which effectively degrades messenger RNA) in patients with non-Hodgkin's lymphoma shows that the treatment is well tolerated.¹⁰ However, only one patient showed objective response while 11 patients had stable disease, and in nine patients the cancer progressed.¹⁰ These types of therapies, however, are likely to be more efficient when combined with chemotherapy or radiation, which triggers apoptosis. It might also be beneficial to combine pro-apoptotic treatment with anti-angiogenesis treatments as hypoxia has been shown to promote apoptosis.¹¹

Tumour heterogeneity and clonal variability will provide an extra challenge for future investigations and successful treatments based on apoptosis. The factors in apoptotic pathways have opened a new exciting dimension in our understanding of how and when cancer treatments succeed or fail—this holds promise for better therapeutic strategies in the future.