Open Access

It has become increasingly clear that caspases, far from being merely cell death effectors, have a much wider range of functions within the cell. These functions are as diverse as signal transduction and cytoskeletal remodeling, and caspases are now known to have an essential role in cell proliferation, migration, and differentiation. There is also evidence that apoptotic cells themselves can direct the behavior of nearby cells through the caspase-dependent secretion of paracrine signaling factors. In some processes, including the differentiation of skeletal muscle myoblasts, both caspase activation in differentiating cells as well as signaling from apoptotic cells has been reported. Here, we review the non-apoptotic outcomes of caspase activity in a range of different model systems and attempt to integrate this knowledge.

After more than twenty years of research, the molecular events of apoptotic cell death can be succinctly stated; different pathways, activated by diverse signals, increase the activity of proteases called caspases that rapidly and irreversibly dismantle condemned cell by cleaving specific substrates. In this time the ideas that apoptosis protects us from tumourigenesis and that cancer chemotherapy works by inducing apoptosis also emerged. Currently, apoptosis research is shifting away from the intracellular events within the dying cell to focus on the effect of apoptotic cells on surrounding tissues. This is producing counterintuitive data showing that our understanding of the role of apoptosis in tumourigenesis and cancer therapy is too simple, with some interesting and provocative implications. Here, we will consider evidence supporting the idea that dying cells signal their presence to the surrounding tissue and, in doing so, elicit repair and regeneration that compensates for any loss of function caused by cell death. We will discuss evidence suggesting that cancer cell proliferation may be driven by inappropriate or corrupted tissue-repair programmes that are initiated by signals from apoptotic cells and show how this may dramatically modify how we view the role of apoptosis in both tumourigenesis and cancer therapy.

Proteins destined for the secretory pathway are processed in the endoplasmic reticulum (ER) where a delicate balance exists between protein folding and degradation of terminally misfolded proteins. Different physiological as well as pathological stress conditions however, can lead to an imbalance between the ER protein folding capacity and protein load, giving rise to an accumulation of misfolded proteins in the ER lumen, a condition dubbed as ‘ER stress’. In an attempt to meet the increased folding demand, cells utilize a conserved signaling pathway, the unfolded protein response (UPR), which is initially charged to re-establish ER homeostasis and support survival. If this mechanism fails, persistent ER stress will eventually cause this cytoprotective UPR to switch into a cell death pathway that can activate mitochondrial apoptosis. As such, the dual function of the UPR in controlling cell fate may play a part in disease development and response to stress signals in conflicting ways. The lethal arm of the UPR may contribute to pathologies that are linked to unscheduled cell death, like diabetes and certain neurodegenerative diseases such as Alzheimer's and Parkinson’s, or be utilized by certain anticancer drugs to kill cancer cells. On the other hand, activation of the pro-survival function of the UPR may assist processes like tumorigenesis and chemoresistance, by endowing cancer cells with an increased capability to adapt to their hostile environment and to cope with cellular damage. Therefore, a better understanding of the different signaling pathways that emanate from the stressed ER and how their integration modulates cell fate decisions represents a crucial requirement to develop new strategies aimed at targeting the UPR for therapeutic purposes.

BACKGROUND Neuronal migration is a crucial process that allows neurons to reach their correct target location to allow the nervous system to function properly. AP-2alpha is a transcription factor essential for neural crest cell migration and its mutation results in apoptosis within this cell population, as demonstrated by genetic models. RESULTS We down-modulated AP-2alpha expression in GN-11 neurons by RNA interference and observe reduced neuron migration following the activation of a specific genetic programme including the Adhesion Related Kinase (Axl) gene. We prove that Axl is able to coordinate migration per se and by ChIP and promoter analysis we observe that its transcription is directly driven by AP-2alpha via the binding to one or more functional AP-2alpha binding sites present in its regulatory region. Analysis of migration in AP-2alpha null mouse embryo fibroblasts also reveals an essential role for AP-2alpha in cell movement via the activation of a distinct genetic programme. CONCLUSION We show that AP-2alpha plays an essential role in cell movement via the activation of cell-specific genetic programmes. Moreover, we demonstrate that the AP-2alpha regulated gene Axl is an essential player in GN-11 neuron migration.

Bcl-2 is known to have dual antiproliferative and antiapoptotic roles. Overexpression of Bcl-2 in the mammary gland using a whey acidic protein (WAP) promoter-driven Bcl-2 transgene inhibits apoptosis in the mammary gland during pregnancy, lactation, and involution, and also counteracts apoptosis induced by overexpression of a mutant p53 transgene (WAP-p53 172 R-L). WAP-Bcl-2 mice and nontransgenic controls were treated with the carcinogen dimethylbenz(a)anthracene (DMBA). Surprisingly, the nontransgenic mice developed mammary tumors with decreased latency. Tumors arising in WAP-Bcl-2 mice displayed substantially reduced levels of proliferation relative to those seen in nontransgenic mice (P < 0.015), perhaps resulting in the observed increase in tumor latency following carcinogen treatment. This WAP-Bcl-2 mouse tumor model reflects the situation seen in some human breast cancers overexpressing Bcl-2, where expression of Bcl-2 has been shown to correlate with a lower proliferative index in tumors.

Background: BCL-2 overexpression is frequently detected in nonmelanoma skin cancer. In normal skin, BCL-2 expression is restricted to the basal cell layer and the hair follicle bulge. Both contain stem cells targeted by carcinogens upon initiation of mouse skin carcinogenesis. It is unknown whether the anti-apoptotic activity of BCL-2 is involved in the susceptibility of this cell type to malignant transformation. If so, extending the pool of BCL-2-expressing cells to suprabasal skin layers should increase the likelihood of skin tumour formation. Materials and Methods: To resolve this issue, we generated a novel transgenic mouse line overexpressing BCL-2 in suprabasal layers of the epidermis. The influence of suprabasal BCL-2 on tumour formation was then tested by chemically inducing skin cancer using the two-stage initiation-promotion protocol. Results: Bcl-2 expression neither influenced the incidence nor the multiplicity of papillomas upon chemical tumour induction with 7,12-dimethylbenz[a]anthracene (DMBA) and 12-O-tetradeca-noylphorbol-13-acetate (TPA), nor their progression to carcinomas. Conclusion: Suprabasal expression of BCL-2 in skin does not increase the formation of papillomas or their malignant progression to squamous cell carcinomas in two-stage mouse skin carcinogenesis.

BACKGROUND Activator protein-2 (AP-2) transcription factors are critically involved in a variety of fundamental cellular processes such as proliferation, differentiation and apoptosis and have also been implicated in carcinogenesis. Expression of the family members AP-2alpha and AP-2gamma is particularly well documented in malignancies of the female breast. Despite increasing evaluation of single AP-2 isoforms in mammary tumors the functional role of concerted expression of multiple AP-2 isoforms in breast cancer remains to be elucidated. AP-2 proteins can form homo- or heterodimers, and there is growing evidence that the net effect whether a cell will proliferate, undergo apoptosis or differentiate is partly dependent on the balance between different AP-2 isoforms. METHODS We simultaneously interfered with all AP-2 isoforms expressed in ErbB-2-positive murine N202.1A breast cancer cells by conditionally over-expressing a dominant-negative AP-2 mutant. RESULTS We show that interference with AP-2 protein function lead to reduced cell number, induced apoptosis and increased chemo- and radiation-sensitivity. Analysis of global gene expression changes upon interference with AP-2 proteins identified 139 modulated genes (90 up-regulated, 49 down-regulated) compared with control cells. Gene Ontology (GO) investigations for these genes revealed Cell Death and Cell Adhesion and Migration as the main functional categories including 25 and 12 genes, respectively. By using information obtained from Ingenuity Pathway Analysis Systems we were able to present proven or potential connections between AP-2 regulated genes involved in cell death and response to chemo- and radiation therapy, (i.e. Ctgf, Nrp1, Tnfaip3, Gsta3) and AP-2 and other main apoptosis players and to create a unique network. CONCLUSIONS Expression of AP-2 transcription factors in breast cancer cells supports proliferation and contributes to chemo- and radiation-resistance of tumor cells by impairing the ability to induce apoptosis. Therefore, interference with AP-2 function could increase the sensitivity of tumor cells towards therapeutic intervention.

One function ascribed to apoptosis is the suicidal destruction of potentially harmful cells, such as cancerous cells. Hence, their growth depends on evasion of apoptosis, which is considered as one of the hallmarks of cancer. Apoptosis is ultimately carried out by the sequential activation of initiator and executioner caspases, which constitute a family of intracellular proteases involved in dismantling the cell in an ordered fashion. In cancer, therefore, one would anticipate caspases to be frequently rendered inactive, either by gene silencing or by somatic mutations. From clinical data, however, there is little evidence that caspase genes are impaired in cancer. Executioner caspases have only rarely been found mutated or silenced, and also initiator caspases are only affected in particular types of cancer. There is experimental evidence from transgenic mice that certain initiator caspases, such as caspase-8 and -2, might act as tumor suppressors. Loss of the initiator caspase of the intrinsic apoptotic pathway, caspase-9, however, did not promote cellular transformation. These data seem to question a general tumor-suppressive role of caspases. We discuss several possible ways how tumor cells might evade the need for alterations of caspase genes. First, alternative splicing in tumor cells might generate caspase variants that counteract apoptosis. Second, in tumor cells caspases might be kept in check by cellular caspase inhibitors such as c-FLIP or XIAP. Third, pathways upstream of caspase activation might be disrupted in tumor cells. Finally, caspase-independent cell death mechanisms might abrogate the selection pressure for caspase inactivation during tumor development. These scenarios, however, are hardly compatible with the considerable frequency of spontaneous apoptosis occurring in several cancer types. Therefore, alternative concepts might come into play, such as compensatory proliferation. Herein, apoptosis and/or non-apoptotic functions of caspases may even promote tumor development. Moreover, experimental evidence suggests that caspases might play non-apoptotic roles in processes that are crucial for tumorigenesis, such as cell proliferation, migration, or invasion. We thus propose a model wherein caspases are preserved in tumor cells due to their functional contributions to development and progression of tumors.