Colonic epithelial cell turnover: possible implications for ulcerative colitis and cancer initiation

In recent years, the interest in cell death regulation and its importance in normal tissue homeostasis and disease have been growing substantially. It is evident that tight regulation of cell numbers by proliferation and apoptosis ensures normal functioning of tissues and organs; such regulation prevents adverse inflammatory responses and precludes the development of cancer cells. Thus regulation of cell numbers and their modality of death are pivotal for the pathogenesis of various diseases of the colon, e.g. acute bacterial colitis, inflammatory bowel diseases (IBD; i.e. Crohn disease (CD) and ulcerative colitis (UC)), and colorectal cancer, including hereditary forms of colon cancer. This review focuses on the regulation of proliferation and death of colonocytes in the inflamed colon during acute and chronic inflammation in UC and development of UCassociated colorectal cancer. Cell death traditionally has been separated into death by necrosis and death by apoptosis (1). The major difference between these two modes of cell death is that apoptosis is not accompanied by leakage of intracellular constituents and thus does not by itself elicit an inflammatory response, whereas necrosis results in cell membrane rupture, often in response to a major physical injury (Fig. 1). Both terms refer to morphologic changes in dying cells. Apoptosis is characterized by cell shrinkage, chromatin condensation, cell membrane blebbing and, ultimately, fragmentation of the cell into membrane-restricted vesicles that are phagocytized rapidly by neighbouring cells or phagocytes of the monocyte linage (1). These changes are accompanied by an equally ordered series of events on the molecular level termed programmed cell death (PCD). PCD can be divided into the extrinsic (i.e. receptordependent) and the intrinsic (i.e. mitochondrion-dependent) apoptosis pathway. The two pathways are outlined in detail in Fig. 2. Receptor-mediated PCD is typically a response to an external stimulus, whereas the intrinsic pathway is a response to a programme initiated by the cell itself. However, interplay between the pathways exists in such a way that the receptormediated apoptosis pathway may recruit the intrinsic pathway and vice versa (2). Both pathways converge in a final common pathway of activated cystein-aspartyl-specific proteases called the caspases that normally exist as inactive zymogens in the cytosol. Caspases are activated by specific proteolysis and have the ability to process and activate other caspases in a cascade-like manner. The downstream caspases, caspases 3, 6 and 7, are activated late in the PCD process and are known as the effector caspases because they proteolyse several structural and homeostatic proteins. The effector caspases thus kill the cell by causing irreversible degradation of DNA and the cytoskeleton and by concomitant exposure of ‘eat me’ signals on the plasmalemma (2). The effector caspases, and especially caspase 3, therefore are normally responsible for the morphologic features of apoptosis. The effector caspases are activated by a set of upstream caspases called initiator caspases that are pathway-specific (Fig. 2). Receptor-mediated PCD typically activates effector caspases through the activation of caspase 8, whereas mitochondrion-mediated PCD depends on activation of caspase 9. The receptor-mediated pathway is activated by the death receptor binding of its ligand. Examples of death receptors on colonocytes are CD95 (Fas receptor, Apo-1), tumour necrosis factor receptor I (TNF-RI) and TNF-related apoptosis-inducing ligand-receptors 1 and 3 (TRAIL-receptors 1 and 3) (3, 4). Their ligands are CD95L (Fas), TNFand TRAIL, respectively. The binding and oligomerization of the death receptors permit binding and processing of procaspase 8 via a number of receptor-specific adaptor proteins. Subsequently, caspase 8 dimerises and activates the effector caspases (2). Interaction between CD95L on an activated cytotoxic T cell and a CD95-expressing enterocyte is an example of receptor-mediated apoptosis of the colon (3, 5). The intrinsic PCD pathway is controlled by the mitochondrion. The mitochondrion has a central role in initiation of the intrinsic pathway because it contains inactive apoptosis initiator molecules, which on release to the cytosol are able to activate the caspase cascade and thus elicit PCD. Release of the apoptosis initiator molecules is regulated by a number of proteins from the B-cell CLL/lymphoma 2 (Bcl-2) family located in the outer mitochondrial membrane (6). Proteins from this family can either prevent or promote PCD. Although the exact mechanisms are still a matter of debate, proapoptotic Bcl-2 family proteins are able to form pore complexes in the outer membrane that permit efflux of the apoptosis initiator proteins. The anti-apoptotic members of the Bcl-2 family seemingly disrupt the pore formation by hetero-oligomerization to the pro-apoptotic members of the Bcl-2 family. The lack of pore formation stabilizes the mitochondrion and thus prevents apoptosis (6). Examples of pro-apoptotic members of the Bcl-2 family are Bcl-2associated X protein (Bax) and Bcl-2 antagonist/killer (Bak), whereas Bcl-2 is an example of an anti-apoptotic Bcl-2 family protein. The apoptosis initiator proteins conREVIEW