Genesis and reversal of the ischemic phenotype in epithelial cells

Ischemic injury to predominantly epithelial organs such as the kidney remains a major cause of morbidity and mortality. For example, in hospitalized patients with acute renal failure, it is estimated that around 50% of all cases are ischemic in origin (1). Nevertheless, progress in the medical management of this and other syndromes in which ischemia occurs has been slow. In part, this may be related to our still rudimentary understanding of the cell biology underlying the ischemic epithelial phenotype and the molecular mechanisms behind the recovery of normal cell and tissue organization. Epithelial tissues, such as the kidney and gut, not only provide a physical barrier between biologic compartments, but mediate vectorial and selective transport of ions, water, and macromolecules between blood and the external environment. These functions depend on the integrity of intercellular junctions (e.g., adherens, tight), the arrangement of lipids and proteins in the plasma membrane into strictly maintained apical and basolateral domains, and productive cell-substratum interactions, all of which are severely affected by ischemia/reperfusion. Although other factors, such as oxidative damage and ion and pH changes, likely play important roles in the generation of the ischemic epithelial phenotype, much of the damage is believed to be due to depletion of cellular ATP (2, 3). Thus, cell culture models using agents that deplete cellular ATP have been used extensively to study ischemic injury in polarized epithelial cells (3). Although the fidelity with which these in vitro models reproduce the lesions observed in vivo has been debated, there is little doubt that these ATP depletion/repletion cell culture models provide valuable insights into the molecular mechanisms underlying ischemic injury and recovery, as similar cellular and molecular lesions are often found in cells of the ischemic whole organ. Many of these lesions are remarkably specific, biochemically definable, and potentially regulated; recovery from these lesions after short-term injury appears to be mediated by a combination of both previously elucidated and potentially novel sorting mechanisms that are transduced by “classical” signaling pathways. Among other cellular and molecular lesions, ischemia and/or ATP depletion induces misfolding and/or aggregation of membrane and secreted proteins (4); disruption of the actin-based cytoskeleton (5); disturbances in apical-basolateral protein polarization (6); mislocalization and degradation of protein components of the intercellular junctions (7, 8); upregulation of a number of genes, including molecular chaperones (4, 9), growth factors and their receptors (10); perturbation of integrin-mediated cell-substratum adhesion (11–13); and induction of programmed and nonprogrammed cell death (2). Alterations in the actin cytoskeleton and integrin-mediated cell-substratum interactions have been extensively reviewed elsewhere (5, 13). Here we focus primarily on recent information on lesions affecting the permeability barrier (intercellular junctions), signaling events involved in the recovery of this barrier, and the roles of molecular chaperones in protecting epithelial cells. The establishment and maintenance of a selectively permeable barrier occur through homotypic interactions of the extracellular domains of multiple transmembrane adhesion molecules between adjacent cells. Examples of such proteins include E-cadherin in the adherens junction (AJ) and the occludin/claudin families in the tight junction (TJ). The intracellular domains of these adhesion molecules also interact (directly or indirectly) with a number of cytoplasmic proteins, including α, β, and γ catenin in the AJ, and zonula occludens-1 (ZO-1), ZO-2, ZO-3, and fodrin in the TJ, providing a functional link to the actin-based cytoskeleton. These interactions also modulate the stability of the adhesion proteins either by maintaining their appropriate conformations to recognize extracellular domains in adjoining cells or perhaps by inhibiting internalization and degradation. Under ischemic conditions, it appears that many of these cellular processes/structures are compromised, promoting junctional protein internalization and degradation, thereby disturbing the cell-cell interactions and the permeability barrier. Identifying molecular mechanisms underlying the cascade of events that induce cellular injury and those involved in the cell’s recovery is key to developing rational therapeutic approaches to diminish the morbidity associated with ischemic injury to epithelial tissues.