Despite acting as a barrier for the organs they encase, epithelial cells turnover at some of the fastest rates in the body. Yet, epithelial cell division must be tightly linked to cell death to preserve barrier function and prevent tumour formation. How do the number of dying cells match those dividing to maintain constant numbers? We previously found that when epithelial cells become too crowded, they activate the stretch-activated channel Piezo1 to trigger extrusion of cells that later die1. Conversely, what controls epithelial cell division to balance cell death at steady state? Here, we find that cell division occurs in regions of low cell density, where epithelial cells are stretched. By experimentally stretching epithelia, we find that mechanical stretch itself rapidly stimulates cell division through activation of the same Piezo1 channel. To do so, stretch triggers cells paused in early G2 to activate calcium-dependent ERK1/2 phosphorylation that activates cyclin B transcription necessary to drive cells into mitosis. Although both epithelial cell division and cell extrusion require Piezo1 at steady state, the type of mechanical force controls the outcome: stretch induces cell division whereas crowding induces extrusion. How Piezo1-dependent calcium transients activate two opposing processes may depend on where and how Piezo1 is activated since it accumulates in different subcellular sites with increasing cell density. In sparse epithelial regions where cells divide, Piezo1 localizes to the plasma membrane and cytoplasm whereas in dense regions where cells extrude, it forms large cytoplasmic aggregates. Because Piezo1 senses both mechanical crowding and stretch, it may act as a homeostatic sensor to control epithelial cell numbers, triggering extrusion/apoptosis in crowded regions and cell division in sparse regions.

Tumor suppressor p53 exhibits an enigmatic phenotype in cells exposed to electrophilic, cyclopentenone prostaglandins of the A and J series. Namely, cells harboring a wild-type p53 gene accumulate p53 protein that is conformationally and functionally impaired. This occurs via an unknown molecular mechanism. We report that electrophilic cyclopentenone prostaglandins covalently modify and inhibit thioredoxin reductase, a selenoprotein that governs p53 and other redox-sensitive transcription factors. This mechanism accounts fully for the unusual p53 phenotype in cells exposed to electrophilic prostaglandins. Based on this mechanism we derived, tested, and affirmed several predictions regarding the kinetics of p53 inactivation; the protective effects of selenium; the structure-activity relationships for inhibition of thioredoxin reductase and impairment of p53 by electrophilic lipids; the susceptibility of hypoxia-inducible factor to inactivation by electrophilic lipids; and the equivalence of chemical inactivation of p53 to deletion of a p53 allele. Chemical precepts dictate that other electrophilic agents should also inhibit thioredoxin reductase and impair its governance of redoxsensitive proteins. Our results provide a novel framework to understand how endogenous and exogenous electrophiles might participate in carcinogenesis; how selenoproteins and selenium might confer protection against cancer; how certain tumors might acquire their paradoxical p53 phenotype; and how chronic inflammation might heighten the risk for cancer.

Electrophilic eicosanoids of the J series, with their distinctive cross-conjugated ␣,␤-unsaturated ketone, inactivate genetically wild type tumor suppressor p53 in a manner analogous to prostaglandins of the A series. Like the prostaglandins of the A series, prostaglandins of the J series have a structural determinant (endocyclic cyclopentenone) that confers the ability to impair the conformation, the phosphorylation, and the transcriptional activity of the p53 tumor suppressor with equivalent potency and efficacy. However, J series prostaglandins have a unique structural determinant (exocyclic ␣,␤-unsaturated ketone) that confers unique efficacy as an apoptotic agonist. In seeking to understand how J series prostaglandins cause apoptosis despite their inactivation of p53, we discovered that they inhibit the ubiquitin isopeptidase activity of the proteasome pathway. In this regard, J series prostaglandins were more efficacious inhibitors than representative members of the A, B, or E series prostaglandins. Disruption of the proteasome pathway with proteasome inhibitors can cause apoptosis independently of p53. Therefore, this finding helps reconcile the p53 transcriptional independence of apoptosis caused by ⌬12-prostaglandin J 2 . This discovery represents a novel mechanism for proteasome pathway inhibition in intact cells. Furthermore, it identifies isopeptidases as novel targets for the development of antineoplastic agents. Certain electrophilic prostaglandins (PG)1 can repress transactivation by NFB and p53 (1-4), two prominent transcription factors that govern the decision of a cell to survive or die (5-7). This transcriptional repression is a pharmacologically unique trait that distinguishes PG of the A and J series from other PG that act via membrane-spanning receptors (8). If the endocyclic ␣,␤-unsaturated ketone shared by A and J series PG confers activity, as hypothesized (1, 2, 4, 9, 10), then two predictions should be valid. First, individual A series and J series PG should act rather uniformly on the cellular processes they affect. Second, their cellular effects should be self-consistent with established models of NFB and p53 function. However, not all experiments affirm these predictions. For example, the A and J series PG both repress NFB transcription and inhibit IB kinase (1, 2); however, only ⌬12-PGJ 2 is anti-inflammatory (11). Likewise, the A and J series PG both repress p53 transcription; however, only the A series PG antagonize p53-dependent apoptosis (Ref. 4 and see below).Herein we report the discovery of a molecular mechanism that clarifies the distinctive cellular effects of cyclopentenone PG. Namely, J series PG preferentially inhibit the ubiquitin isopeptidase activity (ubiquitin-specific protease) of the proteasome pathway. This pathway is the major nonlysosomal degradation pathway in cells (12, 13). The degradation of target proteins via this pathway largely depends on their covalent modification with a ubiquitin polymer. This polymer consists of ubiquitin (8.5 kDa) subunits that are...