Survival of exfoliated epithelial cells: a delicate balance between anoikis and apoptosis

Abstract

The recovery of exfoliated cells from biological fluids is a noninvasive technology which is in high demand in the field of translational research. Exfoliated epithelial cells can be isolated from several body fluids (i.e., breast milk, urines, and digestives fluids) as a cellular mixture (senescent, apoptotic, proliferative, or quiescent cells). The most intriguing are quiescent cells which can be used to derive primary cultures indicating that some phenotypes retain clonogenic potentials. Such exfoliated cells are believed to enter rapidly in anoikis after exfoliation. Anoikis can be considered as an autophagic state promoting epithelial cell survival after a timely loss of contact with extracellular matrix and cell neighbors. This paper presents current understanding of exfoliation along with the influence of methodology on the type of gastrointestinal epithelial cells isolated and, finally, speculates on the balance between anoikis and apoptosis to explain the survival of gastrointestinal epithelial cells in the environment.

Figure 1

The concept of epithelial functional unit. The flow chart summarizes the cellular status of epithelial cells recovered by dietary palmitate on intestinal renewal (left panel) or by cycles of fasting/refeeding on gastric exfoliation (right panel). As shown in central panel, epithelial cell functional units are organized in four compartments (stem cells, proliferative, terminally differentiated, senescent). The isolation of exfoliated cells is dependent on the techniques (manual or induced by some stress), the epithelium itself, the health status and the species. A major difference in the autophagy/phagocytosis process of senescent cells has been found between rat or mouse-exfoliated colonocytes which are lost in the lumen and human colonocytes which are rarely lost in the lumen but actively recycled by neighboring cells [15]. Quiescent epithelial cells can be isolated from different body fluids (breast milk, urines, and digestives fluids). They are believed to enter rapidly in anoikis after exfoliation. Anoikis is considered as an autophagic state promoting epithelial cell survival after a timely loss of contact with extracellular matrix and cell neighbors [16]. Primary cultures can be derived from breast milk indicating that some phenotypes retain clonogenic potentials. Molecular components of circadian clocks are detected in these cells like in any other somatic cells and could provide molecular data on the expression ratios between these proteins provided sampling procedures are standardized. Further work is needed to check for any different rates of exfoliation according to the time of day, seasons, and to differences in phenotypes.

Figure 2

From a physiological point of view, exfoliated epithelial cells resulting from nutritional induction or mild stress are in anoikis by activating a survival mechanism partly using the molecular paths of macroautophagy. Macroautophagy, here referred to as autophagy has been described in amino-acid-free (a) as well as in glucose-free (b) situations. Autophagosome elongation is triggered by lipid modification of LC3 (by phosphatidylethanolamine, PE). The first pathway is linked to growth factors and nutrient-sensing pathways (a), and the second is related to energy-sensing pathway (b). Detachment of extracellular matrix induces both pathways and may activate also the integrated stress response through PERK and eIF2alpha, [16] as such the situation of exfoliated cells is complex and needs more biochemical description to delineate stable molecular tags, useful in assay design, from the labile ones. The experimental starvation of primary or cancerous cells in culture consists in exposing cells to eagle minimum essential medium without serum for few hours. Under these conditions, amino-acid-free situations have been described [37, 38]. Phosphorylation involving Ulk1 in AMPK regulation has been demonstrated [39]. Survivin has also been involved in the inhibition of the conversion of LC3-I to LC3-II form (i.e., acting as an inhibitor of autophagosome formation—[40]) but in cancer cells or stem cells. However, the role of survivin in the turnover of adult cells is still debated [41, 42]. The expression of survivin has been reported in gastric parietal cells both in adult rat and human [38]. Gastric exfoliated epithelial cells of preterm infants do express high amount of survivin suggesting a crucial role of this molecule in the survival of these cells [43, 44]. Glucose starvation occurs at birth [45, 46]. Along with cytokines and drugs, nutritional factors are now considered to be able to alter the balance between cellular survival and death [47].