Temperature-Induced Inactivation of Cytoplasmic Biogel Osmosensing Properties is Associated with Suppression of Regulatory Volume Decrease in A549 Cells

Search for Upstream Cell Volume Sensors: The Role of Plasma Membrane and Cytoplasmic Hydrogel

The plasma membrane plays a prominent role in the regulation of cell volume by mediating selective transport of extra- and intracellular osmolytes. Recent studies show that upstream sensors of cell volume changes are mainly located within the cytoplasm that displays properties of a hydrogel and not in the plasma membrane. Cell volume changes occurring in anisosmotic medium as well as in isosmotic environment affect properties of cytoplasmic hydrogel that, in turn, trigger rapid regulatory volume increase and decrease (RVI and RVD). The downstream signaling pathways include reorganization of 2D cytoskeleton and altered composition of polyphosphoinositides located on the inner surface of the plasma membrane. In addition to its action on physico-chemical properties of cytoplasmic hydrogel, cell volume changes in anisosmotic conditions affect the ionic strength of the cytoplasm and the [Na⁺]_i/[K⁺]_i ratio. Elevated intracellular ionic strength evoked by long term exposure of cells to hypertonic environment resulted in the activation of TonEBP and augmented expression of genes controlling intracellular organic osmolyte levels. The role of Na⁺_i/K⁺_i -sensitive, Ca²⁺_i -mediated and Ca²⁺_i-independent mechanisms of excitation-transcription coupling in cell volume-adjustment remains unknown.

Heat shock exerts anticancer effects on liver cancer via autophagic degradation of aquaporin 5

Previous studies described that the expression of aquaporin 5 (AQP5) was altered in tumors of various organs. AQP5 is attracting attention as a new cancer therapeutic target. In the present study, heat shock-induced changes in AQP5 expression were evaluated by immunofluorescent staining (IF) and western blotting (WB) of liver cancer cells. AQP5 knockdown experiments or a heat shock treatment were conducted, and their effects on cell volume, proliferation, cell cycle, the activity of apoptosis and migration/invasion were compared. Cycloheximide (CHX) chase experiments and double IF of AQP5 and light chain 3B (LC3B) were performed to investigate the mechanisms underlying changes in AQP5 expression. The results showed that IF and WB revealed decrease in AQP5 expression on cellular membranes and in the cytoplasm of heated cells. AQP5 knockdown and heat shock similarly decreased cell volume, suppressed migration/invasion and proliferation, and induced early apoptosis and partial G0/G1 arrest. CHX chase experiments revealed that heat shock accelerated the degradation of AQP5, which was rescued under CHX and the autophagy inhibitor, bafilomycin A1 (BafA1). Double IF showed the co-localization of AQP5 and LC3B on BafA1-treated heated cells. In conclusion, we demonstrated that heat shock decreased AQP5 on cellular membranes and in the cytoplasm by activating autophagic degradation, and heat shock and AQP5 knockdown exerted similar anticancer effects, suggesting that heat shock exerts anticancer effects via the autophagic degradation of AQP5.

Calcium is not required for triggering volume restoration in hypotonically challenged A549 epithelial cells

Maintenance of cell volume is a fundamental housekeeping function in eukaryotic cells. Acute cell swelling activates a regulatory volume decrease (RVD) process with poorly defined volume sensing and intermediate signaling mechanisms. Here, we analyzed the putative role of Ca²⁺ signaling in RVD in single substrate-adherent human lung epithelial A549 cells. Acute cell swelling was induced by perfusion of the flow-through imaging chamber with 50 % hypotonic solution at a defined fluid turnover rate. Changes in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and cell volume were monitored simultaneously with ratiometric Fura-2 fluorescence and 3D reconstruction of stereoscopic single-cell images, respectively. Hypotonic challenge caused a progressive swelling peaking at ∼20 min and followed, during the next 20 min, by RVD of 60 ± 7 % of the peak volume increase. However, at the rate of swelling used in our experiments, these processes were not accompanied by a measurable increment of [Ca²⁺]_i. Loading with intracellular Ca²⁺ chelator BAPTA slightly delayed peak of swelling but did not prevent RVD in 82 % of cells. Further, electrophysiology whole-cell patch-clamp experiments showed that BAPTA did not block activation of volume-regulated anion channel (VRAC) measured as swelling-induced outwardly rectifying 5-nitro-2-(3-phenylpropyl-amino) benzoic acid sensitive current. Together, our data suggest that intracellular Ca²⁺-mediated signaling is not essential for VRAC activation and subsequent volume restoration in A549 cells.

Role of cytoskeleton network in anisosmotic volume changes of intact and permeabilized A549 cells

Recently we found that cytoplasm of permeabilized mammalian cells behaves as a hydrogel displaying intrinsic osmosensitivity. This study examined the role of microfilaments and microtubules in the regulation of hydrogel osmosensitivity, volume-sensitive ion transporters, and their contribution to volume modulation of intact cells. We found that intact and digitonin-permeabilized A549 cells displayed similar rate of shrinkage triggered by hyperosmotic medium. It was significantly slowed-down in both cell preparations after disruption of actin microfilaments by cytochalasin B, suggesting that rapid water release by intact cytoplasmic hydrogel contributes to hyperosmotic shrinkage. In hyposmotic swelling experiments, disruption of microtubules by vinblastine attenuated the maximal amplitude of swelling in intact cells and completely abolished it in permeabilized cells. The swelling of intact cells also triggered ~10-fold elevation of furosemide-resistant (86)Rb+ (K+) permeability and the regulatory volume decrease (RVD), both of which were abolished by Ba2+. Interestingly, RVD and K+ permeability remained unaffected in cytocholasin/vinblastine treated cells demonstrating that cytoskeleton disruption has no direct impact on Ba2+-sensitive K+-channels involved in RVD. Our results show, for the first time, that the cytoskeleton network contributes directly to passive cell volume adjustments in anisosmotic media via the modulation of the water retained by the cytoplasmic hydrogel.