The effects of extracellular nucleotides on [Ca2+]i signalling in a human-derived renal proximal tubular cell line (HKC-8)

Protective effects of dapagliflozin against oxidative stress-induced cell injury in human proximal tubular cells

Elevated reactive oxygen species (ROS) in type 2 diabetes cause cellular damage in many organs. Recently, the new class of glucose-lowering agents, SGLT-2 inhibitors, have been shown to reduce the risk of developing diabetic complications; however, the mechanisms of such beneficial effect are largely unknown. Here we aimed to investigate the effects of dapagliflozin on cell proliferation and cell death under oxidative stress conditions and explore its underlying mechanisms. Human proximal tubular cells (HK-2) were used. Cell growth and death were monitored by cell counting, water-soluble tetrazolium-1 (WST-1) and lactate dehydrogenase (LDH) assays, and flow cytometry. The cytosolic and mitochondrial (ROS) production was measured using fluorescent probes (H2DCFDA and MitoSOX) under normal and oxidative stress conditions mimicked by addition of H₂O₂. Intracellular Ca²⁺ dynamics was monitored by FlexStation 3 using cell-permeable Ca²⁺ dye Fura-PE3/AM. Dapagliflozin (0.1–10 μM) had no effect on HK-2 cell proliferation under normal conditions, but an inhibitory effect was seen at an extreme high concentration (100 μM). However, dapagliflozin at 0.1 to 5 μM showed remarkable protective effects against H₂O₂-induced cell injury via increasing the viable cell number at phase G0/G1. The elevated cytosolic and mitochondrial ROS under oxidative stress was significantly decreased by dapagliflozin. Dapagliflozin increased the basal intracellular [Ca²⁺]_i in proximal tubular cells, but did not affect calcium release from endoplasmic reticulum and store-operated Ca²⁺ entry. The H₂O₂-sensitive TRPM2 channel seemed to be involved in the Ca²⁺ dynamics regulated by dapagliflozin. However, dapagliflozin had no direct effects on ORAI1, ORAI3, TRPC4 and TRPC5 channels. Our results suggest that dapagliflozin shows anti-oxidative properties by reducing cytosolic and mitochondrial ROS production and altering Ca²⁺ dynamics, and thus exerts its protective effects against cell damage under oxidative stress environment.

Lysosomal protease deficiency or substrate overload induces an oxidative-stress mediated STAT3-dependent pathway of lysosomal homeostasis

Diverse cellular processes depend on the lysosomal protease system but how cells regulate lysosomal proteolytic capacity is only partly understood. We show here that cells can respond to protease/substrate imbalance in this compartment by de novo expression of multiple lysosomal hydrolases. This response, exemplified here either by loss of asparagine endopeptidase (AEP) or other lysosomal cysteine proteases, or by increased endocytic substrate load, is not dependent on the transcription factor EB (TFEB) but rather is triggered by STAT3 activation downstream of lysosomal oxidative stress. Similar lysosomal adaptations are seen in mice and cells expressing a constitutively active form of STAT3. Our results reveal how cells can increase lysosomal protease capacity under ‘fed’ rather than ‘starved’ conditions that activate the TFEB system. In addition, STAT3 activation due to lysosomal stress likely explains the hyperproliferative kidney disease and splenomegaly observed in AEP-deficient mice.

How cells regulate their lysosomal proteolytic capacity is only partly understood. Here, the authors show that lysosomal protease deficiency or substrate overload induces lysosomal stress leading to activation of a STAT3-dependent, TFEB-independent pathway of lysosomal hydrolase expression.

Emerging Roles of Diacylglycerol-Sensitive TRPC4/5 Channels

Transient receptor potential classical or canonical 4 (TRPC4) and TRPC5 channels are members of the classical or canonical transient receptor potential (TRPC) channel family of non-selective cation channels. TRPC4 and TRPC5 channels are widely accepted as receptor-operated cation channels that are activated in a phospholipase C-dependent manner, following the G_q/11 protein-coupled receptor activation. However, their precise activation mechanism has remained largely elusive for a long time, as the TRPC4 and TRPC5 channels were considered as being insensitive to the second messenger diacylglycerol (DAG) in contrast to the other TRPC channels. Recent findings indicate that the C-terminal interactions with the scaffolding proteins Na⁺/H⁺ exchanger regulatory factor 1 and 2 (NHERF1 and NHERF2) dynamically regulate the DAG sensitivity of the TRPC4 and TRPC5 channels. Interestingly, the C-terminal NHERF binding suppresses, while the dissociation of NHERF enables, the DAG sensitivity of the TRPC4 and TRPC5 channels. This leads to the assumption that all of the TRPC channels are DAG sensitive. The identification of the regulatory function of the NHERF proteins in the TRPC4/5-NHERF protein complex offers a new starting point to get deeper insights into the molecular basis of TRPC channel activation. Future studies will have to unravel the physiological and pathophysiological functions of this multi-protein channel complex.

Dynamic NHERF interaction with TRPC4/5 proteins is required for channel gating by diacylglycerol

The activation mechanism of the classical transient receptor potential channels TRPC4 and -5 via the G_q/11 protein-phospholipase C (PLC) signaling pathway has remained elusive so far. In contrast to all other TRPC channels, the PLC product diacylglycerol (DAG) is not sufficient for channel activation, whereas TRPC4/5 channel activity is potentiated by phosphatidylinositol 4,5-bisphosphate (PIP₂) depletion. As a characteristic structural feature, TRPC4/5 channels contain a C-terminal PDZ-binding motif allowing for binding of the scaffolding proteins Na⁺/H⁺ exchanger regulatory factor (NHERF) 1 and 2. PKC inhibition or the exchange of threonine for alanine in the C-terminal PDZ-binding motif conferred DAG sensitivity to the channel. Altogether, we present a DAG-mediated activation mechanism for TRPC4/5 channels tightly regulated by NHERF1/2 interaction. PIP₂ depletion evokes a C-terminal conformational change of TRPC5 proteins leading to dynamic dissociation of NHERF1/2 from the C terminus of TRPC5 as a prerequisite for DAG sensitivity. We show that NHERF proteins are direct regulators of ion channel activity and that DAG sensitivity is a distinctive hallmark of TRPC channels.

P2Y receptors and kidney function

Cellular release of nucleotides is of physiological importance to regulate and maintain cell function and integrity. Also in the tubular and collecting duct system of the kidney, nucleotides are released in response to changes in cell volume or luminal flow rate and act in a paracrine and autocrine way on basolateral and luminal P2Y receptors. Recent studies using gene knockout mice assigned a prominent role to G protein-coupled P2Y(2) receptors, which are activated by both ATP and UTP. The antidiuretic hormone, arginine-vasopressin (AVP), and possibly an increase in collecting duct cell volume induce ATP release. The subsequent activation of P2Y(2) receptors inhibits AVP-induced cAMP formation and water reabsorption, which stabilizes cell volume and facilitates water excretion. An increase in NaCl intake enhances luminal release of ATP and UTP in the aldosterone-sensitive distal nephron which by activating apical P2Y(2) receptors and phospholipase C lowers the open probability of the epithelial sodium channel ENaC, thereby facilitating sodium excretion. Thus, the renal ATP/UTP/P2Y(2) receptor system not only serves to preserve cell volume and integrity but is also regulated by stimuli that derive from body NaCl homeostasis. The system also inhibits ENaC activity during aldosterone escape, i.e. when sodium reabsorption via ENaC is inappropriately high. The P2Y(2) receptor tone inhibits the expression and activity of the Na-K-2Cl cotransporter NKCC2 in the thick ascending limb and mediates vasodilation. While the role of other P2Y receptors in the kidney is less clear, the ATP/UTP/P2Y(2) receptor system regulates NaCl and water homeostasis and blood pressure.