Dynamic Control of Mitochondrial Ca2+ Levels as a Survival Strategy of Cancer Cells

Abstract

Cancer cells have increased energy requirements due to their enhanced proliferation activity. This energy demand is, among others, met by mitochondrial ATP production. Since the second messenger Ca2+ maintains the activity of Krebs cycle dehydrogenases that fuel mitochondrial respiration, proper mitochondrial Ca2+ uptake is crucial for a cancer cell survival. However, a mitochondrial Ca2+ overload induces mitochondrial dysfunction and, ultimately, apoptotic cell death. Because of the vital importance of balancing mitochondrial Ca2+ levels, a highly sophisticated machinery of multiple proteins manages mitochondrial Ca2+ homeostasis. Notably, mitochondria sequester Ca2+ preferentially at the interaction sites between mitochondria and the endoplasmic reticulum (ER), the largest internal Ca2+ store, thus, pointing to mitochondrial-associated membranes (MAMs) as crucial hubs between cancer prosperity and cell death. To investigate potential regulatory mechanisms of the mitochondrial Ca2+ uptake routes in cancer cells, we modulated mitochondria–ER tethering and the expression of UCP2 and analyzed mitochondrial Ca2+ homeostasis under the various conditions. Hence, the expression of contributors to mitochondrial Ca2+ regulation machinery was quantified by qRT-PCR. We further used data from The Cancer Genome Atlas (TCGA) to correlate these in vitro findings with expression patterns in human breast invasive cancer and human prostate adenocarcinoma. ER-mitochondrial linkage was found to support a mitochondrial Ca2+ uptake route dependent on uncoupling protein 2 (UCP2) in cancer cells. Notably, combined overexpression of Rab32, a protein kinase A-anchoring protein fostering the ER-mitochondrial tethering, and UCP2 caused a significant drop in cancer cells' viability. Artificially enhanced ER-mitochondrial tethering further initiated a sudden decline in the expression of UCP2, probably as an adaptive response to avoid mitochondrial Ca2+ overload. Besides, TCGA analysis revealed an inverse expression correlation between proteins stabilizing mitochondrial-ER linkage and UCP2 in tissues of human breast invasive cancer and prostate adenocarcinoma. Based on these results, we assume that cancer cells successfully manage mitochondrial Ca2+ uptake to stimulate Ca2+-dependent mitochondrial metabolism while avoiding Ca2+-triggered cell death by fine-tuning ER-mitochondrial tethering and the expression of UCP2 in an inversed manner. Disruption of this equilibrium yields cancer cell death and may serve as a treatment strategy to specifically kill cancer cells.

Sigma-1 Receptor Promotes Mitochondrial Bioenergetics by Orchestrating ER Ca2+ Leak during Early ER Stress

“…Our results in SH-SY5Y cells showed no detectable changes in MAMs after 2 h and 8 h of tunicamycin treatment in control or S1R knock-down cells ( Figure S3a,b ). The possible reasons for discrepancies with published data could be specificities of cell lines used, as most of the reported studies were performed in HeLa cells [ 5 , 25 ].…”

“…S1R seems to play a major role in this adaptation, since knock-down and a pharmacological antagonist of S1R almost eliminated respective responses ( Figure 2 b,c and Figure 3 b,e and Figure S2c ). In search for a possible mechanism of action of S1R, we measured the amount of MAMs, as it has been reported that MAMs are increasing during early or later stages of ER stress [ 5 , 25 ]. Our results in SH-SY5Y cells showed no detectable changes in MAMs after 2 h and 8 h of tunicamycin treatment in control or S1R knock-down cells ( Figure S3a,b ).…”

Near-UV Light Induced ROS Production Initiates Spatial Ca2+ Spiking to Fire NFATc3 Translocation

“…To approach this question, we came up with a strategy to buffer the Ca 2+ spiking through the L-type Ca 2+ channel by increasing the proximity between mitochondria and the sub-plasma membrane region. This was achieved by expressing the AKAP-RFP-CAAX construct [ 19 , 20 , 21 ] in INS-1 cells, which positioned mitochondria in the sub-PM region ( Figure 5 F, red images). By expressing AKAP-RFP-CAAX together with GFP-NFATc3 in Fura-2-loaded INS-1 cells, we monitored GFP-NFATc3 translocation and cytosolic Ca 2+ spiking simultaneously.…”

“…To do this, we came up with a hypothesis that if we buffer Ca 2+ spikes, which originate from L-type Ca 2+ channels at the plasma membrane, but keep the same ROS levels, we would not see the NFATc3 translocation and Ca 2+ oscillations. To test this idea, we positioned mitochondria to the sub-plasma membrane region using the AKAP-RFP-CAAX construct [ 19 , 20 , 21 ] to buffer local Ca 2+ spiking. Interestingly, AKAP-RFP-CAAX-expressing cells did not alter ROS production ( Figure S4a–d ) in response to near-UV light stimulation, but they eliminated near-UV light-induced Ca 2+ spiking as well as NFATc3 translocation ( Figure 5 B,E–G), thus supporting our hypothesis that ROS-induced cytosolic Ca 2+ spiking, and not ROS itself, is required for NFATc3 translocation to the nucleus.…”

Multifunctional Mitochondria-Targeting Nanosystems for Enhanced Anticancer Efficacy

“…Energy metabolism allows for the production of reactive oxygen species (ROS), leading to activation of pyk2 and src protein tyrosine kinases, which ultimately stimulates and promotes tumor development and invasion ( Xu et al, 2015 ). In cancer cells, the extra- and intracellular Ca 2+ pools accumulating in mitochondria influence the permeability of mitochondrial permeability transition pore (mtPTP), which thereby affects tumor cell apoptosis ( Qin et al, 2021 ; Madreiter-Sokolowski et al, 2021 ). (3) Mitochondria are related to multiple drug resistance: Genotoxic drugs trigger mitochondrial shift by means of regulating mitochondrial energy metabolism and upregulating mitochondrial oxidative phosphorylation, which results in drug resistance to chemotherapy ( Lee et al, 2017 ; Lee et al, 2018 ).…”