Regulation of the ATP-sensitive potassium channel subunit, Kir6.2, by a Ca2+-dependent protein kinase C

The activity of ATP-sensitive potassium (K(ATP)) channels is governed by the concentration of intracellular ATP and ADP and is thus responsive to the metabolic status of the cell. Phosphorylation of K(ATP) channels by protein kinase A (PKA) or protein kinase C (PKC) results in the modulation of channel activity and is particularly important in regulating smooth muscle tone. At the molecular level the smooth muscle channel is composed of a sulfonylurea subunit (SUR2B) and a pore-forming subunit Kir6.1 and/or Kir6.2. Previously, Kir6.1/SUR2B channels have been shown to be inhibited by PKC, and Kir6.2/SUR2B channels have been shown to be activated or have no response to PKC. In this study we have examined the modulation of channel complexes formed of the inward rectifier subunit, Kir6.2, and the sulfonylurea subunit, SUR2B. Using a combination of biochemical and electrophysiological techniques we show that this complex can be inhibited by protein kinase C in a Ca(2+)-dependent manner and that this inhibition is likely to be as a result of internalization. We identify a residue in the distal C terminus of Kir6.2 (Ser-372) whose phosphorylation leads to down-regulation of the channel complex. This inhibitory effect is distinct from activation which is seen with low levels of channel activity.     

Voltage-gated potassium channel Kv1.3 regulates GLUT4 trafficking to the plasma membrane via a Ca2+-dependent mechanism

Kv1.3 is a voltage-gated K⁺ channel expressed in insulin-sensitive tissues. We previously showed that gene inactivation or pharmacological inhibition of Kv1.3 channel activity increased peripheral insulin sensitivity independently of body weight by augmenting the amount of GLUT4 at the plasma membrane. In the present study, we further examined the effect Kv1.3 on GLUT4 trafficking and tested whether it occurred via an insulin-dependent pathway. We found that Kv1.3 inhibition by margatoxin (MgTX) stimulated glucose uptake in adipose tissue and skeletal muscle and that the effect of MgTX on glucose transport was additive to that of insulin. Furthermore, whereas the increase in uptake was wortmannin insensitive, it was completely inhibited by dantrolene, a blocker of Ca²⁺ release from intracellular Ca²⁺ stores. In white adipocytes in primary culture, channel inhibition by Psora-4 increased GLUT4 translocation to the plasma membrane. In these cells, GLUT4 protein translocation was unaffected by the addition of wortmannin but was significantly inhibited by dantrolene. Channel inhibition depolarized the membrane voltage and led to sustained, dantrolene-sensitive oscillations in intracellular Ca²⁺ concentration. These results indicate that the apparent increase in insulin sensitivity observed in association with inhibition of Kv1.3 channel activity is mediated by an increase in GLUT4 protein at the plasma membrane, which occurs largely through a Ca²⁺-dependent process. insulin; glucose; diabetes; calcium     

Intracellular localization and physiological function of a rice Ca2+-permeable channel OsTPC1

Two-pore channels (TPCs) are cation channels with a voltage-sensor domain conserved in plants and animals. Rice OsTPC1 is predominantly localized to the plasma membrane (PM), and assumed to play an important role as a Ca²⁺-permeable cation channel in the regulation of cytosolic Ca²⁺ rise and innate immune responses including hypersensitive cell death and phytoalexin biosynthesis in cultured rice cells triggered by a fungal elicitor, xylanase from Trichoderma viride. In contrast, Arabidopsis AtTPC1 is localized to the vacuolar membrane (VM). To gain further insights into the intracellular localization of OsTPC1, we stably expressed OsTPC1-GFP in tobacco BY-2 cells. Confocal imaging and membrane fractionation revealed that, unlike in rice cells, the majority of OsTPC1-GFP fusion protein was targeted to the VM in tobacco BY-2 cells. Intracellular localization and functions of the plant TPC family is discussed.     

Functional coupling of the beta(1) subunit to the large conductance Ca(2+)-activated K(+) channel in the absence of Ca(2+). Increased Ca(2+) sensitivity from a Ca(2+)-independent mechanism

Coexpression of the beta(1) subunit with the alpha subunit (mSlo) of BK channels increases the apparent Ca(2+) sensitivity of the channel. This study investigates whether the mechanism underlying the increased Ca(2+) sensitivity requires Ca(2+), by comparing the gating in 0 Ca(2+)(i) of BK channels composed of alpha subunits to those composed of alpha+beta(1) subunits. The beta(1) subunit increased burst duration approximately 20-fold and the duration of gaps between bursts approximately 3-fold, giving an approximately 10-fold increase in open probability (P(o)) in 0 Ca(2+)(i). The effect of the beta(1) subunit on increasing burst duration was little changed over a wide range of P(o) achieved by varying either Ca(2+)(i) or depolarization. The effect of the beta(1) subunit on increasing the durations of the gaps between bursts in 0 Ca(2+)(i) was preserved over a range of voltage, but was switched off as Ca(2+)(i) was increased into the activation range. The Ca(2+)-independent, beta(1) subunit-induced increase in burst duration accounted for 80% of the leftward shift in the P(o) vs. Ca(2+)(i) curve that reflects the increased Ca(2+) sensitivity induced by the beta(1) subunit. The Ca(2+)-dependent effect of the beta(1) subunit on the gaps between bursts accounted for the remaining 20% of the leftward shift. Our observation that the major effects of the beta(1) subunit are independent of Ca(2+)(i) suggests that the beta(1) subunit mainly alters the energy barriers of Ca(2+)-independent transitions. The changes in gating induced by the beta(1) subunit differ from those induced by depolarization, as increasing P(o) by depolarization or by the beta(1) subunit gave different gating kinetics. The complex gating kinetics for both alpha and alpha+beta(1) channels in 0 Ca(2+)(i) arise from transitions among two to three open and three to five closed states and are inconsistent with Monod-Wyman-Changeux type models, which predict gating among only one open and one closed state in 0 Ca(2+)(i).     

Voltage and Ca2+ activation of single large-conductance Ca2+-activated K+ channels described by a two-tiered allosteric gating mechanism

The voltage- and Ca2+-dependent gating mechanism of large-conductance Ca2+-activated K+ (BK) channels from cultured rat skeletal muscle was studied using single-channel analysis. Channel open probability (Po) increased with depolarization, as determined by limiting slope measurements (11 mV per e-fold change in Po; effective gating charge, q(eff), of 2.3 +/- 0.6 e(o)). Estimates of q(eff) were little changed for intracellular Ca2+ (Ca2+(i)) ranging from 0.0003 to 1,024 microM. Increasing Ca2+(i) from 0.03 to 1,024 microM shifted the voltage for half maximal activation (V(1/2)) 175 mV in the hyperpolarizing direction. V(1/2) was independent of Ca2+(i) for Ca2+(i) < or = 0.03 microM, indicating that the channel can be activated in the absence of Ca2+(i). Open and closed dwell-time distributions for data obtained at different Ca2+(i) and voltage, but at the same Po, were different, indicating that the major action of voltage is not through concentrating Ca2+ at the binding sites. The voltage dependence of Po arose from a decrease in the mean closing rate with depolarization (q(eff) = -0.5 e(o)) and an increase in the mean opening rate (q(eff) = 1.8 e(o)), consistent with voltage-dependent steps in both the activation and deactivation pathways. A 50-state two-tiered model with separate voltage- and Ca2+-dependent steps was consistent with the major features of the voltage and Ca2+ dependence of the single-channel kinetics over wide ranges of Ca2+(i) (approximately 0 through 1,024 microM), voltage (+80 to -80 mV), and Po (10(-4) to 0.96). In the model, the voltage dependence of the gating arises mainly from voltage-dependent transitions between closed (C-C) and open (O-O) states, with less voltage dependence for transitions between open and closed states (C-O), and with no voltage dependence for Ca2+-binding and unbinding. The two-tiered model can serve as a working hypothesis for the Ca2+- and voltage-dependent gating of the BK channel.     

Regulation by GTP of a Ca2+-activated K+ channel in the apical membrane of rabbit cortical collecting duct cells

Ca²⁺-activated K⁺ channels play an important role in Ca²⁺ signal transduction and may be regulated by mechanisms other than a direct effect of Ca²⁺. Inside-out patches of the apical membrane of confluent transformed rabbit cortical collecting duct cells cultured on collagen were subjected to patch clamp analysis. Two types of K⁺ channel, of medium and high conductance, were observed. The latter channel was characterized by a K⁺/Na⁺ permeability ratio of 10, an inwardly rectified current, a conductance of 80 pS at 0 mV, and an open probability dependent on both voltage and Ca²⁺. Guanosine 5-triphosphate (GTP) but not a guanosine 5-diphosphate (GDP) analogue, adenosine 5-triphosphate (ATP), cytidine 5-triphosphate (CTP), or inosine 5-triphosphate (ITP), inhibited the activity of this Ca²⁺-activated K⁺ channel. The inhibitory effect of GTP was dose dependent, with a 50% inhibitory concentration of 10^–5 m in the absence of Mg²⁺. In the presence of Mg²⁺ (1 mm), which is required for the binding of GTP to G proteins, the 50% inhibitory concentration decreased to 3×10^–12 m. Pertussis toxin or cholera toxin (each at 10 ng/ml) did not prevent the inhibitory effect of GTP. After removal of GTP from the medium bathing an inhibited channel, subsequent application of Ca²⁺ failed to activate the channel. Ca²⁺-activated K⁺ channels of smooth muscle cells and proximal tubule cells did not respond to GTP. Thus, the Ca²⁺-activated K⁺ channel in the apical membrane of collecting duct cells is inhibited by GTP, which appears to exert its effect via a G protein that is insensitive to both cholera and pertussis toxins.     

Nuclear sequestration of beta-subunits by Rad and Rem is controlled by 14-3-3 and calmodulin and reveals a novel mechanism for Ca2+ channel regulation

Voltage-gated Ca2+ channels (VDCCs) are heteromultimeric proteins that mediate Ca2+ influx into cells upon membrane depolarization. These channels are involved in various cellular events, including gene expression, regulation of hormone secretion and synaptic transmission. Kir/Gem, Rad, Rem, and Rem2 belong to the RGK family of Ras-related small G proteins. RGK proteins interact with the beta-subunits and downregulate VDCC activity. Kir/Gem was proposed to prevent surface expression of functional Ca2+ channels, while for Rem2 the mechanism remains controversial. Here, we have analyzed the mechanism by which Rad and Rem regulate VDCC activity. We show that, similar to Kir/Gem and Rem2, 14-3-3 and CaM binding regulate the subcellular distribution of Rad and Rem, which both inhibit Ca2+ channel activity by preventing its expression on the cell surface. This function is regulated by calmodulin and 14-3-3 binding only for Rad and not for Rem. Interestingly, nuclear targeting of Rad and Rem can relocalize and sequester the beta-subunit to the nucleus, thus providing a novel mechanism for Ca2+ channel downregulation.     

Ceramide increase cytoplasmic Ca2+ concentration in Jurkat T cells by liberation of calcium from intracellular stores and activation of a store-operated calcium channel

The effect of ceramide on the cytoplasmic Ca2+ concentration ([Ca2+]i) varies depending on the cell type. We have found that in Jurkat human T cells ceramide increases the [Ca2+]i from a thapsigargin-sensitive calcium pool and the subsequent activation of a capacitative Ca2+ entry. This effect occurs both in the presence and in the absence of extracellular calcium. Addition of ceramine, a non-hydrolysable analogue of ceramide, reproduced its effect on the [Ca2+]i ruling out that this is due to the conversion of ceramide to sphingosine. The effect of ceramide was additive to that obtained by sphingosine, but not to the Jurkat T cells specific antibody OKT3. However, different to the latter, ceramide do not induced an elevation of InsP3. The opening of a store operated Ca2+ channel by ceramide was corroborated by experiments of Fura-2 quenching, using Mn2+ as a surrogate for Ca2+ and confirmed by whole-cell recording patch clamp techniques.     

Protein phosphorylation activates the guard cell Ca2+ channel and is a prerequisite for gating by abscisic acid

Protein phosphorylation and cytosolic-free [Ca2+] ([Ca2+]i) contribute to signalling cascades evoked by the water-stress hormone abscisic acid (ABA) that lead to stomatal closure in higher-plant leaves. ABA activates an inward-rectifying Ca2+ channel at the plasma membrane of stomatal guard cells, promoting Ca2+ entry by shifting the voltage-sensitivity of the channels. Because many of these effects could be mediated by kinase/phosphatase action at the membrane, we examined a role for protein (de-)phosphorylation in plasma membrane patches from Vicia guard cells. Ca2+ channel activity decayed rapidly in excised patches, and recovered on adding ATP (K1/2, 1.3 +/- 0.7 mm) but not the non-hydrolyzable analog ATPgammaS. ABA activation of the channel required the presence of ATP and like ABA, the 1/2 A-type protein phosphatase antagonists okadaic acid (OA) and calyculin A (CA) enhanced Ca2+ channel activity by increasing the open probability and number of active channels. Neither ATP nor the antagonists affected the mean open lifetime of the channel, suggesting an action through changes in closed lifetime distributions. Like ABA, OA and CA shifted the voltage-sensitivities of the Ca2+ current and [Ca2+]i increases in intact guard cells towards positive voltages. OA and CA also augmented the [Ca2+]i rise evoked by hyperpolarization and delayed its recovery. These results demonstrate a membrane-delimited interaction between 1/2 A-type protein phosphatase(s) and the Ca2+ channel or associated proteins, and they are consistent with a role for protein (de-)phosphorylation in ABA signalling mediated directly through Ca2+ channel gating that leads to [Ca2+]i increases in the guard cells.     

Activation of the endothelin receptor inhibits the G protein-coupled inwardly rectifying potassium channel by a phospholipase A2-mediated mechanism

To develop a malleable system to model the well-described, physiological interactions between Gq/11 - coupled receptor and Gi/o-coupled receptor signaling, we coexpressed the endothelin A receptor, the mu-opioid receptor, and the G protein-coupled inwardly rectifying potassium channel (Kir 3) heteromultimers in Xenopus laevis oocytes. Activation of the Gi/o-coupled mu-opioid receptor strongly increased Kir 3 channel current, whereas activation of the Gq/11-coupled endothelin A receptor inhibited the Kir 3 response evoked by mu-opioid receptor activation. The magnitude of the inhibition of Kir 3 was channel subtype specific; heteromultimers composed of Kir 3.1 and Kir 3.2 or Kir 3.1 and Kir 3.4 were significantly more sensitive to the effects of endothelin-1 than heteromultimers composed of Kir 3.1 and Kir 3.5. The difference in sensitivity of the heteromultimers suggests that the endothelin-induced inhibition of the opioid- activated current was caused by an effect at the channel rather than at the opioid receptor. The endothelin-1-mediated inhibition was mimicked by arachidonic acid and blocked by the phospholipase A2 inhibitor arachidonoyl trifluoromethyl ketone. Consistent with a possible phospholipase A2-mediated mechanism, the endothelin-1 effect was blocked by calcium chelation with BAPTA-AM and was not affected by kinase inhibition by either staurosporine or genistein. The data suggest the hypothesis that Gq/11-coupled receptor activation may interfere with Gi/o-coupled receptor signaling by the activation of phospholipase A2 and subsequent inhibition of effector function by a direct effect of an eicosanoid on the channel.     

Protein phosphorylation is a prerequisite for intracellular Ca2+ release and ion channel control by nitric oxide and abscisic acid in guard cells

Recent work has indicated that nitric oxide (NO) and its synthesis are important elements of signal cascades in plant-pathogen defence, and are a prerequisite for drought and abscisic acid (ABA) responses in Arabidopsis thaliana and Vicia faba guard cells. NO regulates inward-rectifying K+ channels and Cl- channels of Vicia guard cells via intracellular Ca2+ release. However, its integration with related signals, including the actions of serine-threonine protein kinases, is less well defined. We report here that the elevation of cytosolic-free [Ca2+] ([Ca2+]i) mediated by NO in guard cells is reversibly inhibited by the broad-range protein kinase antagonists staurosporine and K252A, but not by the tyrosine kinase antagonist genistein. The effects of kinase antagonism translate directly to a loss of NO-sensitivity of the inward-rectifying K+ channels and background (Cl- channel) current, and to a parallel loss in sensitivity of the K+ channels to ABA. These results demonstrate that NO-dependent signals can be modulated through protein phosphorylation upstream of intracellular Ca2+ release, and they implicate a target for protein kinase control in ABA signalling that feeds into NO-dependent Ca2+ release.     

External bioenergy-induced increases in intracellular free calcium concentrations are mediated by Na+/Ca2+ exchanger and L-type calcium channel

External bioenergy (EBE, energy emitted from a human body) has been shown to increase intracellular calcium concentration ([Ca²⁺]_i, an important factor in signal transduction) and regulate the cellular response to heat stress in cultured human lymphoid Jurkat T cells. In this study, we wanted to elucidate the underlying mechanisms. A bioenergy specialist emitted bioenergy sequentially toward tubes of cultured Jurkat T cells for one 15-minute period in buffers containing different ion compositions or different concentrations of inhibitors. [Ca²⁺]_i was measured spectrofluorometrically using the fluorescent probe fura-2. The resting [Ca²⁺]_i in Jurkat T cells was 70 ± 3 nM (n = 130) in the normal buffer. Removal of external calcium decreased the resting [Ca²⁺]_i to 52 ± 2 nM (n = 23), indicating that [Ca²⁺] entry from the external source is important for maintaining the basal level of [Ca²⁺]_i. Treatment of Jurkat T cells with EBE for 15 min increased [Ca²⁺]_i by 30 ± 5% (P 0.05, Student t-test). The distance between the bioenergy specialist and Jurkat T cells and repetitive treatments of EBE did not attenuate [Ca²⁺]_i responsiveness to EBE. Removal of external Ca²⁺ or Na⁺, but not Mg²⁺, inhibited the EBE-induced increase in [Ca²⁺]_i. Dichlorobenzamil, an inhibitor of Na⁺/Ca²⁺ exchangers, also inhibited the EBE-induced increase in [Ca²⁺]_i in a concentration-dependent manner with an IC50 of 0.11 ± 0.02 nM. When external [K⁺] was increased from 4.5 mM to 25 mM, EBE decreased [Ca²⁺]_i. The EBE-induced increase was also blocked by verapamil, an L-type voltage-gated Ca²⁺ channel blocker. These results suggest that the EBE-induced [Ca²⁺]_i increase may serve as an objective means for assessing and validating bioenergy effects and those specialists claiming bioenergy capability. The increase in [Ca²⁺]_i is mediated by activation of Na⁺/Ca²⁺ exchangers and opening of L-type voltage-gated Ca²⁺ channels. (Mol Cell Biochem 271: 51–59, 2005)     

Bradykinin stimulates phospholipase D in PC12 cells by a mechanism which is independent of increases in intracellular Ca2+

These experiments were designed to learn the role of bradykinin induced changes in intracellular Ca²⁺ in the activation of phospholipase D activity in PC12 cells. Ionomycin at a concentration of 0.1M caused an increase in intracellular Ca²⁺ comparable to bradykinin, but had no effect on phospholipase D activity. Carbachol, ATP, and thapsigargin also increased intracellular Ca²⁺ but had no effect on phospholipase D activity. Increases in intracellular Ca²⁺ may be a necessary but not a sufficient factor in the activation of phospholipase D. To investigate this issue, the bradykinin induced increase in intracellular Ca²⁺ was blocked by preincubating the cells in Ca²⁺-free media plus EGTA or in media containing the intracellular Ca²⁺ chelator BAPTA/AM. These preincubations completely blocked the bradykinin induced increase in intracellular Ca²⁺ but only attenuated the bradykinin mediated activation of phospholipase D. Physiological increases in intracellular Ca²⁺ apparently do not mediate the effect of bradykinin on phospholipase D.     

Endothelin-1 and insulin activate the steady-state voltage dependent R-type Ca2+ channel in aortic smooth muscle cells via a pertussis toxin and cholera toxin sensitive G-protein

In single rabbit aortic smooth muscle cells, and at a concentration known to induce a maximum sustained increase of intracellular Ca²⁺ via activation of the steady-state voltage dependent R-type Ca²⁺ channels, endothelin-1 (10^-7 M) and insulin (80 U/ml) were found to induce a sustained increase in cytosolic free Ca²⁺ ([Ca]_i) levels that was significantly attenuated by pre-treatment with either pertussis toxin (PTX), cholera toxin (CTX) or removal of extracellular Ca²⁺.However, both PTX and CTX failed to inhibit the sustained depolarization-evoked sustained Ca²⁺ influx and [Ca]_i elevation via activation of the R-type Ca²⁺ channels. Moreover, ET-1 and insulin-evoked sustained increases in Ca²⁺ influx were not attenuated by the selective PKC inhibitor, bisindolylmaleimide (BIS), or the specific L-type Ca²⁺ channel blocker, nifedipine, but were completely reversed by the R-type Ca²⁺ channel blocker, (-) PN 200-110 (isradipine). These data suggest that both insulin and ET-1 activate the nifedipine-insensitive but isradipine-sensitive steady state voltage dependent R-type Ca²⁺ channels present on rabbit VSMCs and these channels are directly coupled to PTX and CTX sensitive G protein(s).     

Role of Ca2+-activated Cl- channels in the mechanism of apoptosis induced by cyclosporin A in a human hepatoma cell line

The mechanism of apoptosis induced by cyclosporin A (CsA) in a human hepatoma cell line was investigated. CsA induced apoptosis in a dose- and time-dependent manner in HepG2 human hepatoma cells. CsA induced Cl- efflux, which was significantly blocked by niflumic acid (NA), a specific inhibitor, and flufenamic acid (FA), 5-nitro-2-(3-phenyl-propylamino)-benzoate (NPPB), and 4,4'-diisothiocyanoto-stibene-2,2'-disulfonic acid (DIDS), non-specific inhibitors of Ca2+-activated Cl- channels (CaCCs), not by calyculin A, an inhibitor of K+,Cl- -cotransport. In addition, CsA did not alter intracellular K+ concentration. Moreover, CsA increased intracellular Ca2+ concentration, and treatment with BAPTA/AM, an intracellular Ca2+ chelator, significantly inhibited the CsA-induced Cl- efflux, indicating that CsA induced Cl- efflux through the activation of CaCCs. Treatment with these CaCC inhibitors (NA, FA, NPPB, and DIDS) markedly prevented the CsA-induced apoptosis. Taken together, these results suggest that CaCCs may mediate apoptosis induced by CsA in HepG2 cells. Furthermore, these results provide a new insight into the novel function of CaCCs in the regulation of cancer cell apoptosis associated with perturbation of intracellular Ca2+ signal.     

Ceramide-1-P induces Ca2+ mobilization in Jurkat T-cells by elevation of Ins(1,4,5)-P3 and activation of a store-operated calcium channel

Sphingolipids comprise a very important class of second messengers involved in cell growth, differentiation, and apoptosis, among other different functions. Recently, these lipids have been implicated in calcium mobilization in different cell lines, including Jurkat T-lymphocytes. However, the effect of each particular sphingolipid appears to be cell-line specific. Among them, the least studied is ceramide-1-P (Cer-1-P). Here, we show that Cer-1-P increased the intracellular Ca(2+) concentration in Jurkat T-cells. Furthermore, laser-scanning confocal microscopy indicated that Ca(2+) is released from the endoplasmic reticulum. An effect on store-operated Ca(2+) channels was evidenced by whole-cell "patch clamp" measurements after Cer-1-P induced Ca(2+) store depletion. The mechanism of action of Cer-1-P resembles that of the Jurkat anti-TCR antibody, but differs from that of ceramide, since Cer-1-P induced an increase in Ins(1,4,5)-P(3).     

A negative feedback regulation of MTORC1 activity by the lysosomal Ca2+ channel MCOLN1 (mucolipin 1) using a CALM (calmodulin)-dependent mechanism

Macroautophagy/autophagy is an evolutionarily conserved pathway that is required for cellular homeostasis, growth and survival. The lysosome plays an essential role in autophagy regulation. For example, the activity of MTORC1, a master regulator of autophagy, is regulated by nutrients within the lysosome. Starvation inhibits MTORC1 causing autophagy induction. Given that MTORC1 is critical for protein synthesis and cellular homeostasis, a feedback regulatory mechanism must exist to restore MTORC1 during starvation. However, the molecular mechanism underlying this feedback regulation is unclear. In this study, we report that starvation activates the lysosomal Ca²⁺ release channel MCOLN1 (mucolipin 1) by relieving MTORC1's inhibition of the channel. Activated MCOLN1 in turn facilitates MTORC1 activity that requires CALM (calmodulin). Moreover, both MCOLN1 and CALM are necessary for MTORC1 reactivation during prolonged starvation. Our data suggest that lysosomal Ca²⁺ signaling is an essential component of the canonical MTORC1-dependent autophagy pathway and MCOLN1 provides a negative feedback regulation of MTORC1 to prevent excessive loss of MTORC1 function during starvation. The feedback regulation may be important for maintaining cellular homeostasis during starvation, as well as many other stressful or disease conditions.