pH-dependent inhibition of native GABA(A) receptors by HEPES

Tumor Microenvironment Acidity Triggers Lipid Accumulation in Liver Cancer via SCD1 Activation

Acidification is recognized as the predominant characteristic of the tumor microenvironment (TME) and contributes to tumor progression. However, the mechanism of extracellular acidic TME directly influences intercellular pathological responses remains unclear. Meanwhile, acidic TME is mainly ascribed to aberrant metabolism of lipids and glucose, but whether and how acidity affects metabolic reprogramming, especially for lipid metabolism, is still unknown. We found that lipid was significantly accumulated in liver cancer cells when exposed to acidic TME. Moreover, proteomic analysis showed that differentially expressed proteins were mainly clustered into fatty acid pathways. Subsequently, we found that acidification increased the expression of SCD1 by activating PI3K/AKT signaling pathway. Interestingly, we found that SCD1 directly bound to PPARα in the acidic TME, which vanished after 2-day reverse incubation in pH7.4 medium, implying extracellular acidosis might influence intercellular function by mediating the binding affinity between SCD1 and PPARα under different pH gradients. In summary, our data revealed that acidosis could significantly trigger fatty acid synthesis to promote liver tumorigenesis by upregulating SCD1 in a PI3K/AKT activation dependent manner and simultaneously promote SCD1 binding to PPARα. Our study not only provides direct mechanistic evidence to support the vital role of acidosis in lipid metabolic reprogramming, but also provides novel insights for determining the binding affinity of functional proteins as a molecular mechanism to better understand the role of the acidic TME in tumor development. Implications: The acidic TME contributes to lipid accumulation in liver cancer by activating the PI3K/AKT signaling pathway and promoting SCD1-PPARα binding.

Locus Coeruleus Acid-Sensing Ion Channels Modulate Sleep-Wakefulness and State Transition from NREM to REM Sleep in the Rat

The locus coeruleus (LC) is one of the essential chemoregulatory and sleep-wake (S-W) modulating centers in the brain. LC neurons remain highly active during wakefulness, and some implicitly become silent during rapid eye movement (REM) sleep. LC neurons are also involved in CO₂-dependent modulation of the respiratory drive. Acid-sensing ion channels (ASICs) are highly expressed in some brainstem chemosensory breathing regulatory areas, but their localization and functions in the LC remain unknown. Mild hypercapnia increases the amount of non-REM (NREM) sleep and the number of REM sleep episodes, but whether ASICs in the LC modulate S-W is unclear. Here, we investigated the presence of ASICs in the LC and their role in S-W modulation and the state transition from NREM to REM sleep. Male Wistar rats were surgically prepared for chronic polysomnographic recordings and drug microinjections into the LC. The presence of ASIC-2 and ASIC-3 in the LC was immunohistochemically characterized. Microinjections of amiloride (an ASIC blocker) and APETx2 (a blocker of ASIC-2 and -3) into the LC significantly decreased wakefulness and REM sleep, but significantly increased NREM sleep. Mild hypercapnia increased the amount of NREM and the number of REM episodes. However, APETx2 microinjection inhibited this increase in REM frequency. These results suggest that the ASICs of LC neurons modulate S-W, indicating that ASICs could play an important role in vigilance-state transition. A mild increase in CO₂ level during NREM sleep sensed by ASICs could be one of the determinants of state transition from NREM to REM sleep.

Modulation of μ-opioid receptor activation by acidic pH is dependent on ligand structure and an ionizable amino acid residue

Background and Purpose

Adverse side effects of conventional opioids can be avoided if ligands selectively activate peripheral opioid receptors in injured tissue. Injury and inflammation are typically accompanied by acidification. In this study, we examined influences of low pH and mutation of the ionizable amino acid residue H297^6.52 on μ‐opioid receptor binding and signalling induced by the μ‐opioid receptor ligands fentanyl, DAMGO, and naloxone.

Experimental Approach

HEK 293 cells stably transfected with μ‐opioid receptors were used to study opioid ligand binding, [³⁵S]‐GTPγS binding, and cAMP reduction at physiological and acidic pH. We used μ‐opioid receptors mutated at H297^6.52 to A (MOR‐H297^6.52A) to delineate ligand‐specific interactions with H297^6.52.

Key Results

Low pH and the mutant receptor MOR‐H297^6.52A impaired naloxone binding and antagonism of cAMP reduction. In addition, DAMGO binding and G‐protein activation were decreased under these conditions. Fentanyl‐induced signalling was not influenced by pH and largely independent of H297^6.52.

Conclusions and Implications

Our investigations indicate that low pH selectively impairs μ‐opioid receptor signalling modulated by ligands capable of forming hydrogen bonds with H297^6.52. We propose that protonation of H297^6.52 at acidic pH reduces binding and subsequent signalling of such ligands. Novel agonists targeting opioid receptors in injured tissue might benefit from lack of hydrogen bond formation with H297^6.52.

1H-NMR spectroscopy shows cellular uptake of HEPES buffer by human cell lines-an effect to be considered in cell culture experiments

HEPES is commonly used in cell culture media as a buffering substance. Compared to the bicarbonate/CO₂ buffer system, it does not require a CO₂ atmosphere, thereby ensuring stable pH values during handling of cell culture media outside of an incubator. Due to its intrinsic charge, HEPES is considered not to be taken up by cells, which was a prerequisite during buffer development for cell culture by Good and colleagues. However, during the last years, evidence has emerged that HEPES seems to be taken up into cells and that it has major effects on cellular functions. Investigating three different cell lines (MCF-7, U2OS, HeLa) showed that all of them accommodated HEPES-containing medium, i.e., they survive and proliferate in the presence of HEPES. Determination of intracellular metabolites revealed the presence of HEPES for all cell lines. Further analysis of MCF-7 cells showed that even 48 h after medium exchange from HEPES-containing medium to HEPES-free medium, intracellular HEPES could still be detected. Thus, contrary to the common view, HEPES is taken up by cells which should be taken into consideration for studies of specific cellular functions. Graphical abstract ᅟ.

Extracellular pH modulates GABAergic neurotransmission in rat hypothalamus

Changes in extracellular pH have a modulatory effect on GABAA receptor function. It has been reported that pH sensitivity of the GABA receptor is dependent on subunit composition and GABA concentration. Most of previous investigations focused on GABA-evoked currents, which only reflect the postsynaptic receptors. The physiological relevance of pH modulation of GABAergic neurotransmission is not fully elucidated. In the present studies, we examined the influence of extracellular pH on the GABAA receptor-mediated inhibitory neurotransmission in rat hypothalamic neurons. The inhibitory postsynaptic currents (IPSCs), tonic currents, and the GABA-evoked currents were recorded with whole-cell patch techniques on the hypothalamic slices from Sprague-Dawley rats at 15-26 postnatal days. The amplitude and frequency of spontaneous GABA IPSCs were significantly increased while the external pH was changed from 7.3 to 8.4. In the acidic pH (6.4), the spontaneous GABA IPSCs were reduced in amplitude and frequency. The pH induced changes in miniature GABA IPSCs (mIPSCs) similar to that in spontaneous IPSCs. The pH effect on the postsynaptic GABA receptors was assessed with exogenously applied varying concentrations of GABA. The tonic currents and the currents evoked by sub-saturating concentration of GABA ([GABA]) (10 μM) were inhibited by acidic pH and potentiated by alkaline pH. In contrast, the currents evoked by saturating [GABA] (1mM) were not affected by pH changes. We also investigated the influence of pH buffers and buffering capacity on pH sensitivity of GABAA receptors on human recombinant α1β2γ2 GABAA receptors stably expressed in HEK 293 cells. The pH influence on GABAA receptors was similar in HEPES- and MES-buffered media, and not dependent on protonated buffers, suggesting that the observed pH effect on GABA response is a specific consequence of changes in extracellular protons. Our data suggest that the hydrogen ions suppress the GABAergic neurotransmission, which is mediated by both presynaptic and postsynaptic mechanisms.