Apigenin and Structurally Related Flavonoids Allosterically Potentiate the Function of Human α7-Nicotinic Acetylcholine Receptors Expressed in SH-EP1 Cells

Abstract

Phytochemicals, such as monoterpenes, polyphenols, curcuminoids, and flavonoids, are known to have anti-inflammatory, antioxidant, neuroprotective, and procognitive effects. In this study, the effects of several polyhydroxy flavonoids, as derivatives of differently substituted 5,7-dihydroxy-4H-chromen-4-one including apigenin, genistein, luteolin, kaempferol, quercetin, gossypetin, and phloretin with different lipophilicities (cLogP), as well as topological polar surface area (TPSA), were tested for induction of Ca2+ transients by α7 human nicotinic acetylcholine (α7 nACh) receptors expressed in SH-EP1 cells. Apigenin (10 μM) caused a significant potentiation of ACh (30 μM)-induced Ca2+ transients, but did not affect Ca2+ transients induced by high K+ (60 mM) containing solutions. Co-application of apigenin with ACh was equally effective as apigenin preincubation. However, the effect of apigenin significantly diminished by increasing ACh concentrations. The flavonoids tested also potentiated α7 nACh mediated Ca2+ transients with descending potency (highest to lowest) by genistein, gossypetin, kaempferol, luteolin, phloretin, quercetin, and apigenin. The specific binding of α7 nACh receptor antagonist [125I]-bungarotoxin remained unchanged in the presence of any of the tested polyhydroxy flavonoids, suggesting that these compounds act as positive allosteric modulators of the α7-nACh receptor in SH-EP1 cells. These findings suggest a clinical potential for these phytochemicals in the treatment of various human diseases from pain to inflammation and neural disease.

Probing function in ligand-gated ion channels without measuring ion transport

“…5 provides an excellent description of the ligand–receptor interaction, and thus, the toxin’s half-saturation concentration is a good estimate of its K D from the closed state. Our estimate of the latter’s value agrees most closely with those of Oz and coworkers (1.18 nM; Shabbir et al, 2021 ), Sullivan and coworkers (0.71 nM; Gopalakrishnan et al, 1995 ), and Lindstrom and coworkers (0.81 nM; Peng et al, 1994 ) for the same heterologously expressed receptor in intact SH-EP1 cells, HEK-293 cell-membrane homogenates, and detergent micelles, respectively.…”

“…For example, for the chicken α7-AChR (in the context of an ECD–TMD chimera having the rat serotonin-receptor type 3A’s [5-HT 3A R’s] TMD), values of the α-BgTx dissociation equilibrium constant ( K D ) from the closed-channel conformation of 70 pM ( Rangwala et al, 1997 ) and 4.2 nM ( Pittel et al, 2010 ) have been reported for receptors on resuspended cells. Similarly, for the wild-type human α7-AChR, values of this K D have been reported to be 0.8 nM ( Peng et al, 1994 ) and 4 nM ( Tillman et al, 2016 ) in detergent-solubilized receptors; 0.7 nM ( Gopalakrishnan et al, 1995 ) and 7 nM ( Peng et al, 2005 ) in resuspended membrane homogenates; and 1.2 nM ( Shabbir et al, 2021 ), 2.7 nM ( daCosta et al, 2015 ), and 26 nM ( Sine et al, 2019 ) in resuspended cells. Moreover, values of the Hill coefficient of small-molecule ligands reported in the literature of these ion channels often run counter to theoretical expectations, with values significantly different from unity for antagonists and values significantly lower than unity for agonists.…”

Kaempferol Ameliorates Oxygen-Glucose Deprivation/Reoxygenation-Induced Neuronal Ferroptosis by Activating Nrf2/SLC7A11/GPX4 Axis

“…As shown in Figure 1B,C, OGD/R obviously decreased SLC7A11 and GPX4 protein levels, which were rescued by kaempferol (10 µM). The concentration of kaempferol used in the present study was determined based upon published studies [24,25]. Consistent with the result of GPX4 expression, reduced GPX4 activity was detected in neurons exposed to OGD/R (Figure 1D).…”

Probing function in ligand-gated ion channels without measuring ion transport

“…Moreover, under these particular conditions, Equation 5 provides an excellent description of the ligand–receptor interaction, and thus, the toxin’s half-saturation concentration is a good estimate of its K D from the closed state. Our estimate of the latter’s value agrees most closely with those of Oz and coworkers (1.18 ± 0.29 nM; (Shabbir et al, 2021)), Sullivan and coworkers (0.71 ± 0.11 nM; (Gopalakrishnan et al, 1995)), and Lindstrom and coworkers (0.81 nM; (Peng et al, 1994)) for the same receptor in intact SH-EP1 cells, HEK-293 cell-membrane homogenates, and detergent micelles, respectively.…”

“…For example, for the chicken α7-AChR [in the context of an ECD–TMD chimera having the rat serotonin-receptor type 3A’s (5-HT 3A R’s) TMD], values of the α-BgTx dissociation equilibrium constant ( K D ) from the closed-channel conformation of 70 pM (Rangwala et al, 1997) and 4.2 nM (Pittel et al, 2010) have been reported for receptors on resuspended cells. Similarly, for the wild-type human α7-AChR, values of this K D have been reported to be 0.8 nM (Peng et al, 1994) and 4 nM (Tillman et al, 2016) in detergent-solubilized receptors, 0.7 nM (Gopalakrishnan et al, 1995) and 7 nM (Peng et al, 2005) in resuspended membrane homogenates, and 1.2 nM (Shabbir et al, 2021), 2.7 nM (daCosta et al, 2015) and 26 nM (Sine et al, 2019) in resuspended cells. Moreover, values of the Hill coefficient of small-molecule ligands reported in the literature of these ion channels often run counter theoretical expectations, with values significantly different from unity for antagonists, and values significantly lower than unity for agonists.…”