Differential sensitivity of rat voltage-sensitive sodium channel isoforms to pyrazoline-type insecticides

Discovery of new fluorescent thiazole-pyrazoline derivatives as autophagy inducers by inhibiting mTOR activity in A549 human lung cancer cells

A series of fluorescent thiazole–pyrazoline derivatives was synthesized and their structures were characterized by ¹H NMR, ¹³C NMR, and HRMS. Biological evaluation demonstrated that these compounds could effectively inhibit the growth of human non-small cell lung cancer (NSCLC) A549 cells in a dose- and time-dependent manner in vitro and inhibit tumor growth in vivo. The structure–activity relationship (SAR) of the compounds was analyzed. Further mechanism research revealed they could induce autophagy and cell cycle arrest while had no influence on cell necrosis. Compound 5e inhibited the activity of mTOR via FKBP12, which could be reversed by 3BDO, an mTOR activator and autophagy inhibitor. Compound 5e inhibited growth, promoted autophagy of A549 cells in vivo. Moreover, compound 5e showed good selectivity with no influence on normal vascular endothelial cell growth and the normal chick embryo chorioallantoic membrane (CAM) capillary formation. Therefore, our research provides potential lead compounds for the development of new anticancer drugs against human lung cancer.

Molecular Mechanism of Action and Selectivity of Sodium Ch annel Blocker Insecticides

Sodium channel blocker insecticides (SCBIs) are a relatively new class of insecticides that are represented by two commercially registered compounds, indoxacarb and metaflumizone. SCBIs, like pyrethroids and DDT, target voltage-gated sodium channels (VGSCs) to intoxicate insects. In contrast to pyrethroids, however, SCBIs inhibit VGSCs at a distinct receptor site that overlaps those of therapeutic inhibitors of sodium channels, such as local anesthetics, anticonvulsants and antiarrhythmics. This review will recount the development of the SCBI insecticide class from its roots as chitin synthesis inhibitors, discuss the symptoms of poisoning and evidence supporting inhibition of VGSCs as their mechanism of action, describe the current model for SCBI-induced inhibition of VGSCs, present a model for the receptor for SCBIs on VGSCs, and highlight differences between data collected from mammalian and insect experimental models.

Voltage-Gated Sodium Channels as Insecticide Targets

Voltage-gated sodium channels are critical for the generation and propagation of action potentials. They are the primary target of several classes of insecticides, including DDT, pyrethroids and sodium channel blocker insecticides (SCBIs). DDT and pyrethroids preferably bind to open sodium channels and stabilize the open state, causing prolonged currents. In contrast, SCBIs block sodium channels by binding to the inactivated state. Many sodium channel mutations are associated with knockdown resistance (kdr) to DDT and pyrethroids in diverse arthropod pests. Functional characterization of kdr mutations together with computational modelling predicts dual pyrethroid receptor sites on sodium channels. In contrast, the molecular determinants of the SCBI receptor site remain largely unknown. In this review, we summarize current knowledge about the molecular mechanisms of action of pyrethroids and SCBIs, and highlight the differences in the molecular interaction of these insecticides with insect versus mammalian sodium channels.

Indoxacarb, Metaflumizone, and Other Sodium Channel Inhibitor Insecticides: Mechanism and Site of Action on Mammalian Voltage-Gated Sodium Channels

Sodium channel inhibitor (SCI) insecticides were discovered almost four decades ago but have only recently yielded important commercial products (eg., indoxacarb and metaflumizone). SCI insecticides inhibit sodium channel function by binding selectively to slow-inactivated (non-conducting) sodium channel states. Characterization of the action of SCI insecticides on mammalian sodium channels using both biochemical and electrophysiological approaches demonstrates that they bind at or near a drug receptor site, the "local anesthetic (LA) receptor." This mechanism and site of action on sodium channels differentiates SCI insecticides from other insecticidal agents that act on sodium channels. However, SCI insecticides share a common mode of action with drugs currently under investigation as anticonvulsants and treatments for neuropathic pain. In this paper we summarize the development of the SCI insecticide class and the evidence that this structurally diverse group of compounds have a common mode of action on sodium channels. We then review research that has used site-directed mutagenesis and heterologous expression of cloned mammalian sodium channels in Xenopus laevis oocytes to further elucidate the site and mechanism of action of SCI insecticides. The results of these studies provide new insight into the mechanism of action of SCI insecticides on voltage-gated sodium channels, the location of the SCI insecticide receptor, and its relationship to the LA receptor that binds therapeutic SCI agents.

Compound-specific effects of mutations at Val787 in DII-S6 of Nav 1.4 sodium channels on the action of sodium channel inhibitor insecticides

Sodium channel inhibitor (SCI) insecticides are hypothesized to inhibit voltage-gated sodium channels by binding selectively to the slow-inactivated state. Replacement of valine at position 787 in the S6 segment of homology domain II of the rat Na(v)1.4 sodium channel by lysine (V787K) enchances slow inactivation of this channel whereas replacement by alanine or cysteine (V787A and V787C) inhibits slow inactivation. To test the hypothesis that SCI insecticides bind selectively to the slow-inactivated state, we constructed mutated Na(v)1.4/V787A, Na(v)1.4/V787C, and Na(v)1.4/V787K cDNAs, expressed wildtype and mutated channels with the auxiliary β1 subunit in Xenopus oocytes, and used the two-electrode voltage clamp technique to examine the effects of these mutations on channel inhibition by four SCI insecticides (indoxacarb, its bioactivated metabolite DCJW, metaflumizone, and RH3421). Mutations at Val787 affected SCI insecticide sensitivity in a manner that was independent of mutation-induced changes in slow inactivation gating. Sensitivity to inhibition by 10 μM indoxacarb was significantly increased in all three mutated channels, whereas sensitivity to inhibition by 10 μM metaflumizone was significantly reduced in Na(v)1.4/V787A channels and completely abolished in Na(v)1.4/V787K channels. The effects of Val787 mutations on metaflumizone were correlated with the hydrophobicity of the substituted amino acid rather than the extent of slow inactivation. None of the mutations at Val787 significantly affected the sensitivity to inhibition by DCJW or RH3421. These results demonstrate that the impact of mutations at Val787 on sodium channel inhibition by SCI insecticides depend on the specific insecticide examined and is independent of mutation-induced changes in slow inactivation gating. We propose that Val787 may be a unique determinant of metaflumizone binding.

Role of the local anesthetic receptor in the state-dependent inhibition of voltage-gated sodium channels by the insecticide metaflumizone

Sodium channel inhibitor (SCI) insecticides selectively target voltage-gated sodium (Na(v)) channels in the slow-inactivated state by binding at or near the local anesthetic receptor within the sodium channel pore. Metaflumizone is a new insecticide for the treatment of fleas on domesticated pets and has recently been reported to block insect sodium channels in the slow-inactivated state, thereby implying that it is also a member of the SCI class. Using the two-electrode voltage-clamp technique, we examined metaflumizone inhibition of rat Na(v)1.4 sodium channels expressed in Xenopus laevis oocytes. Metaflumizone selectively inhibited Na(v)1.4 channels at potentials that promoted slow inactivation and shifted the voltage dependence of slow inactivation in the direction of hyperpolarization. Metaflumizone perfusion at a hyperpolarized holding potential also shifted the conductance-voltage curve for activation in the direction of depolarization and antagonized use-dependent lidocaine inhibition of fast-inactivated sodium channels, actions not previously observed with other SCI insecticides. We expressed mutated Na(v)1.4/F1579A and Na(v)1.4/Y1586A channels to investigate whether metaflumizone shares the domain IV segment S6 (DIV-S6) binding determinants identified for other SCI insecticides. Consistent with previous investigations of SCI insecticides on rat Na(v)1.4 channels, the F1579A mutation reduced sensitivity to block by metaflumizone, whereas the Y1586A mutation paradoxically increased the sensitivity to metaflumizone. We conclude that metaflumizone selectively inhibits slow-inactivated Na(v)1.4 channels and shares DIV-S6 binding determinants with other SCI insecticides and therapeutic drugs. However, our results suggest that metaflumizone interacts with resting and fast-inactivated channels in a manner that is distinct from other compounds in this insecticide class.

Coexpression with Auxiliary β Subunits Modulates the Action of Tefluthrin on Rat Na(v)1.6 and Na(v)1.3 Sodium Channels

We expressed the rat Na(v)1.3 and Na(v)1.6 sodium channel α subunit isoforms in Xenopus oocytes either alone or with the rat β1 and β2 auxiliary subunits in various combinations and assessed the sensitivity of the expressed channels to resting and use-dependent modification by the pyrethroid insecticide tefluthrin using the two-electrode voltage clamp technique. Coexpression with the β1 and β2 subunits, either individually or in combination, did not affecting the resting sensitivity of Na(v)1.6 channels to tefluthrin. Modification by tefluthrin of Na(v)1.6 channels in the absence of β subunits was not altered by the application of trains of high-frequency depolarizing prepulses. By contrast, coexpression of the Na(v)1.6 channel with the β1 subunit enhanced the extent of channel modification twofold following repeated depolarization. Coexpression of Na(v)1.6 with the β2 subunit also slightly enhanced modification following repeated depolarization, but coexpression of Na(v)1.6 with both β subunits caused enhanced modification following repeated depolarization that was indistinguishable from that found with Na(v)1.6+β1 channels. In contrast to Na(v)1.6, the resting modification by tefluthrin of Na(v)1.3 channels expressed in the absence of β subunits was reduced by repeated depolarization. However, tefluthrin modification of the Na(v)1.3 α subunit expressed with both β subunits was enhanced 1.7-fold by repeated depolarization, thereby confirming that β subunit modulation of use-dependent effects was not confined to the Na(v)1.6 isoform. These results show that the actions of pyrethroids on mammalian sodium channels in the Xenopus oocyte expression system are determined in part by the interactions of the sodium channel α subunit with the auxiliary β subunits that are part of the heteromultimeric sodium channel complexes found in neurons and other excitable cells.

Role of the sixth transmembrane segment of domain IV of the cockroach sodium channel in the action of sodium channel blocker insecticides

Sodium channel blocker insecticides (SCBIs), such as indoxacarb and metaflumizone, are a new class of insecticides with a mechanism of action different from those of other insecticides that target sodium channels. SCBIs block sodium channels in a manner similar to local anesthetics (LAs) such as lidocaine. Several residues, particularly F1579 and Y1586, in the sixth transmembrane segment (S6) of domain IV (IV) of rat Na(v)1.4 sodium channels are required for the action of LAs and SCBIs and may form part of overlapping receptor sites. However, the binding site for SCBIs in insect sodium channels remains undefined. We used site-directed mutagenesis, the Xenopus laevis oocyte expression system, and the two-electrode voltage clamp technique to study the effects on SCBI activity of mutating F1817 and Y1824 (analogous to those residues identified in mammalian sodium channels) to alanine, in the voltage-sensitive sodium channel of the German cockroach, Blattella germanica. The mutant channels showed no effect or a marked increase in channel sensitivity to both DCJW (the active metabolite of indoxacarb) and metaflumizone. Thus, it appeared that although the F1817 residue plays a role in the action of SCBIs and that both residues are involved in LA activity in mammalian sodium channels, neither F1817 nor Y1824 are integral determinants of SCBI binding on insect sodium channels. Our results suggest that the receptor site of SCBIs on insect sodium channels may be significantly different from that on mammalian sodium channels.

Point mutations at the local anesthetic receptor site modulate the state-dependent block of rat Nav1.4 sodium channels by pyrazoline-type insecticides

Pyrazoline-type insecticides (PTIs) selectively block sodium channels at membrane potentials that promote slow sodium channel inactivation and are proposed to interact with a site that overlaps the local anesthetic (LA) receptor site. Mutagenesis studies identified two amino acid residues in the S6 segment of homology domain IV (Phe-1579 and Tyr-1586 in the rat Na(v)1.4 sodium channel) as principal elements of the LA receptor. To test the hypothesis that PTIs bind to the LA receptor, we constructed mutated Na(v)1.4/F1579A and Na(v)1.4/Y1586A cDNAs, expressed native and mutated channels in Xenopus oocytes, and examined the effects of these mutations on channel block by three PTIs (indoxacarb, its bioactivation product DCJW, and RH3421) by two-electrode voltage clamp. DCJW and RH3421 had no effect on Na(v)1.4 channels held at -120mV but caused a slowly developing block upon depolarization to -30mV. Estimated IC(50) values following 15min of exposure were 1 and 4muM for DCJW and RH3421, respectively. Indoxacarb failed to block Na(v)1.4 channels under all experimental conditions. Sensitivity to block by DCJW and RH3421 at -30mV was significantly reduced in Na(v)1.4/F1579A channels, a finding that is consistent with the impact of this mutation on drug binding. In contrast to its effect on drug binding, the Y1586A mutation increased the sensitivity of Na(v)1.4 channels held at -30mV to all three compounds, conferring modest sensitivity to indoxacarb and increasing sensitivity to DCJW and RH3421 by 58- and 16-fold, respectively. These results provide direct evidence for the action of PTIs at the LA receptor.

Insect sodium channels and insecticide resistance

Voltage-gated sodium channels are essential for the generation and propagation of action potentials (i.e., electrical impulses) in excitable cells. Although most of our knowledge about sodium channels is derived from decades of studies of mammalian isoforms, research on insect sodium channels is revealing both common and unique aspects of sodium channel biology. In particular, our understanding of the molecular dynamics and pharmacology of insect sodium channels has advanced greatly in recent years, thanks to successful functional expression of insect sodium channels in Xenopus oocytes and intensive efforts to elucidate the molecular basis of insect resistance to insecticides that target sodium channels. In this review, I discuss recent literature on insect sodium channels with emphases on the prominent role of alternative splicing and RNA editing in the generation of functionally diverse sodium channels in insects and the current understanding of the interactions between insect sodium channels and insecticides.