Anticancer/antiviral agent Akt inhibitor-IV massively accumulates in mitochondria and potently disrupts cellular bioenergetics.

Inhibitors of the PI3-kinase/Akt (protein kinase B) pathway are under investigation as anticancer and antiviral agents. Akt inhibitor-IV (ChemBridge 5233705, CAS 681281-88-9, AKTIV), a small molecule reported to inhibit this pathway, exhibits potent anticancer and broad-spectrum antiviral activity. However, depending on concentration, this cationic benzimidazole derivative exhibits paradoxical positive or negative effects on the phosphorylation of Akt that are not well understood. To elucidate its mechanism of action, we investigated its spectroscopic properties. This compound proved to be sufficiently fluorescent (excitation λmax = 388 nm, emission λmax = 460 nm) to enable examination of its uptake and distribution in living mammalian cells. Despite a low quantum yield of 0.0016, imaging of HeLa cells treated with AKTIV (1 μM, 5 min) by confocal laser scanning microscopy, with excitation at 405 nm, revealed extensive accumulation in mitochondria. Treatment of Jurkat lymphocytes with 1 μM AKTIV for 15 min caused accumulation to over 250 μM in these organelles, whereas treatment with 5 μM AKTIV yielded concentrations of over 1 mM in mitochondria, as analyzed by flow cytometry. This massive loading resulted in swelling of these organelles, followed by their apparent disintegration. These effects were associated with profound disruption of cellular bioenergetics including mitochondrial depolarization, diminished mitochondrial respiration, and release of reactive oxygen species. Because mitochondria play key roles in both cancer proliferation and viral replication, we conclude that the anticancer and antiviral activities of AKTIV predominantly result from its direct and immediate effects on the structure and function of mitochondria.

Akt inhibitor-IV (ChemBridge 5233705, CAS 681281-88-9, AKTIV, Figure 1), a cationic benzimidazole derivative, exhibits a wide range of biological activities. This small molecule was first identified in a chemical genetic screen as an inhibitor of nuclear export of the FOXO1a protein.[1] In U2OS cancer cells, low concentrations of AKTIV (IC50 = 0.625 μM) blocked nuclear export of this protein with concomitant inhibitory effects on cellular proliferation (IC50 < 1.25 μM).[1] At a higher concentration of 10 μM, phosphorylation of the serine-threonine kinase Akt (protein kinase B) on residues Ser473 and Thr308 was suppressed. Because phosphorylation of FOXO1a by Akt promotes nuclear export, AKTIV was initially proposed to block nuclear export of this protein by inhibiting a kinase in the PI3-kinase (PI3K)/Akt pathway.[1] Other studies of AKTIV confirmed its potent cytotoxic activity (typical IC50 values <2 μM) against a wide range of cancer and other cell lines.[2−7] Although some inhibitors of the PI3K/Akt pathway show promise as anticancer agents,[8,9] more recent studies[10] of AKTIV concluded that this compound does not directly block the activity of any known kinases within this signaling cascade. Moreover, AKTIV paradoxically increases phosphorylation of Akt when added to BHK-21 cells at 1–2 μM.[10] High concentrations (10 μM) of this small molecule also activate the unfolded protein response (UPR) and trigger cellular blebbing and apoptosis in HEK293T cells.[11]

Figure 1

Structure of Akt inhibitor-IV (AKTIV).

In addition to its major effects on cellular proliferation, AKTIV exhibits broad-spectrum antiviral activity. Viruses inhibited by this compound include vesicular stomatitis virus (VSV), respiratory syncytial virus, vaccinia virus in infected BHK-21 cells,[10] and parainfluenzavirus-5 (PIV5) in infected HeLa cells.[12−14] However, the mechanistic basis for this activity is not well understood, the importance of the PI3K/Akt pathway in viral replication is controversial, and AKTIV has been reported to block the replication of negative-strand RNA viruses through a Akt-independent mechanism.[10]

To probe the structural features associated with this highly biologically active scaffold, we previously reported the synthesis of AKTIV and a collection of analogues.[13] We demonstrated that this compound and some analogues exhibit selective anticancer activity against human HeLa carcinoma cells when compared with normal human bronchial/tracheal epithelial (NBHE) cells. We further confirmed its antiviral effects against recombinant parainfluenzavirus-5 in HeLa cells. Although the photophysical properties of these compounds have not been previously characterized, during our prior studies of analogues of AKTIV, we noticed that some of these compounds were qualitatively fluorescent in solution, and this observation led us to hypothesize that the intrinsic fluorescence of Akt inhibitor-IV, similar to other fluorescent molecular probes,[15,16] might reveal aspects of its biological mechanism of action. Using this approach, we found that AKTIV rapidly accumulates to high levels in mitochondria of treated mammalian cells and profoundly affects the morphology of these organelles. Treatment of cancer cell lines with AKTIV rapidly triggers extensive mitochondrial dysfunction, and this new understanding of its mechanism of action explains many of the diverse biological activities of this potent small molecule.

Results and Discussion

During our prior studies of AKTIV,[13] we observed that dilute solutions of this compound are qualitatively fluorescent when irradiated with ultraviolet light. To examine the photophysical properties of this compound in detail, we obtained absorbance, fluorescence excitation, and fluorescence emission spectra in aqueous buffer (PBS, pH 7.4) and ethanol. As shown in Figure 2, the spectral properties of AKTIV in both of these solvents were very similar, indicating that unlike some common fluorophores, such as dansyl, NBD, and certain coumarins,[17] its fluorescence properties are not highly sensitive to environmental conditions. In these solvents, AKTIV was found to strongly absorb in the UV–violet region (absorbance λmax, PBS = 390 nm), with a relatively high extinction coefficient (ε390 nm) in PBS of 54 000 M–1 cm–1. Upon excitation with UV (λmax, ex. = 388 nm, Figure 2), AKTIV emitted blue fluorescence with λmax = 460 nm (Figure 2). However, the quantum yield (Φ) of AKTIV was only 0.0016, as determined by comparison with the integrated emission of the coumarin-derived fluorophore Pacific Blue (Φ = 0.75, λmax,abs = 405 nm, λmax,em = 450 nm).[18,19] When the brightness of these two fluorophores was compared, defined as the product of the quantum yield and extinction coefficient, AKTIV was 260-fold less bright than Pacific Blue (ε405nm = 30 000 M–1 cm–1).[19] However, the relatively close match of the maximal excitation wavelength (388 nm) of AKTIV to the 405 nm laser line of many confocal laser scanning microscopes and flow cytometers led us to conclude that the intrinsic fluorescence of this compound might enable imaging and analysis of its subcellular distribution in living mammalian cells.

Figure 2

Photophysical properties of AKTIV. The absorbance (Abs.) spectrum, excitation (Ex.) spectrum, and fluorescence emission (Em.) spectrum were obtained in PBS (pH 7.4) and ethanol. Values for the extinction coefficient (ε), and quantum yield (Φ) in PBS are listed above.

To investigate the utility of AKTIV as a fluorescent probe, HeLa cervical carcinoma cells were briefly treated (5 min) with a low concentration (1 μM) of this compound. Imaging by confocal laser scanning microscopy, with excitation at 405 nm, revealed fluorescence in defined subcellular structures of treated cells (Figure 3). These structures exhibited a tubular morphology and were identified as mitochondria by costaining with the spectrally orthogonal probe MitoTracker Deep Red-FM. To examine if autofluorescence of NADH[20] in these organelles contributed to this signal, control experiments in the absence of AKTIV were used to verify that the fluorescence observed in mitochondria upon excitation at 405 nm was exclusively derived from this compound (Figure 3). Because of its low quantum yield, the visualization of AKTIV in mitochondria of treated HeLa cells suggested that this compound accumulates to high levels in these organelles.

Figure 3

Confocal laser scanning and DIC micrographs of living HeLa cells. Cells in panel A were treated with MitoTracker Deep Red FM (100 nM) and AKTIV (1 μM) for 5 min. Cells in panel B were treated with MitoTracker Deep Red FM (100 nM, 5 min) alone to confirm the absence of autofluorescence under these conditions. The instrument settings and imaging parameters are identical in panels A and B. Scale bar = 20 μm.

As shown in Figure 4, AKTIV shows structural similarities to MitoTracker Deep Red-FM,[21] JC-1,[22] rhodamine 123,[23] MKT-007,[24,25] and F16.[26] These types of lipophilic delocalized cations are known to accumulate in mitochondria[27,28] and are driven into this organelle by the high negative electrochemical potential that exists across the inner mitochondrial membrane (typically −120 to −180 mV, depending on cell type).[29−32] This negative membrane potential is critical for production of ATP by the mitochondrial F0F1-ATP synthase and is generated by pumping of protons across the mitochondrial inner membrane by the respiratory chain.

Figure 4

Comparison of the structure of AKTIV with other small molecules that selectively accumulate in mitochondria of mammalian cells.

To quantify the extent of bioaccumulation of small molecules in mitochondria, we initially analyzed the fluorescence of bead standards upon excitation at 405 and 488 nm. This allowed standard curves of molecular equivalents of the fluorophores Cascade Blue (for AKTIV) and fluorescein (for rhodamine 123) to be constructed. Human Jurkat cells were briefly treated with AKTIV or rhodamine 123 for 15 min, subjected to flow cytometry under the same conditions, and the number of fluorophores per cell was calculated based on the brightness of Cascade Blue (Φ = 0.54, ε399 nm = 28 000 M–1 cm–1)[33] and fluorescein (ΦpH 9 = 0.93, ε490 nm= 76 900 M–1 cm–1).[34] As shown in Figure 5, based on the published[35] average cell volume (663.7 μm3) and mitochondrial volume (33.4 μm3) of Jurkat lymphocytes, concentrations of rhodamine 123 and AKTIV in mitochondria were determined. These values were further corrected to account for the quenching of these fluorophores that occurs at high concentrations (data shown in Figure S1 and methods provided in the Supporting Information). These studies revealed that after treatment of cells with culture media containing 0.05 to 5 μM of these compounds, rhodamine 123 was estimated to reach concentrations of 1–548 μM in mitochondria. Under the same conditions, AKTIV was estimated to accumulate to 17–1435 μM in these organelles. Correspondingly, when 1 μM of AKTIV was added to cells for 15 min, over 250-fold bioaccumulation in mitochondria was observed. This rapid, selective, and massive accumulation in mitochondria provides strong evidence that these organelles are a major and direct target of the biological effects of this small molecule.

Figure 5

Concentrations of rhodamine 123 (left bars) and AKTIV (right bars) in mitochondria of Jurkat lymphocytes after treatment for 15 min at 37 °C. Treated cells were analyzed by flow cytometry with excitation at 488 nm (rhodamine 123) or 405 nm (AKTIV). Cellular fluorescence was converted to concentration using a standard curve constructed with SpheroTech Rainbow Ultra beads and a ratio of mitochondrial volume to total cell volume of 0.053. These concentrations were corrected to account for quenching of fluorophores observed at high concentrations. Error bars represent the standard deviation.

To probe the effects of AKTIV on mitochondria, we examined the morphology of these organelles in HeLa cells by confocal laser scanning microscopy. In living cells, mitochondria constantly undergo fusion and fission events, and these dynamic processes play key roles in mitochondrial biogenesis, cellular energetics, apoptosis, and mitochondrial morphology.[36,37] Compared to the vehicle (0.1% DMSO) control, when HeLa cells were treated with AKTIV (1 μM) for 20 min, rapid and extensive swelling of the normally tubular mitochondria was observed (Figure 6). This swelling was followed by similarly rapid disintegration of these organelles into smaller structures (videos showing the time course of these effects are provided in the Supporting Information). As a positive control, hydrogen peroxide was added to induce oxidative stress and disrupt the mitochondrial network.[38] In HeLa cells treated with H2O2 (1 mM) for 20–40 min, fragmentation of the mitochondrial network was observed, but this treatment did not cause the swelling or disintegration associated with AKTIV (Figure 6). These results indicate that AKTIV profoundly disrupts the morphology of mitochondria of HeLa cells, inducing distinctive phenotype of swelling followed by disintegration of these organelles.

Figure 6

Confocal laser scanning micrographs of living HeLa cells treated with MitoTracker Deep Red FM (100 nM). Cells were further treated with DMSO (0.1%, panels A–C), AKTIV (1 μM, panels D–F), or hydrogen peroxide (1 mM, panels G–I) at 37 °C. The same field of cells was imaged at the three times shown. Scale bar = 20 μm.

Elevated mitochondrial membrane potential is a hallmark of cancer.[30,39,40] To examine the effect of AKTIV on cell lines that differ in mitochondrial membrane potential, we compared the toxicity of this compound with an uncharged desethyl analogue (DEAKTIV)[13] toward two cancer cell lines (HeLa and Jurkat) and the normal monkey kidney cell line CV-1. The DEAKTIV analogue was chosen as a control because it lacks the fixed positive charge needed to drive accumulation into mitochondria. As shown in Figure 7, AKTIV was highly toxic toward HeLa (IC50 = 320 ± 30 nM) and Jurkat (IC50 = 340 ± 30 nM) cells after treatment for 48 h, but under these conditions, this compound was less toxic toward normal CV-1 cells (IC50= 870 ± 90 nM), which have been shown to exhibit a lower inherent mitochondrial membrane potential.[41] The uncharged but structurally similar DEAKTIV analogue was not toxic to any of these cell lines (IC50 > 10 μM), providing evidence that the positive charge of AKTIV drives this compound into mitochondria. This interpretation was further supported by confocal imaging of HeLa cells treated with the rapid mitochondrial depolarizer cyanide 3-chlorophenylhydrazine (CCCP, 50 μM, 15 min). This treatment inhibited the uptake of AKTIV by mitochondria, indicating that the negative membrane potential of these organelles is required for accumulation of this compound (confocal microscopy images provided in Figure S2 of the Supporting Information).

Figure 7

Panel A: Cytotoxicity of AKTIV and the uncharged DEAKTIV analogue toward two cancer cell lines (HeLa, Jurkat) and a normal cell line (CV-1) after 48 h. Panel B: Effects of treatment with these compounds for 2 h on the polarization of mitochondria as assayed with the ratiometric fluorescent probe JC-1 (1 μM).

To examine the functional consequences of treatment with AKTIV, the fluorescent probe JC-1 (Figure 4) was used to measure effects on the membrane potential of mitochondria. This probe undergoes a shift in fluorescence emission from green to red as it accumulates in mitochondria, and by analysis of this red/green ratio, one can quantify the extent of mitochondrial depolarization. As shown in Figure 7, treatment with AKTIV for 2 h depolarized mitochondria of HeLa cells (IC50 = 920 ± 30 nM) and Jurkat cells (IC50 = 750 ± 30 nM) at submicromolar concentrations, whereas treatment with DEAKTIV was inconsequential.

Mitochondria predominantly produce cellular ATP during aerobic respiration.[30] To determine whether AKTIV inhibits this process, we used a MitoXpress Xtra-HS assay (Luxcel)[42] to measure the consumption of oxygen by mitochondria in HeLa cells (Figure 8). Consistent with previous studies,[43] CCCP, an uncoupling agent that disrupts mitochondrial electron transport chain activity by bypassing efflux of protons through the ATP synthase complex, accelerated consumption of O2 compared to vehicle control. In contrast, AKTIV (2 μM) completely blocked consumption of O2 over the 90 min time course of the experiment. This inhibition was comparable to treatment with 10 μM of the mitochondrial complex I inhibitor rotenone.[43] Addition of LY294002 (10 μM), a well-characterized inhibitor of PI3K/Akt pathway,[44] showed no effect on O2 consumption, demonstrating that blocking this pathway under these conditions does not affect mitochondrial respiration. This control experiment provides evidence that the inhibition of mitochondrial respiration by AKTIV might be upstream of its effects on the PI3K/Akt pathway.

Figure 8

Panel A: Consumption of oxygen by HeLa cells over 90 min as quantified with a MitoXpress XS assay. In negative control experiments with cells treated with DMSO (0.1%) or AKTIV (2 μM) but without addition of the Pt-porphyrin-based MitoXpress probe, no time-resolved fluorescence was observed (data not shown). Panel B: Fluorescence of the ROS sensor H2DCFDA (2 μM) in Jurkat cells. Cells were treated for 30 min followed by analysis by flow cytometry. Error bars represent standard errors of the mean.

Reactive oxygen species (ROS) are byproducts of the mitochondrial electron transport chain.[45] High levels of ROS indicate mitochondrial dysfunction and can induce apoptosis.[46] To investigate whether treatment with AKTIV induces changes in cellular ROS, which could link mitochondrial dysfunction to downstream redox-sensitive biological effects such as activation of Akt,[47] we assayed levels of ROS in Jurkat lymphocytes with the fluorescent probe H2DCFDA. As shown in Figure 8, treatment of Jurkat cells with AKTIV (30 min) increased cellular ROS in a dose-dependent manner by up to 59-fold at 2 μM compared with the vehicle control. Treatment with hydrogen peroxide (30 min) as a positive control increased ROS by 24-fold at 1 μM and 93-fold at 10 μM. Higher concentrations of hydrogen peroxide (50 μM) showed reduced effects that were associated with cytotoxicity. As another control, treatment with rotenone (30 min), an agent previously shown to elevate cellular ROS in HepG2 cells,[48] increased ROS in a dose-dependent manner by up to 16-fold at 50 μM. Given that phosphorylation of Akt is redox-sensitive[47] and can be activated by treatment of cells with hydrogen peroxide, these results suggest that the mitochondrial dysfunction and production of ROS by AKTIV may be mechanistically linked to the previously reported[10] activation of Akt.

Conclusions

Based on its effects on the nuclear export of FOXO1a, AKTIV was initially proposed[1] to function as an inhibitor of the PI3K/Akt pathway. However, a more recent investigation by Connor[10] concluded that this compound does not inhibit any known kinases in this signaling pathway; moreover, AKTIV increases phosphorylation of Akt kinase when added to BHK-21 cells at concentrations that elicit major biological effects. To investigate its mechanism of action, we used intrinsic fluorescence to examine the subcellular localization of AKTIV. Imaging of treated HeLa cells by confocal microscopy with excitation at 405 nm revealed mitochondria as a major target of this potent anticancer/antiviral agent. When cancer cell lines were treated with 1 μM of this compound, AKTIV rapidly accumulated to over 250 μM in these organelles within 15 min and caused profound mitochondrial dysfunction. Low concentrations of AKTIV triggered swelling, disintegration, and depolarization of mitochondria, elevation of ROS, and essentially complete inhibition of cellular consumption of oxygen. This rapid elevation of ROS provides a potential mechanistic link between mitochondrial dysfunction induced by AKTIV and increased phosphorylation[10] of the redox-sensitive[47] Akt kinase observed at low concentrations. These studies revealed a novel mechanism of action of AKTIV and provide a new rationale for its anticancer and broad-spectrum antiviral activities.

Due to alterations in glucose metabolism, cancer cells commonly exhibit elevated mitochondrial membrane potentials of at least 60 mV compared with normal cells.[30,39,40] By accumulating in hyperpolarized mitochondria, delocalized lipophilic cations such as rhodamine 123,[23,41] dequinalinium,[49] F16,[26] MKT-077,[24,25] rosamines,[50] and others[27,51,52] can exhibit selective anticancer activity.[53,54] These types of compounds can affect a number of different mitochondrial targets involved in cellular proliferation including mitochondrial polarization,[26] the NADH-ubiquinone reductase,[55] the F0F1 ATPase,[27] and pro-apoptotic signals such as Bcl-2 family members.[56] Consequently, the selective anticancer activity of AKTIV can be explained by its profound effects on hyperpolarized mitochondria in the absence of downstream effects on the PI3K/Akt pathway.

Inhibitors of mitochondrial respiration suppress the de novo synthesis of pyrimidines.[57] This mechanism of action was recently reported[58] to be responsible for the broad spectrum antiviral activity of the natural product antimycin A, a mitochondrial complex III inhibitor, against RNA viruses. This antiviral activity of antimycin A and analogues was observed at nanomolar concentrations that are not toxic to mammalian host cells. Other compounds similarly shown to exhibit antiviral activity by promoting mitochondrial dysfunction include the complex III inhibitor myxothiazole, the complex I inhibitor rotenone, the proton ionophore CCCP, and the ATP synthase inhibitor oligomycin.[58] Some of these mitochondrial electron transport chain inhibitors exhibit antiviral activity under conditions where ATP production is not suppressed.[58] Based on this precedent, we conclude that the broad spectrum antiviral activity of AKTIV relates to its immediate and direct effects on mitochondria either through suppression of de novo synthesis of pyrimidines, the generation of mitochondrial ROS, and/or depletion of cellular ATP. Consequently, identification of mitochondria as a major target of AKTIV, which are profoundly affected within minutes of treatment of mammalian cells at concentrations of ∼1−5 μM, provides a unifying explanation for many of the diverse biological activities of this potent small molecule.