Induction of G2/M phase arrest by squamocin in chronic myeloid leukemia (K562) cells

Squamocin is one of the annonaceous acetogenins and has been reported to have anticancer activity. Squamocin was found to inhibit the growth of K562 cells in a time- and dose-dependent manner. Cell cycle analysis showed G2/M phase arrest in K562 cells following 24 h exposure to squamocin. During the G2/M arrest, cyclin-dependent kinase inhibitors (CDKIs), p21 and p27 were increased in a dose-dependent manner. Analysis of the cell cycle regulatory proteins demonstrated that squamocin did not change the steady-state levels of Cdk2, Cdk4, cyclin A, cyclin B1, cyclin D3 and cyclin E, but decreased the protein levels of Cdk1 and Cdc25C. These results suggest that squamocin inhibits the proliferation of K562 cells via G2/M arrest in association with the induction of p21, p27 and the reduction of Cdk1 and Cdc25C kinase activities.     

Remarkable substituent effect: β-aminosquamocin, a potent dual inhibitor of mitochondrial complexes I and III

The introduction of a primary amine function on the terminal α,β-unsaturated lactone of squamocin 1, a common structural hallmark of annonaceous acetogenins, shifted this specific inhibitor of mitochondrial complex I into a potent dual inhibitor of complexes I and III. The mechanism of action of β-aminosquamocin 2, against these two respiratory targets, is studied and discussed in view of current structure-activity relationship knowledge in the acetogenin series.     

Squamocin mode of action to stimulate biofilm formation of Pseudomonas plecoglossicida J26, a PAHs degrading bacterium

Squamocin, an annonaceous acetogenin (ACG) extracted from Annona cherimolia (Annonaceae), has been shown to increase biofilm production of Pseudomonas plecoglossicida J26 (closely related to P. plecoglossicida), a polycyclic aromatic hydrocarbon degrading bacterium. PAHs have become priority pollutants for bioremediation due to their carcinogenicity and toxicity. The effect of various stressful stimuli (naphthalene, octanol, HCl, and NaCl) on cell growth, biofilm formation and autoinducer production of P. plecoglossicida were evaluated and compared with the effect of squamocin to establish its mode of action on biofilm formation. All stressors that inhibited growth stimulated autoinducer production while squamocin was growth stimulant at concentrations above 2.5 μg ml⁻¹. Despite structural similarities, squamocin is not an autoinducer agonist. It indirectly stimulates autoinducer production and increases P. plecoglossicida J26 cell growth. This is the first report on an ACG mode of action in the formation of biofilm in a naphthalene-degrading strain.     

Selective cytotoxicity of squamocin on T24 bladder cancer cells at the S-phase via a Bax-, Bad-, and caspase-3-related pathways

Annonaceous acetogenins are a group of potential anti-neoplastic agents isolated from Annonaceae plants. We purified squamocin, a cytotoxic bis-tetrahydrofuran acetogenin, from the seeds of Annona reticulata and analyzed its biologic effects on cancer cells. We showed that squamocin was cytotoxic to all the cancer lines tested. Furthermore, squamocin arrested T24 bladder cancer cells at the G1 phase and caused a selective cytotoxicity on S-phase-enriched T24 cells. It induced the expression of Bax and Bad pro-apoptotic genes, enhanced caspase-3 activity, cleaved the functional protein of PARP and caused cell apoptosis. These results suggest that squamocin is a potentially promising anticancer compound.     

Involvement of caspase-3 activation in squamocin-induced apoptosis in leukemia cell line HL-60

Annonaceous acetogenins have potent antitumor effect in vitro and in vivo. Squamocin is one of the annonaceous acetogenins and has been reported to have antiproliferative effect on cancer cells. Our results from this study showed that squamocin inhibited proliferation of HL-60 cells with IC50 value of 0.17 microg/ml and induced apoptosis of HL-60 cells. Investigation of the mechanism of squamocin-induced apoptosis revealed that treatment of HL-60 cells with squamocin resulted in extensive nuclear condensation. DNA fragmentation, cleavage of the death substrate poly (ADP-ribose) polymerase (PARP) and induction of caspase-3 activity. Pretreatment of HL-60 cells with caspase-3 specific inhibitor DEVD-CHO prevented squamocin-induced DNA fragmentation, PARP cleavage and cell death. The expression levels of protein bcl-2, bax have no change in response to squamocin treatment in HL-60 cells, whereas stress-activated protein kinase (SAPK/JNK) was activated after treatment with squamocin in HL-60 cells. These results suggest that apoptosis of HL-60 cells induced by squamocin requires caspase-3 activation and is related to SAPK activation.     

Squamocin-O(1) and squamocin-O(2), new adjacent bis-tetrahydrofuran acetogenins from the seeds of Annona squamosa

Two bis-tetrahydrofuran acetogenins, squamocin-O(1) (1) and squamocin-O(2) (2), were isolated from a MeOH extract of seeds of Annona squamosa L. Their structures were determined by spectral means including precursor-ion scanning mass spectral analysis for their aminal derivatives. The configurations at the oxymethine chiral centers were assigned as 12R,15R,16R,19R,20R,23R,24S,28S,36S for 1 and 12S,15R,16R,19R,20R,23R,24S, 28S,36S for 2, based on 1H NMR analysis of their Mosher's ester derivatives and CD data.