Abstract
Therapeutic agents used in the treatment of cancer are known to develop resistance against cancer cells. Hence, there is a continuing need to investigate novel agents for the treatment and management of cancer. Antitumor activity of greensporone C (GC), a new resorcylic acid lactone isolated from an organic extract of a culture of a Halenospora sp. freshwater fungus, was subjected for screening against a panel of leukemic cell lines (K562, U937, and AR320). In all the three cell lines, cell proliferation was inhibited in dose-dependent fashion. GC further arrested the cells in SubG0 phase in dose-dependent manner. Annexin V/PI dual staining data confirmed apoptotic death of treated K562 and U937 leukemic cells. Treatment with GC suppressed constitutively phosphorylated AKT and downregulated expression of inhibitor of apoptotic proteins XIAP, cIAP-1, and cIAP-2. In summation to this, GC-treated leukemic cells upregulated protein expression of pro-apoptotic proteins, Bax with concomitant decrease in expression of anti-apoptotic proteins including Bcl-2 and Bcl-xL. Upregulation of Bax was associated with cytochrome c release which was confirmed from the collapse of mitochondrial membrane. Released cytochrome c further activated caspase cascade which in turn initiated apoptosis process. Anticancer activity of this isolated fungal compound GC was potentiated via stimulating production of reactive oxygen species (ROS) along with depletion of reduced glutathione (GSH) levels in K562 and U937 leukemic cells. Pretreatment of these cells with N-acetyl cysteine prevented GC-induced depletion of reduced GSH level and mitochondrial-caspase-induced apoptosis. Altogether, our data show that GC modulates the apoptotic response of human leukemic cells and raises the possibility of its use as a novel therapeutic strategy for hematological malignancies.
Highlights
MATERIALS AND METHODSNatural products are and have been playing vital role in the area of drug discovery
We investigated the functional role of Greensporone C (GC) on caspase activation in leukemic cells
An increase in γ-H2AX-a marker for DNA double-stranded breakage was observed in response to GC treatment in leukemic cells suggesting a role of DNA damage in GC-mediated apoptosis (Figures 2E,F)
Summary
MATERIALS AND METHODSNatural products are and have been playing vital role in the area of drug discovery. From approximately 0.5 million secondary metabolites (natural products) that have been described to date, about 14% (70,000) were of microbial origin. Of about 33,500 bioactive microbial natural compounds, 47% were of fungal origin (Bills and Gloer, 2016). Fungal compounds have been gaining a lot of importance and recognition in the area of anti-cancer drug discovery worldwide with some being currently under investigation in different phases of clinical trials (Kinghorn et al, 2016). From estimated 5.1 million species of fungi on earth (Blackwell, 2011), only about 99,000 species have been described (Blackwell, 2011), and a smaller fraction of these were explored for bioactive secondary metabolites (Kinghorn et al, 2016). Freshwater fungi, which flourishes in freshwater ecosystems and is primarily involved in the decomposition of submerged plant debris represents an even less studied area in mycology with only about 3,000 species being identified so far (Jones et al, 2014)
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