Natural Polyphyllins (I, II, D, VI, VII) Reverses Cancer Through Apoptosis, Autophagy, Mitophagy, Inflammation, and Necroptosis

Abstract

Cancer is the second leading cause of mortality worldwide. Conventional therapies, including surgery, radiation, and chemotherapy, have limited success because of secondary resistance. Therefore, safe, non-resistant, less toxic, and convenient drugs are urgently required. Natural products (NPs), primarily sourced from medicinal plants, are ideal for cancer treatment because of their low toxicity and high success. NPs cure cancer by regulating different pathways, such as PI3K/AKT/mTOR, ER stress, JNK, Wnt, STAT3, MAPKs, NF-kB, MEK-ERK, inflammation, oxidative stress, apoptosis, autophagy, mitophagy, and necroptosis. Among the NPs, steroid saponins, including polyphyllins (I, II, D, VI, and VII), have potent pharmacological, analgesic, and anticancer activities for the induction of cytotoxicity. Recent research has demonstrated that polyphyllins (PPs) possess potent effects against different cancers through apoptosis, autophagy, inflammation, and necroptosis. This review summarizes the available studies on PPs against cancer to provide a basis for future research.

Keywords: apoptosis; autophagy; inflammation; natural products; necroptosis; polyphyllins; saponins.

Figure 1

Molecular anticancer mechanisms of PPs. (A) In cancer cells, PPD, PPI, II, VI, and VII induce ROS generation, inhibit MMP, upregulate Bax, Bak, Bim, and tBid, and downregulate Bcl-2 and Bcl-xl, resulting in mitochondrial membrane permeability, allowing Cyt-c and AIF to enter the cytoplasm from the mitochondria. When Cyt-c and AIF accumulate in the cytoplasm, they cause the activation of caspase-3, caspase-9, and PARTP, leading to cell apoptosis. (B) In mitochondrial-independent pathway, PPII, VI, and VII upregulate FAS, DR3, and DR5 and downregulate DcR3, which further activate caspase-8, caspase-3, and PARO and cause cancer cell apoptosis. (C) In the STAT3 pathway, PPI and PPVII downregulate the Malat1 and IL-6 activation of STAT3 and cause apoptosis. (D) In the Wnt/β-catenin pathway, PPI inhibits Wnt5A, GSk-3B, and β-catenin and its translocation into the nucleus, leading to cell apoptosis.

Figure 2

Anticancer molecular mechanisms of PPs. (A) In the PI3K/AKT/mTOR pathway, PI3K, PI3Kc2b, Akt, and mTOR are downregulated while Bax and caspase-3,9 and upregulated, leading to cell apoptosis. (B) In the ER stress pathway, PPI upregulates UPR, p-ERK, Bip, elF2α, ATF-4, and CHOP. PPD upregulates Bip/GRP78, PDI, and CHOP. Once CHOP becomes activated, it enters the nucleus and regulates UPR target genes, resulting in apoptosis. (C) In the JNK signaling pathway, PPI and PPD activate JNK and c-jun expression and result in cell apoptosis, while JNK inhibitor (SP600125) reverses the apoptosis.

Figure 3

PPs induce cell cycle arrest in cancer cells. (A) PPI increases ROS generation and p21 expression while downregulating CB1, CG1 and c-myc expression, causing G2/M phase cell cycle arrest. (B) PPD, PPVI, and PPVII upregulate the expression of ATM, which increases p53, p21Waf1/Cip1, and p27 expression while activating chk1/2 and Cdc25C. The activated p27 and Cdc25C further inhibit cyclinB1 and CDK1 and cause G2/M phase cell cycle arrest. (C) PPI+FC upregulates the expression of p53, p21, and p27, which further inhibit CDK1, Cyclin E, CD1, and CDk4, resulting in G1 phase cell cycle arrest. PPI+FC also causes the G1 phase cell cycle arrest by inhibiting PCNA. (D) PPI, VI, and VII activate p53, p21, and p27, which further inhibit the expression of cmy-c, cyclin-B1, CDK1, Cyclin D1, Cyclin A, and CDK2 and lead to S phase cell cycle arrest.

Figure 4

PPs inhibit cancer cell proliferation and induce apoptosis in cancer cells through the NF-kB and MAPK pathways. (A) In the NF-kB pathway, PPI+FC or Cisplatin inhibit the IKBα, which further inhibit the NF-kB p65, on one side its translocation to nucleus while on other side they inhibit the HOTAIR, MUCIN1 which further inhibit the HOTAIR translocation into nucleus. Once they inhibit the p65 and HOTAIR translocation into nucleus, they further inhibit VEGE, MMP-9, MUCIN1, DNMT1 and EZH2 which lead to inhibition of cell proliferation. (B) In the MAPK pathway, PPD and PPVII inhibit AKT, JNK1/2, and ERK1/2 expression, which further inhibit caspase-8, Bcl-2, and Bcl-xl while upregulating Bak and Bax, resulting in mitochondrial membrane permeability, allowing Cyt-c to translocate from the mitochondria into the cytoplasm. PPD and PPVII activate p38MAPK, thereby increasing Cyt-c expression; as a result, caspase-9 also becomes activated. PPD and PPVII also activate p38 and caspase-9, respectively. Upon activation, caspase-9 further activates caspase-3 and PARP, triggering the apoptosis of cancer cells.

Figure 5

PPI induces protective autophagy through the PI3K/AKT/mTOR pathway by inhibiting AKT (S473), mTOR (S2448), p70S6K (T389), and 4EBP1 (T37/46) and increasing the conversion of LC3I conversion into LC3II. EGF markedly increases the phosphorylation of Akt and p70S6K and reverses LC3II, suggesting the autophagy is due to the PI3K/AKT/mTOR pathway. In the PI3K/AKT/mTOR pathway, PPVII inhibits PI3K Akt and mTOR activated mTOR (ser2448), total mTOR Raptor, Rictor and GβL in a dose-dependent manner. PPVII also increases AMPK phosphorylation, which further inhibits mTOR and induces autophagy in HepG-2 cells by modulating the AMPK/mTOR pathway. PPG or PPVII induces autophagy through the Akt, p38MAPK, ERK1/2, and JNK pathways. PPG or PPVII activates JNK1/2 and inhibits Akt, p38 MAPK, and ERK1/2, which further increase the conversion of LC3I to LC3II, P62 degradation, and formation of LC3-positive structures or LC3 puncta. Furthermore, PPVII treatment decreases the level of total Bcl-2 decreases but increases those of p-Bcl-2 and Beclin. The AKT inhibitor (LY294002) and JNK1/2 inhibitor (SP600125) increase, suggesting that PPG induces autophagy via the Akt, p38 MAPK, and JNK1/2 pathways. Moreover, LY294002, U0126, and SP600125 significantly attenuate PG-induced LC3-II activation, suggesting that the activation of ERK1/2 and JNK1/2 is involved in PG-induced autophagy.

Figure 6

PPs inhibit inflammation and induce necroptosis and mitophagy in cancer cells. (A) PPVII inhibits the inflammation by inhibiting the LPS-induced expression of TNF-α, IL-1B, and IL-6. PPVII also inhibits the inflammation through the NF-kB and MAPK pathways. In the NF-kB pathway, PPVII inhibit the IKB-α, p65 and its translocation to the nucleus which further inhibit iNOS, COX-2, and MMP-9 and inhibit inflammation. In the MAPK pathway, PPVII inhibits p38-MAPK, ERK1/2, and JNK1/2, resulting in inhibition of inflammation. (B) PPD induces necroptosis in cancer cells, whereas necrosulfonamide reverses this inhibition. (C) PPI induces the translocation of DRP1 to the mitochondria by dephosphorylating DRP1 at ser-637, causing mitochondrial fission. PPI increases the stabilization of full-length PINK1 at the surface of the mitochondria, leading to P62, PARK2, ubiquitin, and LC3B-II recruitment to the mitochondria and then to mitophagy. PPI-induced mitophagy is suppressed markedly by PINK1 knockdown. DRP1 suppression through shRNA or mdivi-1 inhibits the knockdown of PINK1 and PPI-induced mitochondrial fragmentation. These results suggest that PINK1 depletion leads to fission and mitochondrial fragmentation.