The PPARγ agonist Troglitazone induces autophagy, apoptosis and necroptosis in bladder cancer cells

Bladder cancer is a major public health problem worldwide, with relatively high morbidity. However, there are few studies on drug development for bladder cancer. Troglitazone (TZ) is a synthetic ligand of peroxisome proliferator-activated receptor-γ, and it can induce apoptosis and autophagy in a variety of cancer cells. Several studies have indicated that TZ affects both cell growth and differentiation progress and has an inhibitory effect on urinary cancer cells. However, this drug's effect on bladder cancer cells remains largely unknown. Here, we report that TZ induced autophagy and enhanced apoptosis in T24 cells. Autophagic blockage resulted in the attenuation of TZ-dependent apoptosis. Necrostatin-1, an inhibitor of necroptosis, was found to reduce light chain 3 (LC3)-II accumulation and partially rescue the loss of cell viability induced by TZ. It was demonstrated that TZ activated autophagy concurrent with the activation of the adenosine monophosphate-dependent protein kinase (AMPK) signaling pathway. AMPK inhibition led to a reduction in LC3-II levels, which were responsive to TZ treatment. Overall, TZ induced multiple types of programmed cell death in bladder cancer cells, and while the autophagy induced by the agonist contributed to the apoptotic process, crosstalk or switching between the different types of cell death likely occurred.

PFKFB3 Inhibition Attenuates Oxaliplatin-Induced Autophagy and Enhances Its Cytotoxicity in Colon Cancer Cells

6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase isoform 3 (PFKFB3), a glycolytic enzyme highly expressed in cancer cells, has been reported to participate in regulating metabolism, angiogenesis, and autophagy. Although anti-cancer drug oxaliplatin (Oxa) effectively inhibits cell proliferation and induces apoptosis, the growing resistance and side-effects make it urgent to improve the therapeutic strategy of Oxa. Although Oxa induces the autophagy process, the role of PFKFB3 in this process remains unknown. In addition, whether PFKFB3 affects the cytotoxicity of Oxa has not been investigated. Here, we show that Oxa-inhibited cell proliferation and migration concomitant with the induction of apoptosis and autophagy in SW480 cells. Both inhibition of autophagy by small molecule inhibitors and siRNA modification decreased the cell viability loss and apoptosis induced by Oxa. Utilizing quantitative PCR and immunoblotting, we observed that Oxa increased PFKFB3 expression in a time- and dose-dependent manner. Meanwhile, suppression of PFKFB3 attenuated both the basal and Oxa-induced autophagy, by monitoring the autophagic flux and phosphorylated-Ulk1, which play essential roles in autophagy initiation. Moreover, PFKFB3 inhibition further inhibited the cell proliferation/migration, and cell viability decreased by Oxa. Collectively, the presented data demonstrated that PFKFB3 inhibition attenuated Oxa-induced autophagy and enhanced its cytotoxicity in colorectal cancer cells.

Sunitinib induces genomic instability of renal carcinoma cells through affecting the interaction of LC3-II and PARP-1

Deficiency of autophagy has been linked to increase in nuclear instability, but the role of autophagy in regulating the formation and elimination of micronuclei, a diagnostic marker for genomic instability, is limited in mammalian cells. Utilizing immunostaining and subcellular fractionation, we found that either LC3-II or the phosphorylated Ulk1 localized in nuclei, and immunoprecipitation results showed that both LC3 and Unc-51-like kinase 1 (Ulk1) interacted with γ-H2AX, a marker for the DNA double-strand breaks (DSB). Sunitinib, a multi-targeted receptor tyrosine kinase inhibitor, was found to enhance the autophagic flux concurring with increase in the frequency of micronuclei accrued upon inhibition of autophagy, and similar results were also obtained in the rasfonin-treated cells. Moreover, the punctate LC3 staining colocalized with micronuclei. Unexpectedly, deprivation of SQSTM1/p62 alone accumulated micronuclei, which was not further increased upon challenge with ST. Rad51 is a protein central to repairing DSB by homologous recombination and treatment with ST or rasfonin decreased its expression. In several cell lines, p62 appeared in the immunoprecipites of Rad51, whereas LC3, Ulk1 and p62 interacted with PARP-1, another protein involved in DNA repair and genomic stability. In addition, knockdown of either Rad51 or PARP-1 completely inhibited the ST-induced autophagic flux. Taken together, the data presented here demonstrated that both LC3-II and the phosphorylated Ulk1 localized in nuclei and interacted with the proteins essential for nuclear stability, thereby revealing a more intimate relationship between autophagy and genomic stability.

6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase isoform 3 spatially mediates autophagy through the AMPK signaling pathway

6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase isoform 3 (PFKFB3), is a critical enzyme for glycolysis and highly expressed in cancer cells. It plays an essential role in regulating metabolism, angiogenesis, and inflammation. Although PFKFB3 is involved in modulating autophagy, its regulatory role appears to be either positive or negative, which remains to be clarified. Unlike other PFK-2/FBPase isoforms, PFKFB3 can localize in both nucleus and cytoplasm, leading to the speculation that subcellular localization of PFKFB3 may play a regulatory role in autophagy. Here, we found that either a PFKFB3 inhibitor or PFKFB3 silencing by siRNA, suppressed the basal and the H2O2-induced autophagy concomitantly with the inhibition of AMPK activity. While overexpression of the wild type PFKFB3 promoted the H2O2-induced autophagy, the K472/473A mutated PFKFB3, which lost nuclear localizing property, inhibited the autophagic process. Although the K472/473A mutated PFKFB3 stimulated more lactate production, it decreased the activity of AMPK compared to the wild type PFKFB3. Moreover, PFKFB3 similarly regulates the autophagy induced by rasfonin, a fungal secondary metabolite that downregulates the activity of AMPK. Compound C, a widely used AMPK inhibitor, induced the autophagic process but reduced the H2O2-dependent autophagy. Collectively, the data demonstrated that PFKFB3 localizing in nucleus is essential for its regulatory role in autophagy, and PFKFB3 at least positively regulated the H2O2-induced autophagy through the AMPK signaling pathway, which likely played dual roles in the process.

The role of PFKFB3 in maintaining colorectal cancer cell proliferation and stemness

Since generally confronting with the hypoxic and stressful microenvironment, cancer cells alter their glucose metabolism pattern to glycolysis to sustain the continuous proliferation and vigorous biological activities. Bifunctional 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) isoform 3 (PFKFB3) functions as an effectively modulator of glycolysis and also participates in regulating angiogenesis, cell death and cell stemness. Meanwhile, PFKFB3 is highly expressed in a variety of cancer cells, and can be activated by several regulatory factors, such as hypoxia, inflammation and cellular signals. In colorectal cancer (CRC) cells, PFKFB3 not only has the property of high expression, but also probably relate to inflammation-cancer transformation. Recent studies indicate that PFKFB3 is involved in chemoradiotherapy resistance as well, such as breast cancer, endometrial cancer and CRC. Cancer stem cells (CSCs) are self-renewable cell types that contribute to oncogenesis, metastasis and relapse. Several studies indicate that CSCs utilize glycolysis to fulfill their energetic and biosynthetic demands in order to maintain rapid proliferation and adapt to the tumor microenvironment changes. In addition, elevated PFKFB3 has been reported to correlate with self-renewal and metastatic outgrowth in numerous kinds of CSCs. This review summarizes our current understanding of PFKFB3 roles in modulating cancer metabolism to maintain cell proliferation and stemness, and discusses its feasibility as a potential target for the discovery of antineoplastic agents, especially in CRC.

AICAR enhances the cytotoxicity of PFKFB3 inhibitor in an AMPK signaling-independent manner in colorectal cancer cells

Numerous studies have shown that 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase isoform 3 (PFKFB3), a pivotal enzyme in modulating glycolysis, plays vital roles in various physiological processes. PFKFB3 activity could be regulated by several factors, such as hypoxia and AMPK signaling; however, it could also function as upstream of AMPK signaling. Here, we showed that PFKFB3 inhibitor PFK-15 induced cell viability loss and apoptosis. Deprivation of PFKFB3 inhibited autophagy, while enhanced the ubiquitin-proteasome degradation pathway. Furthermore, PFK-15 reduced both the AMPK and AKT-mTORC1 signaling pathways, as the attenuated phosphorylation level of kinases themselves and their substrates. The addition of AICAR rescued the AMPK activity and autophagy, but enhanced PFK-15-induced cell viability loss. In fact, AICAR promoted the cytotoxicity of PFK-15 even in the AMPKα1/2-silenced cells, indicating AICAR might function in an AMPK-independent manner. Nevertheless, AICAR further reduced the AKT-mTORC1 activity down-regulated by PFK-15. Moreover, it failed to enhance PFK-15's cytotoxicity in the AKT1/2-silenced cells, indicating AKT-mTORC1 participated during these processes. Collectively, the presented data demonstrated that PFK-15 inhibited cell viability, AMPK, and AKT-mTORC1 signaling, and AICAR probably enhanced the cell viability loss aroused by PFK-15 in an AKT-dependent and AMPK-independent manner, thereby revealing a more intimate relationship among PFKFB3, AMPK, and AKT-mTORC1 signaling pathways.

Proteasome inhibition attenuates rasfonin-induced autophagy concurring with the upregulation of caspase-dependent apoptosis

Two major protein quality control mechanisms exist in eukaryotic cells, the ubiquitin-proteasome system (UPS) and the autophagy-lysosome system. Generally, the inhibition of UPS is believed to enhance autophagic pathway; nevertheless, the crosstalk between these two degradation systems may be much more complicated. Rasfonin, a 2-pyrone derivative of fungal secondary metabolites, is demonstrated to have the antitumor effect and can function as an autophagy inducer. Here, we reported that rasfonin activated multiple cell death pathways, including caspase-dependent apoptosis. Using electroscopy and microscopy, we observed rasfonin increased the formation of autophagosome. In immunoblotting assay, rasfonin enhanced autophagic flux concomitant with the upregulation of ubiquitination. MG132, an inhibitor of proteasome, attenuated rasfonin-dependent autophagy, whereas its presentation stimulated rasfonin-induced cleavage of poly (ADP-ribose) polymerase, a marker of caspase-dependent apoptosis. Together, we demonstrated that rasfonin induced the activation of both UPS and autophagic pathway, and the inhibition of UPS attenuated rasfonin-induced autophagy and enhanced the cytotoxicity of rasonin by upregulation of caspase-dependent apoptosis.

Necroptosis pathway blockage attenuates PFKFB3 inhibitor-induced cell viability loss and genome instability in colorectal cancer cells

Cancer cells prone to utilize aerobic glycolysis other than oxidative phosphorylation to sustain its continuous cell activity in the stress microenvironment. Meanwhile, cancer cells generally suffer from genome instability, and both radiotherapy and chemotherapy may arouse DNA strand break, a common phenotype of genome instability. Glycolytic enzyme PFKFB3 (6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase isoform 3), plays essential roles in variety physiology and pathology processes, and generally maintain high level in cancer cells. Although this protein has been reported to involve in genome instability, its role remains unclear and controversial. Here, we showed that PFK-15, a PFKFB3 inhibitor, obviously induced apoptosis, cell viability loss, and inhibited cell proliferation/migration. Besides, PFK-15 was also found to induce necroptosis, as it not only up-regulated the phosphorylated RIP1, RIP3 and MLKL, but also enhanced the interaction between RIP3 and RIP1/MLKL, all of which are characterization of necroptosis induction. Both genetically and pharmacologically deprivation of necroptosis attenuated the cytotoxic effect of PFK-15. Besides, PFK-15 increased the γ-H2AX level and micronuclei formation, markers for genome instability, and inhibition of necroptosis attenuated these phenotypes. Collectively, the presented data demonstrated that PFK-15 induced genome instability and necroptosis, and deprivation of necroptosis attenuated cytotoxicity and genotoxicity of PFK-15 in colorectal cancer cells, thereby revealing a more intimate relationship among PFKFB3, necroptosis and genome instability.