YY2-DRP1 Axis Regulates Mitochondrial Fission and Determines Cancer Stem Cell Asymmetric Division.

Cancer stem cells (CSCs) are associated with tumor progression, recurrence, and therapeutic resistance. To maintain their pool while promoting tumorigenesis, CSCs divide asymmetrically, producing a CSC and a highly proliferative, more differentiated transit-amplifying cell. Exhausting the CSC pool has been proposed as an effective antitumor strategy; however, the mechanism underlying CSC division remains poorly understood, thereby largely limiting its clinical application. Here, through cross-omics analysis, yin yang 2 (YY2) is identified as a novel negative regulator of CSC maintenance. It is shown that YY2 is downregulated in stem-like tumor spheres formed by hepatocarcinoma cells and in liver cancer, in which its expression is negatively correlated with disease progression and poor prognosis. Furthermore, it is revealed that YY2 overexpression suppressed liver CSC asymmetric division, leading to depletion of the CSC pool and decreased tumor-initiating capacity. Meanwhile, YY2 knock-out in stem-like tumor spheres caused enrichment in mitochondrial functions. Mechanistically, it is revealed that YY2 impaired mitochondrial fission, and consequently, liver CSC asymmetric division, by suppressing the transcription of dynamin-related protein 1. These results unravel a novel regulatory mechanism of mitochondrial dynamic-mediated CSCs asymmetric division and highlight the role of YY2 as a tumor suppressor and a therapeutic target in antitumor treatment.

YY2/PHGDH axis suppresses tumorigenesis by inhibiting tumor cell de novo serine biosynthesis.

Metabolic reprogramming is one of the key features of tumors facilitating their rapid proliferation and adaptation to harsh microenvironments. Yin Yang 2 (YY2) has recently been reported as a tumor suppressor downregulated in various types of tumors; however, the molecular mechanisms underlying its tumor-suppressive activity remain poorly understood. Furthermore, the involvement of YY2 in tumor cell metabolic reprogramming remains unclear. Herein, we aimed to elucidate the novel regulatory mechanism of YY2 in the suppression of tumorigenesis. Using transcriptomic analysis, we uncovered an unprecedented link between YY2 and tumor cell serine metabolism. YY2 alteration could negatively regulate the expression level of phosphoglycerate dehydrogenase (PHGDH), the first enzyme in the serine biosynthesis pathway, and consequently, tumor cell de novo serine biosynthesis. Mechanistically, we revealed that YY2 binds to the PHGDH promoter and suppresses its transcriptional activity. This, in turn, leads to decreased production of serine, nucleotides, and cellular reductants NADH and NADPH, which subsequently suppresses tumorigenic potential. These findings reveal a novel function of YY2 as a regulator of the serine metabolic pathway in tumor cells and provide new insights into its tumor suppressor activity. Furthermore, our findings suggest the potential of YY2 as a target for metabolic-based antitumor therapeutic strategies.

Homeostasis Imbalance of YY2 and YY1 Promotes Tumor Growth by Manipulating Ferroptosis.

Ferroptosis is a type of programmed cell death caused by disruption of redox homeostasis and is closely linked to amino acid metabolism. Yin Yang 2 (YY2) and its homolog Yin Yang 1 (YY1) are highly homologous, especially in their zinc-finger domains. Furthermore, they share a consensus DNA binding motif. Increasing evidences have demonstrated the tumor suppressive effect of YY2, in contrast with the oncogenic YY1; however, little is known about the biological and pathological functions of YY2. Here, it is determined that YY2 induces tumor cell ferroptosis and subsequently suppresses tumorigenesis by inhibiting solute carrier family 7 member 11 (SLC7A11) transcription, leading to the decreased glutathione biosynthesis. Furthermore, YY2 and YY1 bind competitively to the same DNA binding site in the SLC7A11 promoter and antagonistically regulate tumor cell ferroptosis, thus suggesting the molecular mechanism underlying their opposite regulation on tumorigenesis. Moreover, mutations of YY2 zinc-finger domains in clinical cancer patients abrogate YY2/SLC7A11 axis and tumor cell ferroptosis. Together, these results provide a new insight regarding the regulatory mechanism of ferroptosis, and a mechanistic explanation regarding the tumor suppressive effect of YY2. Finally, these findings demonstrate that homeostasis between YY1 and YY2 is crucial for maintaining redox homeostasis in tumor cells.

Transcription Factor YY1 Promotes Cell Proliferation by Directly Activating the Pentose Phosphate Pathway.

Tumor cells alter their metabolism to meet their demand for macromolecules and support a high rate of proliferation as well as cope with oxidative stress. The transcription factor yin yang 1 (YY1) is upregulated in various types of tumors and is crucial for tumor cell proliferation and metastasis. However, its role in tumor cell metabolic reprogramming is poorly understood. Here, we show that YY1 alters tumor cell metabolism by activating glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in the pentose phosphate pathway. By stimulating the pentose phosphate pathway, YY1 enhanced production of nucleotides and DNA synthesis, decreased intracellular reactive oxygen species levels, and promoted antioxidant defense by supplying increased reducing power in the form of NADPH. Importantly, YY1-mediated regulation of the pentose phosphate pathway in tumor cells occurred not through p53, but rather through direct activation of G6PD transcription by YY1. Regulation of pentose phosphate pathway activity through G6PD was strongly related to YY1-induced proliferation of tumor cells and tumorigenesis. Together, our results describe a novel role for YY1 in regulating G6PD in a p53-independent manner, which links its function in tumorigenesis to metabolic reprogramming in tumor cells.Significance: This study reveals a novel role for YY1 in regulating G6PD and activating the pentose phosphate pathway, linking its function in tumorigenesis to metabolic reprogramming. Cancer Res; 78(16); 4549-62. ©2018 AACR.

Transcription factor Yin Yang 2 is a novel regulator of the p53/p21 axis.

Yin Yang 2 (YY2) is a multifunctional zinc-finger transcription factor that belongs to YY family. Unlike the well-characterized YY1, our understanding regarding the biological functions of YY2 is still very limited. Here we found for the first time that in contrast to YY1, which had been reported to be oncogenic, the expression level of YY2 in tumor cells and/or tissues was downregulated compared with its expression level in the normal ones. We also demonstrated that YY2 exerts biological function contrary to YY1 in cell proliferation. We elucidated that YY2 positively enhances p21 expression, and concomitantly, its silencing promotes cells to enter G2/M phase and enhances cell proliferation. Furthermore, we found that YY2 regulation on p21 occurs p53-dependently. Finally, we identified a novel YY2 binding site in the promoter region of tumor suppressor p53. We found that YY2 binds to the p53 promoter and activates its transcriptional activity, and subsequently, regulates cell cycle progression via p53/p21 axis. Taken together, our study not only identifies YY2 as a novel tumor suppressor gene that plays a pivotal role in cell cycle regulation, but also provides new insights regarding the regulatory mechanism of the conventional p53/p21 axis.

Transcription factor YY1 contributes to tumor growth by stabilizing hypoxia factor HIF-1α in a p53-independent manner.

In response to hypoxic stress, hypoxia-inducible factor (HIF)-1α is a critical transcription factor regulating fundamental cellular processes, and its elevated expression level and activity are associated with poor outcomes in most malignancies. The transcription factor Yin Yang 1 (YY1) is an important negative regulator of the tumor suppressor factor p53. However, the role of YY1 under tumor hypoxic condition is poorly understood. Herein, we show that inhibition of YY1 reduced the accumulation of HIF-1α and its activity under hypoxic condition, and consequently downregulated the expression of HIF-1α target genes. Interestingly, our results revealed that the downregulation of HIF-1α by inhibiting YY1 is p53-independent. Functionally, the in vivo experiments revealed that inhibition of YY1 significantly suppressed growth of metastatic cancer cells and lung colonization and also attenuated angiogenesis in a p53-null tumor. Collectively, our findings unraveled a novel mechanism by which YY1 inhibition disrupts hypoxia-stimulated HIF-1α stabilization in a p53-independent manner. Therefore, YY1 inhibition could be considered as a potential tumor therapeutic strategy to give consistent clinical outcomes independent of p53 status.

Yin Yang 1 induces transcriptional activity of p73 through cooperation with E2F1.

The transcription factor p73 is a structural homologue of p53 and plays an important role in tumorigenesis, differentiation and development. However, the regulation of p73 pathway has not been wholly understood. Here we reported that YY1-silencing resulted in significant reductions in the activities of the p73 promoters and the endogenous p73 expression level, conversely, overexpression of YY1 could induce the activities of them. Furthermore, we showed that YY1 and E2F1 have synergistic effect on p73 promoter activity. The results of YY1-silencing and E2F1-silencing alone revealed that both factors are involved in the doxorubicin-induced activation of p73 promoter. Immunofluorescence staining and co-immunoprecipitation assays demonstrated that cooperation of YY1 and E2F1 is concomitant with physical interaction in nuclei. The results presented here suggested the cooperative transcriptional regulation of p73 by YY1 and E2F1, and might provide a new regulation mechanism by the YY1 network on tumorigenesis, differentiation and development.

Cooperative regulation of p73 promoter by Yin Yang 1 and E2F1.

The transcriptional factor Yin Yang 1 (YY1) plays multifunctional role in various biological processes. Here we report that the silencing of YY1 using siRNA expression vectors in U2OS cells led to a significant decrease in transcriptional activity of p73, which is a member of p53 family. Consistently, overexpression of YY1 could enhance the transcriptional activity of p73 in a dose-dependent manner. Moreover, synergistic cooperation between YY1 and E2F1, through a mechanism involving a physical interaction, was observed in the regulation of p73 promoter. In conclusion, our results provide new insights into the function of YY1, and might clarify the complex regulation mechanism by YY1 network in diverse biological processed, such as development, differentiation and cancer biology.

Cloning of adiponectin receptors that mediate antidiabetic metabolic effects.

Adiponectin (also known as 30-kDa adipocyte complement-related protein; Acrp30) is a hormone secreted by adipocytes that acts as an antidiabetic and anti-atherogenic adipokine. Levels of adiponectin in the blood are decreased under conditions of obesity, insulin resistance and type 2 diabetes. Administration of adiponectin causes glucose-lowering effects and ameliorates insulin resistance in mice. Conversely, adiponectin-deficient mice exhibit insulin resistance and diabetes. This insulin-sensitizing effect of adiponectin seems to be mediated by an increase in fatty-acid oxidation through activation of AMP kinase and PPAR-alpha. Here we report the cloning of complementary DNAs encoding adiponectin receptors 1 and 2 (AdipoR1 and AdipoR2) by expression cloning. AdipoR1 is abundantly expressed in skeletal muscle, whereas AdipoR2 is predominantly expressed in the liver. These two adiponectin receptors are predicted to contain seven transmembrane domains, but to be structurally and functionally distinct from G-protein-coupled receptors. Expression of AdipoR1/R2 or suppression of AdipoR1/R2 expression by small-interfering RNA supports our conclusion that they serve as receptors for globular and full-length adiponectin, and that they mediate increased AMP kinase and PPAR-alpha ligand activities, as well as fatty-acid oxidation and glucose uptake by adiponectin.