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Maf1 is a novel target of PTEN and PI3K signaling that negatively regulates oncogenesis and lipid metabolism.

Palian BM, Rohira AD, Johnson SA, He L, Zheng N, Dubeau L, Stiles BL, Johnson DL - PLoS Genet. (2014)

Bottom Line: PTEN-mediated changes in Maf1 expression are mediated by PTEN acting on PI3K/AKT/FoxO1 signaling, revealing a new pathway that regulates RNA pol III-dependent genes.We further identify lipogenic enzymes as a new class of Maf1-regulated genes whereby Maf1 occupancy at the FASN promoter opposes SREBP1c-mediated transcription activation.Together, these results establish a new biological role for Maf1 as a downstream effector of PTEN/PI3K signaling and reveal that Maf1 is a key element by which this pathway co-regulates lipid metabolism and oncogenesis.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology, Keck School of Medicine, University of Southern California, and the Norris Comprehensive Cancer Center, Los Angeles, California, United States of America.

ABSTRACT
Maf1 was initially identified as a transcriptional repressor of RNA pol III-transcribed genes, yet little is known about its other potential target genes or its biological function. Here, we show that Maf1 is a key downstream target of PTEN that drives both its tumor suppressor and metabolic functions. Maf1 expression is diminished with loss of PTEN in both mouse models and human cancers. Consistent with its role as a tumor suppressor, Maf1 reduces anchorage-independent growth and tumor formation in mice. PTEN-mediated changes in Maf1 expression are mediated by PTEN acting on PI3K/AKT/FoxO1 signaling, revealing a new pathway that regulates RNA pol III-dependent genes. This regulatory event is biologically relevant as diet-induced PI3K activation reduces Maf1 expression in mouse liver. We further identify lipogenic enzymes as a new class of Maf1-regulated genes whereby Maf1 occupancy at the FASN promoter opposes SREBP1c-mediated transcription activation. Consistent with these findings, Maf1 inhibits intracellular lipid accumulation and increasing Maf1 expression in mouse liver abrogates diet-mediated induction of lipogenic enzymes and triglycerides. Together, these results establish a new biological role for Maf1 as a downstream effector of PTEN/PI3K signaling and reveal that Maf1 is a key element by which this pathway co-regulates lipid metabolism and oncogenesis.

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Maf1 controls intracellular lipid accumulation and de novo lipogenesis.(A) Maf1 represses intracellular lipid accumulation. Huh-7 cells infected with nsRNA (left) or Maf1 shRNA (right) were stained with Oil Red O and Mayer's Hematoxylin. Intracellular lipid droplets were detected as red spheres and nuclei are shown in purple. Magnification 40x. (B) Maf1 overexpression inhibits diet-induced lipogenic gene expression. Male C57BL mice injected with adenovirus overexpressing either Maf1-HA or LacZ were fed a high carbohydrate diet for 2 or 4 days. RNA was isolated from livers and qRT-PCR was performed with primers specific for FASN and ACC1. Fold changes are statistically significant: Student t-test, FASN; 2 day p = 0.0067, 4 day p = 0.0028; ACC1; 2 day p = 0.006, 4 day p = 0.021. (C) Protein lysates were isolated from the mouse livers fed a high carbohydrate diet for 4 days and were subjected to immunoblot analysis using antibodies against FASN, LacZ, HA (ectopic Maf1-HA), or vinculin. Representative samples shown. (D) Maf1 overexpression reduces triglyceride accumulation. Triglycerides were isolated from the livers of mice that were fed a high carbohydrate diet for 4 days and quantified. Values represent means ±S.E. (n = 4). Fold changes are statistically significant with p<0.018.
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pgen-1004789-g006: Maf1 controls intracellular lipid accumulation and de novo lipogenesis.(A) Maf1 represses intracellular lipid accumulation. Huh-7 cells infected with nsRNA (left) or Maf1 shRNA (right) were stained with Oil Red O and Mayer's Hematoxylin. Intracellular lipid droplets were detected as red spheres and nuclei are shown in purple. Magnification 40x. (B) Maf1 overexpression inhibits diet-induced lipogenic gene expression. Male C57BL mice injected with adenovirus overexpressing either Maf1-HA or LacZ were fed a high carbohydrate diet for 2 or 4 days. RNA was isolated from livers and qRT-PCR was performed with primers specific for FASN and ACC1. Fold changes are statistically significant: Student t-test, FASN; 2 day p = 0.0067, 4 day p = 0.0028; ACC1; 2 day p = 0.006, 4 day p = 0.021. (C) Protein lysates were isolated from the mouse livers fed a high carbohydrate diet for 4 days and were subjected to immunoblot analysis using antibodies against FASN, LacZ, HA (ectopic Maf1-HA), or vinculin. Representative samples shown. (D) Maf1 overexpression reduces triglyceride accumulation. Triglycerides were isolated from the livers of mice that were fed a high carbohydrate diet for 4 days and quantified. Values represent means ±S.E. (n = 4). Fold changes are statistically significant with p<0.018.

Mentions: Since the amount of lipogenic enzyme activity controls de novo lipogenesis and intracellular lipid accumulation, and FASN expression is limiting for lipogenesis, we further examined the biological consequence of Maf1-mediated repression on lipid biogenesis. As decreased Maf1 expression in Huh-7 cells resulted in an increase in both FASN and ACC1 protein expression (Fig. 5A), we asked whether Maf1-mediated changes in these lipogenic enzymes would affect intracellular lipid accumulation. Compared with control cells expressing non-silencing RNA, cells expressing Maf1 shRNA displayed an increase in the number of visible lipid droplets (Fig. 6A). These results support the idea that Maf1 negatively regulates intracellular lipid accumulation, at least in part, through its ability to directly repress the transcription of lipogenic enzymes.


Maf1 is a novel target of PTEN and PI3K signaling that negatively regulates oncogenesis and lipid metabolism.

Palian BM, Rohira AD, Johnson SA, He L, Zheng N, Dubeau L, Stiles BL, Johnson DL - PLoS Genet. (2014)

Maf1 controls intracellular lipid accumulation and de novo lipogenesis.(A) Maf1 represses intracellular lipid accumulation. Huh-7 cells infected with nsRNA (left) or Maf1 shRNA (right) were stained with Oil Red O and Mayer's Hematoxylin. Intracellular lipid droplets were detected as red spheres and nuclei are shown in purple. Magnification 40x. (B) Maf1 overexpression inhibits diet-induced lipogenic gene expression. Male C57BL mice injected with adenovirus overexpressing either Maf1-HA or LacZ were fed a high carbohydrate diet for 2 or 4 days. RNA was isolated from livers and qRT-PCR was performed with primers specific for FASN and ACC1. Fold changes are statistically significant: Student t-test, FASN; 2 day p = 0.0067, 4 day p = 0.0028; ACC1; 2 day p = 0.006, 4 day p = 0.021. (C) Protein lysates were isolated from the mouse livers fed a high carbohydrate diet for 4 days and were subjected to immunoblot analysis using antibodies against FASN, LacZ, HA (ectopic Maf1-HA), or vinculin. Representative samples shown. (D) Maf1 overexpression reduces triglyceride accumulation. Triglycerides were isolated from the livers of mice that were fed a high carbohydrate diet for 4 days and quantified. Values represent means ±S.E. (n = 4). Fold changes are statistically significant with p<0.018.
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pgen-1004789-g006: Maf1 controls intracellular lipid accumulation and de novo lipogenesis.(A) Maf1 represses intracellular lipid accumulation. Huh-7 cells infected with nsRNA (left) or Maf1 shRNA (right) were stained with Oil Red O and Mayer's Hematoxylin. Intracellular lipid droplets were detected as red spheres and nuclei are shown in purple. Magnification 40x. (B) Maf1 overexpression inhibits diet-induced lipogenic gene expression. Male C57BL mice injected with adenovirus overexpressing either Maf1-HA or LacZ were fed a high carbohydrate diet for 2 or 4 days. RNA was isolated from livers and qRT-PCR was performed with primers specific for FASN and ACC1. Fold changes are statistically significant: Student t-test, FASN; 2 day p = 0.0067, 4 day p = 0.0028; ACC1; 2 day p = 0.006, 4 day p = 0.021. (C) Protein lysates were isolated from the mouse livers fed a high carbohydrate diet for 4 days and were subjected to immunoblot analysis using antibodies against FASN, LacZ, HA (ectopic Maf1-HA), or vinculin. Representative samples shown. (D) Maf1 overexpression reduces triglyceride accumulation. Triglycerides were isolated from the livers of mice that were fed a high carbohydrate diet for 4 days and quantified. Values represent means ±S.E. (n = 4). Fold changes are statistically significant with p<0.018.
Mentions: Since the amount of lipogenic enzyme activity controls de novo lipogenesis and intracellular lipid accumulation, and FASN expression is limiting for lipogenesis, we further examined the biological consequence of Maf1-mediated repression on lipid biogenesis. As decreased Maf1 expression in Huh-7 cells resulted in an increase in both FASN and ACC1 protein expression (Fig. 5A), we asked whether Maf1-mediated changes in these lipogenic enzymes would affect intracellular lipid accumulation. Compared with control cells expressing non-silencing RNA, cells expressing Maf1 shRNA displayed an increase in the number of visible lipid droplets (Fig. 6A). These results support the idea that Maf1 negatively regulates intracellular lipid accumulation, at least in part, through its ability to directly repress the transcription of lipogenic enzymes.

Bottom Line: PTEN-mediated changes in Maf1 expression are mediated by PTEN acting on PI3K/AKT/FoxO1 signaling, revealing a new pathway that regulates RNA pol III-dependent genes.We further identify lipogenic enzymes as a new class of Maf1-regulated genes whereby Maf1 occupancy at the FASN promoter opposes SREBP1c-mediated transcription activation.Together, these results establish a new biological role for Maf1 as a downstream effector of PTEN/PI3K signaling and reveal that Maf1 is a key element by which this pathway co-regulates lipid metabolism and oncogenesis.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology, Keck School of Medicine, University of Southern California, and the Norris Comprehensive Cancer Center, Los Angeles, California, United States of America.

ABSTRACT
Maf1 was initially identified as a transcriptional repressor of RNA pol III-transcribed genes, yet little is known about its other potential target genes or its biological function. Here, we show that Maf1 is a key downstream target of PTEN that drives both its tumor suppressor and metabolic functions. Maf1 expression is diminished with loss of PTEN in both mouse models and human cancers. Consistent with its role as a tumor suppressor, Maf1 reduces anchorage-independent growth and tumor formation in mice. PTEN-mediated changes in Maf1 expression are mediated by PTEN acting on PI3K/AKT/FoxO1 signaling, revealing a new pathway that regulates RNA pol III-dependent genes. This regulatory event is biologically relevant as diet-induced PI3K activation reduces Maf1 expression in mouse liver. We further identify lipogenic enzymes as a new class of Maf1-regulated genes whereby Maf1 occupancy at the FASN promoter opposes SREBP1c-mediated transcription activation. Consistent with these findings, Maf1 inhibits intracellular lipid accumulation and increasing Maf1 expression in mouse liver abrogates diet-mediated induction of lipogenic enzymes and triglycerides. Together, these results establish a new biological role for Maf1 as a downstream effector of PTEN/PI3K signaling and reveal that Maf1 is a key element by which this pathway co-regulates lipid metabolism and oncogenesis.

Show MeSH
Related in: MedlinePlus