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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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Increased Maf1 expression suppresses cellular transformation and tumorigenesis, consistent with its diminished expression in human cancer tissues.Nuclear Maf1 expression is decreased in PTEN negative human prostate and liver cancers. Immunohistochemistry of frozen human liver tissue (A) and paraffin-embedded human prostate tissue (B) with Maf1 or PTEN antibodies. Photomicrographs show representative staining of cancerous tissue (right) and adjacent normal tissue (left). Insets represent enlargements of areas highlighted. Scale bars represent 50 µm. (C) Increased Maf1 expression results in repression of RNA polymerase III-dependent transcription. Immunoblot analysis of protein lysates from Huh7 stable cell lines with vector or human Maf1-HA expression plasmid using HA (ectopic Maf1-HA), Maf1, or β-actin antibodies (left). qRT-PCR was performed using RNA isolated from stable cell lines with primers for pre-tRNALeu, pre-tRNAiMet, 7SL RNA, using GAPDH as an internal control (right). Values are the mean ±S.E. (n≥3). Fold changes in transcripts were statistically different from vector controls (Student t-test, pre-tRNALeu and pre-tRNAiMet, p = 0.0001, 7SL RNA, p = 0.0025). (D) Effect of increased Maf1 expression on Huh7 cell doubling time. Stable cell lines described in “C” grown on duplicate plates were trypsinized and counted daily. Values are the mean ±S.E. (n≥3), p = 0.0001. (E) Increased Maf1 expression represses anchorage-independent growth. Stable cell lines were analyzed for growth in soft agar. Colonies>100 uM were counted. Values are the means ±S.E. (n≥3), p = 0.005. (F) Tumor growth rate is repressed and visible tumor formation is delayed in mice with cells expressing increased Maf1. Two independent stable cell lines expressing Maf1 were injected subcutaneously into the groins of nude mice (10 mice per group). Calculated tumor growth rates shown (left). The day of first visible tumors noted are shown in the table (right). Values shown are the means ±S.E., p = 0.02.
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pgen-1004789-g002: Increased Maf1 expression suppresses cellular transformation and tumorigenesis, consistent with its diminished expression in human cancer tissues.Nuclear Maf1 expression is decreased in PTEN negative human prostate and liver cancers. Immunohistochemistry of frozen human liver tissue (A) and paraffin-embedded human prostate tissue (B) with Maf1 or PTEN antibodies. Photomicrographs show representative staining of cancerous tissue (right) and adjacent normal tissue (left). Insets represent enlargements of areas highlighted. Scale bars represent 50 µm. (C) Increased Maf1 expression results in repression of RNA polymerase III-dependent transcription. Immunoblot analysis of protein lysates from Huh7 stable cell lines with vector or human Maf1-HA expression plasmid using HA (ectopic Maf1-HA), Maf1, or β-actin antibodies (left). qRT-PCR was performed using RNA isolated from stable cell lines with primers for pre-tRNALeu, pre-tRNAiMet, 7SL RNA, using GAPDH as an internal control (right). Values are the mean ±S.E. (n≥3). Fold changes in transcripts were statistically different from vector controls (Student t-test, pre-tRNALeu and pre-tRNAiMet, p = 0.0001, 7SL RNA, p = 0.0025). (D) Effect of increased Maf1 expression on Huh7 cell doubling time. Stable cell lines described in “C” grown on duplicate plates were trypsinized and counted daily. Values are the mean ±S.E. (n≥3), p = 0.0001. (E) Increased Maf1 expression represses anchorage-independent growth. Stable cell lines were analyzed for growth in soft agar. Colonies>100 uM were counted. Values are the means ±S.E. (n≥3), p = 0.005. (F) Tumor growth rate is repressed and visible tumor formation is delayed in mice with cells expressing increased Maf1. Two independent stable cell lines expressing Maf1 were injected subcutaneously into the groins of nude mice (10 mice per group). Calculated tumor growth rates shown (left). The day of first visible tumors noted are shown in the table (right). Values shown are the means ±S.E., p = 0.02.

Mentions: Given that liver and prostate tissues from Pten conditional mutant mice exhibit reductions in Maf1 expression, we tested whether Maf1 expression might be similarly deregulated in PTEN-deficient human cancers. Three matched normal tissues and hepatocellular carcinomas that were PTEN negative were examined by immunostaining for potential changes in Maf1 expression. These tissues were also hepatitis B and C negative. A representative case is shown (Fig. 2A). Maf1 was predominantly localized in the nuclei of normal liver tissue. In contrast, matched tumor tissue displayed a marked reduction in nuclear Maf1 expression. We further examined whether Maf1 expression might be deregulated in human prostate cancer where PTEN is frequently lost. Tissue specimens from four human prostate cancer cases that did not express PTEN were examined by immunostaining. A representative case is shown (Fig. 2B). Compared with the predominant nuclear staining of Maf1 in normal prostate epithelium, the adjacent cancerous tissue from the same individual exhibited a substantial decrease in Maf1 expression, correlated with loss of PTEN in the malignant tissue. Together, these results are the first to suggest the idea that Maf1 expression may be deregulated in human cancer. Given that a reduction in Maf1 expression is similarly observed in Pten- mice prostate and liver (Fig. 1A), this suggests that the loss of PTEN drives the alterations in Maf1 expression.


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)

Increased Maf1 expression suppresses cellular transformation and tumorigenesis, consistent with its diminished expression in human cancer tissues.Nuclear Maf1 expression is decreased in PTEN negative human prostate and liver cancers. Immunohistochemistry of frozen human liver tissue (A) and paraffin-embedded human prostate tissue (B) with Maf1 or PTEN antibodies. Photomicrographs show representative staining of cancerous tissue (right) and adjacent normal tissue (left). Insets represent enlargements of areas highlighted. Scale bars represent 50 µm. (C) Increased Maf1 expression results in repression of RNA polymerase III-dependent transcription. Immunoblot analysis of protein lysates from Huh7 stable cell lines with vector or human Maf1-HA expression plasmid using HA (ectopic Maf1-HA), Maf1, or β-actin antibodies (left). qRT-PCR was performed using RNA isolated from stable cell lines with primers for pre-tRNALeu, pre-tRNAiMet, 7SL RNA, using GAPDH as an internal control (right). Values are the mean ±S.E. (n≥3). Fold changes in transcripts were statistically different from vector controls (Student t-test, pre-tRNALeu and pre-tRNAiMet, p = 0.0001, 7SL RNA, p = 0.0025). (D) Effect of increased Maf1 expression on Huh7 cell doubling time. Stable cell lines described in “C” grown on duplicate plates were trypsinized and counted daily. Values are the mean ±S.E. (n≥3), p = 0.0001. (E) Increased Maf1 expression represses anchorage-independent growth. Stable cell lines were analyzed for growth in soft agar. Colonies>100 uM were counted. Values are the means ±S.E. (n≥3), p = 0.005. (F) Tumor growth rate is repressed and visible tumor formation is delayed in mice with cells expressing increased Maf1. Two independent stable cell lines expressing Maf1 were injected subcutaneously into the groins of nude mice (10 mice per group). Calculated tumor growth rates shown (left). The day of first visible tumors noted are shown in the table (right). Values shown are the means ±S.E., p = 0.02.
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pgen-1004789-g002: Increased Maf1 expression suppresses cellular transformation and tumorigenesis, consistent with its diminished expression in human cancer tissues.Nuclear Maf1 expression is decreased in PTEN negative human prostate and liver cancers. Immunohistochemistry of frozen human liver tissue (A) and paraffin-embedded human prostate tissue (B) with Maf1 or PTEN antibodies. Photomicrographs show representative staining of cancerous tissue (right) and adjacent normal tissue (left). Insets represent enlargements of areas highlighted. Scale bars represent 50 µm. (C) Increased Maf1 expression results in repression of RNA polymerase III-dependent transcription. Immunoblot analysis of protein lysates from Huh7 stable cell lines with vector or human Maf1-HA expression plasmid using HA (ectopic Maf1-HA), Maf1, or β-actin antibodies (left). qRT-PCR was performed using RNA isolated from stable cell lines with primers for pre-tRNALeu, pre-tRNAiMet, 7SL RNA, using GAPDH as an internal control (right). Values are the mean ±S.E. (n≥3). Fold changes in transcripts were statistically different from vector controls (Student t-test, pre-tRNALeu and pre-tRNAiMet, p = 0.0001, 7SL RNA, p = 0.0025). (D) Effect of increased Maf1 expression on Huh7 cell doubling time. Stable cell lines described in “C” grown on duplicate plates were trypsinized and counted daily. Values are the mean ±S.E. (n≥3), p = 0.0001. (E) Increased Maf1 expression represses anchorage-independent growth. Stable cell lines were analyzed for growth in soft agar. Colonies>100 uM were counted. Values are the means ±S.E. (n≥3), p = 0.005. (F) Tumor growth rate is repressed and visible tumor formation is delayed in mice with cells expressing increased Maf1. Two independent stable cell lines expressing Maf1 were injected subcutaneously into the groins of nude mice (10 mice per group). Calculated tumor growth rates shown (left). The day of first visible tumors noted are shown in the table (right). Values shown are the means ±S.E., p = 0.02.
Mentions: Given that liver and prostate tissues from Pten conditional mutant mice exhibit reductions in Maf1 expression, we tested whether Maf1 expression might be similarly deregulated in PTEN-deficient human cancers. Three matched normal tissues and hepatocellular carcinomas that were PTEN negative were examined by immunostaining for potential changes in Maf1 expression. These tissues were also hepatitis B and C negative. A representative case is shown (Fig. 2A). Maf1 was predominantly localized in the nuclei of normal liver tissue. In contrast, matched tumor tissue displayed a marked reduction in nuclear Maf1 expression. We further examined whether Maf1 expression might be deregulated in human prostate cancer where PTEN is frequently lost. Tissue specimens from four human prostate cancer cases that did not express PTEN were examined by immunostaining. A representative case is shown (Fig. 2B). Compared with the predominant nuclear staining of Maf1 in normal prostate epithelium, the adjacent cancerous tissue from the same individual exhibited a substantial decrease in Maf1 expression, correlated with loss of PTEN in the malignant tissue. Together, these results are the first to suggest the idea that Maf1 expression may be deregulated in human cancer. Given that a reduction in Maf1 expression is similarly observed in Pten- mice prostate and liver (Fig. 1A), this suggests that the loss of PTEN drives the alterations in Maf1 expression.

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