Functional studies by site-directed mutagenesis on the role of Sp1 in the expression of the pyruvate kinase M and aldolase A genes

Role of sp transcription factors in the regulation of cancer cell metabolism

Cancer cells exhibit altered metabolism characterized by the generation of adenosine triphosphate by glycolysis and generation of fatty acids by de novo synthesis. The majority of genes involved in these pathways have binding sites for specificity protein (Sp) transcription factors in their promoters. Studies showing that Sp transcription factors, particularly Sp1, are involved in the regulation in cancer cells of hexokinase, pyruvate kinase, lactate dehydrogenase, fatty acid synthase, and hypoxia-inducible factor-1α are reviewed. Glycolysis and lipogenesis in cancers are also known to be stimulated by the constitutive activation of the PI3K/Akt signaling pathway. Evidence is presented for the notion that Sp transcription factors may act in concert with Akt to regulate the abnormal metabolism of cancer cells.

Sodium butyrate induces transcription from the G alpha(i2) gene promoter through multiple Sp1 sites in the promoter and by activating the MEK-ERK signal transduction pathway

Sodium butyrate, an erythroid differentiation inducer and a histone deacetylase inhibitor, increases G alpha(i2) levels in differentiating K562 cells. Here we show that sodium butyrate induces G alpha(i2) gene transcription via sequences at -50/-36 and -92/-85 in the G alpha(i2) gene promoter. Both sequences contain core sequence motif for Sp1 binding; electrophoretic mobility shift as well as supershift assays confirmed binding to Sp1. Transcription from the G alpha(i2) gene promoter was also activated by two other histone deacetylase inhibitors, trichostatin A and Helminthsporium carbonium toxin (HC toxin), which also induce erythroblastic differentiation in K562 cells. However, hydroxyurea, a potent erythroid differentiation inducer in these cells, did not activate transcription from this gene promoter, indicating that promoter activation is inducer-specific. Mutations within the Sp1 sites at -50/-36 and -92/-85 in the G alpha(i2) gene promoter substantially decreased transcriptional activation by sodium butyrate, trichostatin A, or HC toxin. Transfection with constitutively activated ERKs indicated that this promoter can be activated through the MEK-ERK signal transduction pathway. Inhibition of the MEK-ERK pathway with U0126 or reduction in the expression of endogenous ERK with an antisense oligonucleotide to ERK significantly inhibited sodium butyrate- and HC toxin-induced transcription but had no effect on trichostatin A-induced transcription. Inhibition of the JNK and p38 MAPKs, using selective inhibitors, had no effect on sodium butyrate-induced transcription. In cells in which sodium butyrate induction of promoter activation had been inhibited by various concentrations of U0126, constitutively activated ERK2 reversed this inhibition. These results show that the MEK-ERK signal transduction pathway is important in butyrate signaling, which eventually converges in the cell nucleus.

Increased formation of reactive oxygen species due to glucose depletion in primary cultures of rat thymocytes inhibits proliferation

Glycolytic glucose degradation to lactate is the only way of energy production in mammalian cells that does not use oxygen, and is thus a means of reducing oxidative stress. The aim of this work was to study the effect on proliferation of progressive glucose depletion during the cell cycle of stimulated cultured rat thymocytes. It has been shown that under conditions of limited glucose supply, these cells increase the generation of reactive oxygen species. Because of the sensitivity of the stimulating transcription factor Sp1 to redox changes with respect to its DNA binding, gene expression of glycolytic enzymes is markedly suppressed. As a consequence, cell division does not occur in glucose depleted thymocytes.

Sp family members and nuclear factor-Y cooperatively stimulate transcription from the rat pyruvate kinase M gene distal promoter region via their direct interactions

The three distal transcriptional regulatory elements of the rat pyruvate kinase M gene, referred to as boxes A, B, and C, are located around -270 base pairs upstream from the transcriptional initiation site. Electrophoretic mobility shift assays with specific competitors and antibodies show that both box A and box B bind to Sp1 and Sp3 and that box C binds nuclear factor-Y (NF-Y). Luciferase reporter assays revealed that although box A and box B alone have no independent effect on luciferase activities, box C alone stimulates transcription. However, the inclusion of all three elements lead to maximal activity because of a synergistic effect, mainly between box B and box C, suggesting that functional synergism between Sp1/Sp3 and NF-Y is critical for the pyruvate kinase M (PKM) gene distal promoter activity. In fact, co-transfection of a dominant negative mutant of NF-YA (NF-YA29) resulted in a decrease in reporter activity in a box C-dependent manner. In addition, the overexpression of Sp1 or Sp3 and NF-Y in Drosophila SL2 cells synergistically stimulated PKM gene distal promoter activity. Using a mammalian two-hybrid system in HeLa cells, it was shown that both Sp1 and Sp3 interacted with NF-YA but not NF-YB and NF-YC. Moreover, glutathione S-transferase pull-down assays revealed that only in vitro translated (35)S-labeled NF-YA interacted with both Sp1 and Sp3 in vitro. A subunit interaction domain of NF-YA, which forms a heterotrimer with NF-YB and NF-YC, is not required for these interactions with Sp1 or Sp3. Thus, we conclude that Sp1, Sp3, and NF-Y stimulate the transcription of the PKM gene via their interactions.