Abstract
Insulin-dependent glucose homeostasis is highly sensitive to the levels of insulin-responsive glucose transporter 4 (GLUT4) expression in adipocytes. The level of GLUT4 protein expression is highly dependent on the rate of GLUT4 gene transcription. GLUT4 gene transcription is decreased in a variety of physiologic states of insulin resistance including type 2 diabetes, obesity, and prolonged fasting. GLUT4 gene expression in adipocytes is differentiation-dependent, with full expression delayed until late in the differentiation program. In this paper, we have tested the hypothesis that differentiation-dependent GLUT4 gene expression in 3T3-L1 adipocytes is dependent on the nuclear concentration of a class II histone deacetylase (HDAC) protein, HDAC5. We have tested this hypothesis by reducing the levels of class II HDACs in the nuclear compartment of 3T3-L1 preadipocytes using two experimental approaches. First, preadipocytes were treated with phenylephrine, an α-adrenergic receptor agonist, to drive HDACS out of the nuclear compartment. Also, the class II HDAC concentrations were reduced using siRNA knockdown. In each case, reduction of nuclear class II HDAC concentration resulted in increased expression of endogenous GLUT4 mRNA in preadipocytes. Together, our data indicate that class II HDAC expression is the major regulatory mechanism for inhibiting GLUT4 expression in the predifferentiated state.
Highlights
We have previously shown that the human glucose transporter 4 (GLUT4) promoter, when expressed in transgenic mice, is governed by two cis-acting domains: a MEF2-binding domain [10] and Domain I [11, 12]
Using both transgenic and cultured cell models, we have shown that maximal GLUT4 transcriptional activation is achieved when MEF2 proteins are bound to the MEF2 domain and GLUT4 enhancer factor (GEF) is bound to Domain I (10 –12)
We have shown that HDAC5 can form a protein complex with GEF informing us of the molecular basis by which MEF2, GEF, and HDAC5 regulate the GLUT4 promoter [17]
Summary
We have previously shown that the human GLUT4 promoter, when expressed in transgenic mice, is governed by two cis-acting domains: a MEF2-binding domain [10] and Domain I [11, 12]. We have shown that HDAC5 can form a protein complex with GEF informing us of the molecular basis by which MEF2, GEF, and HDAC5 regulate the GLUT4 promoter [17]. We tested the hypothesis that nuclear HDAC5 levels in preadipocytes are responsible for repression of GLUT4 gene transcription prior to differentiation. Using a variety of techniques to reduce nuclear HDAC expression, we were able to induce GLUT4 mRNA expression in preadipocytes, a state in which GLUT4 is not normally expressed, confirming that HDAC-mediated repression of the GLUT4 promoter is a major mechanism responsible for regulation of differentiation-dependent GLUT4 gene expression
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