Paradoxical Effect of Mitochondrial Respiratory Chain Impairment on Insulin Signaling and Glucose Transport in Adipose Cells

Abstract

Adipocyte function is crucial for the control of whole body energy homeostasis. Pathway analysis of differentiating 3T3-L1 adipocytes reveals that major metabolic pathways induced during differentiation involve mitochondrial function. However, it is not clear why differentiated white adipocytes require enhanced respiratory chain activity relative to pre-adipocytes. To address this question, we used small interference RNA to interfere with the induction of the transcription factor Tfam, which is highly induced between days 2 and 4 of differentiation and is crucial for replication of mitochondrial DNA. Interference with Tfam resulted in cells with decreased respiratory chain capacity, reflected by decreased basal oxygen consumption, and decreased mitochondrial ATP synthesis, but no difference in many other adipocyte functions or expression levels of adipose-specific genes. However, insulin-stimulated GLUT4 translocation to the cell surface and subsequent glucose transport are impaired in Tfam knockdown cells. Paradoxically, insulin-stimulated Akt phosphorylation is significantly enhanced in these cells. These studies reveal independent links between mitochondrial function, insulin signaling, and glucose transport, in which impaired respiratory chain activity enhances insulin signaling to Akt phosphorylation, but impairs GLUT4 translocation. These results indicate that mitochondrial respiratory chain dysfunction in adipocytes can cause impaired insulin responsiveness of GLUT4 translocation by a mechanism downstream of the Akt protein kinase.A large body of evidence has pointed to a close relationship between ectopic fat accumulation in tissues such as muscle and liver and the development of insulin resistance (1-3). The primary defense against such ectopic lipid accumulation is a well functioning adipose tissue, capable of sequestering excess calories in the form of stored triglycerides (4). In addition to this crucial role, adipose tissue is an endocrine organ that controls whole body energy homeostasis by secreting multiple cytokines that signal to other tissues (5, 6). The central role of adipose tissue in energy homeostasis is underscored by recent findings indicating that adipose tissue is a primary locus for the alterations induced by caloric restriction that accompany longevity (7,8). Thus, the cell biological mechanism involved in optimal adipose tissue development and function are crucial for the control of whole organism energy homeostasis and the determination of life span.Adipocyte differentiation is accompanied by an expansion of mitochondrial mass (9, 10), but the functional role of the relatively high levels of mitochondria in white adipocytes compared with those in adipose stroma and other tissues is not clear. High mitochondria levels may be required for the support of adipocyte-specific ATP-requiring processes (11), or to support metabolic functions such as glyceroneogenesis, which is required for triglyceride deposition (12, 13). White adipocyte mitochondria levels in rodents and humans change ma...

Adenylate Kinase 2 Links Mitochondrial Energy Metabolism to the Induction of the Unfolded Protein Response

“…This alteration of respiratory chain function did not impair secretory function as adiponectin secretion was unaffected (29). Consistent with these previous findings, siRNA-targeted depletion led to a decrease in Tfam mRNA expression and mtDNA content by 60 and 40%, respectively (Fig.…”

“…Previous work from our laboratory has shown that Tfam, a factor involved in mitochondrial DNA transcription and replication (28), can be silenced during 3T3-L1 adipocyte differentiation, mitigating the increase in oxidative phosphorylation that accompanies mitochondrial biogenesis (29). Thus, we investigated the effects of both Tfam and AK2 depletion on the induction of the UPR.…”

Mesodermal developmental gene Tbx15 impairs adipocyte differentiation and mitochondrial respiration

“…Mitochondria play an important role in controlling adipocyte fatty acid storage by regulating fatty acid degradation through oxidation, as well as their synthesis (24,25). Mitochondrial mass, measured by Mitotracker Green FM dye accumulation and fluorescence-activated cell sorting analysis, was reduced by 25 ± 2% (P < 0.05) in 3T3-L1-Tbx15 preadipocytes (day 0) compared with controls ( Fig.…”

“…Indeed, during the early stages of adipocyte differentiation, mitochondrial mass increases rapidly (32), and agents that inhibit mitochondrial respiration during this early stage impair differentiated functions (24). Overexpression of Tbx15 significantly reduces preadipocyte mitochondrial mass, mitochondrial basal respiration, and mitochondrial respiratory capacity, which leads to decreased adipocyte differentiation and reduced triglyceride accumulation.…”

“…In adipose tissue, mitochondrial respiration appears to be required for fatty acid synthesis preceding triglyceride synthesis (24,32). Indeed, during the early stages of adipocyte differentiation, mitochondrial mass increases rapidly (32), and agents that inhibit mitochondrial respiration during this early stage impair differentiated functions (24).…”