Multiphasic Regulation of Systemic and Peripheral Organ Metabolic Responses to Cardiac Hypertrophy.

BACKGROUND: Reduced fat oxidation in hypertrophied hearts coincides with a shift of carnitine palmitoyl transferase I from muscle to increased liver isoforms. Acutely increased carnitine palmitoyl transferase I in normal rodent hearts has been shown to recapitulate the reduced fat oxidation and elevated atrial natriuretic peptide message of cardiac hypertrophy. METHODS AND RESULTS: Because of the potential for reduced fat oxidation to affect cardiac atrial natriuretic peptide, and thus, induce adipose lipolysis, we studied peripheral and systemic metabolism in male C57BL/6 mice model of transverse aortic constriction in which left ventricular hypertrophy occurred by 2 weeks without functional decline until 16 weeks (ejection fraction, -45.6%; fractional shortening, -22.6%). We report the first evidence for initially improved glucose tolerance and insulin sensitivity in response to 2 weeks transverse aortic constriction versus sham, linked to enhanced insulin signaling in liver and visceral adipose tissue (epididymal white adipose tissue [WAT]), reduced WAT inflammation, elevated adiponectin, mulitilocular subcutaneous adipose tissue (inguinal WAT) with upregulated oxidative/thermogenic gene expression, and downregulated lipolysis and lipogenesis genes in epididymal WAT. By 6 weeks transverse aortic constriction, the metabolic profile reversed with impaired insulin sensitivity and glucose tolerance, reduced insulin signaling in liver, epididymal WAT and heart, and downregulation of oxidative enzymes in brown adipose tissue and oxidative and lipogenic genes in inguinal WAT. CONCLUSIONS: Changes in insulin signaling, circulating natriuretic peptides and adipokines, and varied expression of adipose genes associated with altered insulin response/glucose handling and thermogenesis occurred prior to any functional decline in transverse aortic constriction hearts. The findings demonstrate multiphasic responses in extracardiac metabolism to pathogenic cardiac stress, with early iWAT browning providing potential metabolic benefits.

Transcription regulator TRIP-Br2 mediates ER stress-induced brown adipocytes dysfunction.

In contrast to white adipose tissue, brown adipose tissue (BAT) is known to play critical roles for both basal and inducible energy expenditure. Obesity is associated with reduction of BAT function; however, it is not well understood how obesity promotes BAT dysfunction, especially at the molecular level. Here we show that the transcription regulator TRIP-Br2 mediates ER stress-induced inhibition of lipolysis and thermogenesis in BAT. Using in vitro, ex vivo, and in vivo approaches, we demonstrate that obesity-induced inflammation upregulates brown adipocytes TRIP-Br2 expression via the ER stress pathway and amelioration of ER stress in mice completely abolishes high fat diet-induced upregulation of TRIP-Br2 in BAT. We find that increased TRIP-Br2 significantly inhibits brown adipocytes thermogenesis. Finally, we show that ablation of TRIP-Br2 ameliorates ER stress-induced inhibition on lipolysis, fatty acid oxidation, oxidative metabolism, and thermogenesis in brown adipocytes. Taken together, our current study demonstrates a role for TRIP-Br2 in ER stress-induced BAT dysfunction, and inhibiting TRIP-Br2 could be a potential approach for counteracting obesity-induced BAT dysfunction.

Lipodystrophy and severe metabolic dysfunction in mice with adipose tissue-specific insulin receptor ablation.

OBJECTIVE: Insulin signaling plays pivotal roles in the development and metabolism of many tissues and cell types. A previous study demonstrated that ablation of insulin receptor (IR) with aP2-Cre markedly reduced adipose tissues mass and protected mice from obesity. However, multiple studies have demonstrated widespread non-adipocyte recombination of floxed alleles in aP2-Cre mice. These findings underscore the need to re-evaluate the role of IR in adipocyte and systemic metabolism with a more adipose tissue-specific Cre mouse line. METHODS: We generated and phenotyped a new adipose tissue-specific IR mouse model using the adipose tissue-specific Adipoq-Cre line. RESULTS: Here we show that the Adipoq-Cre-mediated IR KO in mice leads to lipodystrophy and metabolic dysfunction, which is in stark contrast to the previous study. In contrast to white adipocytes, absence of insulin signaling does not affect development of marrow and brown adipocytes, but instead is required for lipid accumulation particularly for the marrow adipocytes. Lipodystrophic IR KO mice have profound insulin resistance, hyperglycemia, organomegaly, and impaired adipokine secretion. CONCLUSIONS: Our results demonstrate differential roles for insulin signaling for white, brown, and marrow adipocyte development and metabolic regulation.