Inherent plasticity of brown adipogenesis in white fat of mice allows for recovery from effects of post-natal malnutrition

Abstract

Interscapular brown adipose tissue (iBAT) is formed during fetal development and stable for the life span of the mouse. In addition, brown adipocytes also appear in white fat depots (wBAT) between 10 and 21 days of age in mice maintained at a room temperature of 23 °C. However, this expression is transient. By 60 days of age the brown adipocytes have disappeared, but they can re-emerge if the adult mouse is exposed to the cold (5 °C) or treated with β3-adrenergic agonists. Since the number of brown adipocytes that can be induced in white fat influences the capacity of the mouse to resist the obese state, we determined the effects of the nutritional conditions on post-natal development (birth to 21 days) of wBAT and its long-term effects on diet-induced obesity (DIO). Under-nutrition caused essentially complete suppression of wBAT in inguinal fat at 21 days of age, as indicated by expression of Ucp1 and genes of mitochondrial structure and function based upon microarray and qRT-PCR analysis, whereas over-nutrition had no discernible effects on wBAT induction. Surprisingly, the suppression of wBAT at 21 days of age did not affect DIO in adult mice maintained at 23 °C, nor did it affect the reduction in obesity or cold tolerance when DIO mice were exposed to the cold at 5 °C for one week. Gene expression analysis indicated that mice raised under conditions that suppressed wBAT at 21 days of age were able to normally induce wBAT as adults. Therefore, neither severe hypoleptinemia nor hypoinsulinemia during suckling permanently impaired brown adipogenesis in white fat. In addition, energy balance studies of DIO mice exposed to cold indicates that mice with reduced adipose stores preferentially increased food intake, whereas those with larger adipose tissue depots preferred to utilize energy from their adipose stores.

Figure 1. Genomic analysis of regulatory factors for wBAT induction during early development.

A. Heat map of changes in gene expression in a K-means cluster of 180 genes with an Ucp1-like profile in inguinal fat of mice at 5, 10, 21, 56 and 112 days of age under control (C), under-nutrition (U) and over-nutrition (O) conditions from birth to 21 days of age. From 21 days of age until 56 days of age all mice were fed a low fat chow diet and from 56 to 112 days of age they were fed a high fat diet as described . B. Expression profiles of Ucp1/BAT marker genes; C. a set of genes encoding regulatory factors with profiles similar to Ucp1; D. A set of putative regulatory factors for Ucp1 expression in BAT in which their profiles do not correspond to that of Ucp1.

Figure 2. Genomic analysis of genes of respiration and oxidative phosphorylation.

Comparison of expression profiles of genes in the Ucp1 K-means cluster that encode components of the respiration, Complexes I to IV, and ATP synthase, Complex V, during control (graphs on the left) and under-nutrition (graphs on the right) dietary conditions as described in the Legend to Figure 1 and EXPERIMENTAL PROCEDURES.

Figure 3. Adiposity and energy balance profiles during over- and under-nutrition.

A. Body weight profile of C57BL/6 J mice under control, over and under-nutrition conditions. N = 19 for control, 23 for over-nutrition and 14 for the under-nutrition groups at 23°C. At 98 days, body weights between groups were significantly different at p<0.01. B. Changes in body weight in the 3 nutritional groups during the 11^th week on a high fat diet at 23°C; C. Food intake in the 3 nutritional groups at on the 11^th week of the high fat diet at 23°C; D. Changes in body weight in the 3 nutritional groups when maintained at 4°C and 23°C during the 12^th week. E. Food intake in the 3 nutritional groups during the 12^th week when maintained at 4°C and 23°C; F. Tabulation of energy balance in the 3 nutritional groups during the 12^th week of high diet when mice were maintained at 4°C. The number of animals (N) for Control, LUN and LON groups were 10, 7 and 12 respectively. For Panel B through F data bars or numbers (Panel F) with the same or no letter/symbol were not significantly different from each other at P<0.05. Data is presented as the mean ± standard error.

Figure 4. mRNA levels for genes important for brown fat function in inguinal fat and iBAT at 21 days of age after exposure to under-nutrition and over-nutrition (left-hand panel) and in 112 day old mice after 8 weeks of a high fat diet and exposure to the cold for 7 days (right-hand panel).

Total RNA was isolated from tissues and analyzed by qRT-PCR using low density array with the ABI 7900 analyzer as described . The data is given as the mean±standard error. The number of samples for inguinal fat was from 11–12 and for iBAT from 8–10. Significance was determined by ANOVA. Groups with the same letter were not different from each other.