ATF4 expression in thermogenic adipocytes is required for cold-induced thermogenesis in mice via FGF21-independent mechanisms

In brown adipose tissue (BAT), short-term cold exposure induces the activating transcription factor 4 (ATF4), and its downstream target fibroblast growth factor 21 (FGF21). Induction of ATF4 in BAT in response to mitochondrial stress is required for thermoregulation, partially by increasing FGF21 expression. In the present study, we tested the hypothesis that Atf4 and Fgf21 induction in BAT are both required for BAT thermogenesis under physiological stress by generating mice selectively lacking either Atf4 (ATF4 BKO) or Fgf21 (FGF21 BKO) in UCP1-expressing adipocytes. After 3 days of cold exposure, core body temperature was significantly reduced in ad-libitum-fed ATF4 BKO mice, which correlated with Fgf21 downregulation in brown and beige adipocytes, and impaired browning of white adipose tissue. Conversely, despite having reduced browning, FGF21 BKO mice had preserved core body temperature after cold exposure. Mechanistically, ATF4, but not FGF21, regulates amino acid import and metabolism in response to cold, likely contributing to BAT thermogenic capacity under ad libitum-fed conditions. Importantly, under fasting conditions, both ATF4 and FGF21 were required for thermogenesis in cold-exposed mice. Thus, ATF4 regulates BAT thermogenesis under fed conditions likely in a FGF21-independent manner, in part via increased amino acid uptake and metabolism.

GDF15 is required for cold-induced thermogenesis and contributes to improved systemic metabolic health following loss of OPA1 in brown adipocytes

We previously reported that mice lacking the protein optic atrophy 1 (OPA1 BKO) in brown adipose tissue (BAT) display induction of the activating transcription factor 4 (ATF4), which promotes fibroblast growth factor 21 (FGF21) secretion as a batokine. FGF21 increases metabolic rates under baseline conditions but is dispensable for the resistance to diet-induced obesity (DIO) reported in OPA1 BKO mice (Pereira et al., 2021). To determine alternative mediators of this phenotype, we performed transcriptome analysis, which revealed increased levels of growth differentiation factor 15 (GDF15), along with increased protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) levels in BAT. To investigate whether ATF4 induction was mediated by PERK and evaluate the contribution of GDF15 to the resistance to DIO, we selectively deleted PERK or GDF15 in OPA1 BKO mice. Mice with reduced OPA1 and PERK levels in BAT had preserved ISR activation. Importantly, simultaneous deletion of OPA1 and GDF15 partially reversed the resistance to DIO and abrogated the improvements in glucose tolerance. Furthermore, GDF15 was required to improve cold-induced thermogenesis in OPA1 BKO mice. Taken together, our data indicate that PERK is dispensable to induce the ISR, but GDF15 contributes to the resistance to DIO, and is required for glucose homeostasis and thermoregulation in OPA1 BKO mice by increasing energy expenditure.

ATF4 Expression in Thermogenic Adipocytes is Required for Cold-Induced Thermogenesis in Mice via FGF21-Independent Mechanisms

In brown adipose tissue (BAT), short-term cold exposure induces the activating transcription factor 4 (ATF4), and its downstream target fibroblast growth factor 21 (FGF21). Induction of ATF4 in BAT in response to mitochondrial stress is required for thermoregulation, partially via upregulation of FGF21. In the present study, we tested the hypothesis that Atf4 and Fgf21 induction in BAT are both required for BAT thermogenesis by generating mice selectively lacking either Atf4 ( ATF4 BKO ) or Fgf21 (FGF21 BKO) in UCP1-expressing adipocytes. After 3 days of cold exposure, core body temperature was significantly reduced in ad-libitum -fed ATF4 BKO mice, which correlated with Fgf21 downregulation in brown and beige adipocytes, and impaired browning of white adipose tissue (WAT). Conversely, despite having reduced browning, FGF21 BKO mice had preserved core body temperature after cold exposure. Mechanistically, ATF4, but not FGF21, regulates amino acid import and metabolism in response to cold, likely contributing to BAT thermogenic capacity under ad libitum -fed conditions. Importantly, under fasting conditions, both ATF4 and FGF21 were required for thermogenesis in cold-exposed mice. Thus, ATF4 regulates BAT thermogenesis by activating amino acid metabolism in BAT in a FGF21-independent manner.

Utility of routine screening fetal echocardiogram in pregnancies conceived by in vitro fertilization

OBJECTIVE

To study the incidence and clinical significance of congenital heart defects (CHDs) detected by fetal echocardiography in pregnancies conceived by in vitro fertilization (IVF).

DESIGN

Cohort study comparing a prospectively maintained database of all fetal echocardiograms from 2012 to 2018 and pooled data from the Connecticut Birth Defects Registry and statewide hospital discharge data.

SETTING

Large tertiary care center.

PATIENT(S)

A total of 181,749 live births and 9,252 fetal echocardiograms were analyzed. Fetal echocardiograms in patients with a previous child with a CHD, a family history of CHD, medication exposure, diabetes, anomaly in previous pregnancy, cardiac or other abnormality noted on previous ultrasound, or monochorionic twins were excluded from the final analysis.

INTERVENTION(S)

Treatment with IVF.

MAIN OUTCOME MEASURE(S)

Incidence of CHD and odds ratios with 95% confidence intervals (CIs). Infant outcomes for cases of CHD were evaluated for clinically significant disease, defined a priori as disease requiring any medical or surgical intervention or continued follow-up with pediatric cardiology.

RESULT(S)

Fetal echocardiography was performed in 2,230 IVF pregnancies, of which 2,040 were without other known risk factors for CHD. The mean gestational age at the time of fetal echocardiography was 22.2 ± 1.4 weeks. The odds ratio for CHD in the IVF group compared with statewide population rates was 1.4 (95% CI 0.9-2.1). CHD was diagnosed in 26 fetuses, of which 21 were clinically insignificant ventricular septal defects. One fetal echocardiogram was concerning for pulmonary stenosis that was not present at birth. Four defects were clinically significant, indicating that 510 fetal echocardiograms were performed for every diagnosis of one clinically significant CHD in the IVF group.

CONCLUSION(S)

The incidence of CHD in IVF pregnancies without other risk factors is not significantly different from baseline population rates, and most CHDs diagnosed by fetal echocardiography in this group are clinically insignificant. Routine screening with fetal echocardiography in all IVF pregnancies provides limited utility beyond routine prenatal care and need not be recommended without the presence of other risk factors.