Latent TGFß-binding proteins regulate UCP1 expression and function via TGFß2.

OBJECTIVE: Activation of brown adipose tissue (BAT) in humans has been proposed as a new treatment approach for combating obesity and its associated diseases, as BAT participates in the regulation of energy homeostasis as well as glucose and lipid metabolism. Genetic contributors driving brown adipogenesis in humans have not been fully understood. METHODS: Profiling the gene expression of progenitor cells from subcutaneous and deep neck adipose tissue, we discovered new secreted factors with potential regulatory roles in white and brown adipogenesis. Among these, members of the latent transforming growth factor beta-binding protein (LTBP) family were highly expressed in brown compared to white adipocyte progenitor cells, suggesting that these proteins are capable of promoting brown adipogenesis. To investigate this potential, we used CRISPR/Cas9 to generate LTBP-deficient human preadipocytes. RESULTS: We demonstrate that LTBP2 and LTBP3 deficiency does not affect adipogenic differentiation, but diminishes UCP1 expression and function in the obtained mature adipocytes. We further show that these effects are dependent on TGFß2 but not TGFß1 signaling: TGFß2 deficiency decreases adipocyte UCP1 expression, whereas TGFß2 treatment increases it. The activity of the LTBP3-TGFß2 axis that we delineate herein also significantly correlates with UCP1 expression in human white adipose tissue (WAT), suggesting an important role in regulating WAT browning as well. CONCLUSIONS: These results provide evidence that LTBP3, via TGFß2, plays an important role in promoting brown adipogenesis by modulating UCP1 expression and mitochondrial oxygen consumption.

Ltbp4 regulates Pdgfrß expression via TGFß-dependent modulation of Nrf2 transcription factor function.

Latent transforming growth factor beta binding protein 4 (LTBP4) belongs to the fibrillin/LTBP family of proteins and plays an important role as a structural component of extracellular matrix (ECM) and local regulator of TGFß signaling. We have previously reported that Ltbp4S knock out mice (Ltbp4S-/-) develop centrilobular emphysema reminiscent of late stage COPD, which could be partially rescued by inactivating the antioxidant protein Sestrin 2 (Sesn2). More recent studies showed that Sesn2 knock out mice upregulate Pdgfrß-controlled alveolar maintenance programs that protect against cigarette smoke induced pulmonary emphysema. Based on this, we hypothesized that the emphysema of Ltbp4S-/- mice is primarily caused by defective Pdgfrß signaling. Here we show that LTBP4 induces Pdgfrß signaling by inhibiting the antioxidant Nrf2/Keap1 pathway in a TGFß-dependent manner. Overall, our data identified Ltbp4 as a major player in lung remodeling and injury repair.

Latent TGF-ß binding protein-1 deficiency decreases female fertility.

The four latent transforming growth factor-ß (TGF-ß) binding proteins LTBP1-4 are extracellular matrix-associated proteins playing a critical role in the activation of TGF-ß. The LTBP1 gene forms two major transcript variants (i.e. Ltbp1S and Ltbp1L) that are derived from different promoters. We have previously shown the importance of LTBP1 in vivo by using three different Ltbp1 null mice that were either deleted for exons 1 and 2 (Ltbp1L knockout), exon 5 (Ltbp1ΔEx5), or exon 8 (Ltbp1ΔEx8). While the Ltbp1L knockout and the Ltbp1ΔEx8 are perinatal lethal and die of cardiovascular abnormalities, the Ltbp1ΔEx5 is viable because it expresses a short form of Ltbp1L that lacks 55 amino acids (Δ55 variant of Ltbp1) formed by splicing out exon 5, while lacking the Ltbp1S variant. Since only the Ltbp1ΔEx5 mouse is viable, we have used this model to address aspects of puberty, fertility, age-dependent reproduction, and ovary function. We report for the first time a function of LTBP1 in female reproduction. The Ltbp1ΔEx5 females showed impaired fertility associated with delayed sexual maturity (p = 0.0074) and ovarian cyst formation in females older than 40 weeks (p = 0.0204).

Inactivation of sestrin 2 induces TGF-beta signaling and partially rescues pulmonary emphysema in a mouse model of COPD.

Chronic obstructive pulmonary disease (COPD) is a leading cause of morbidity and mortality worldwide. Cigarette smoking has been identified as one of the major risk factors and several predisposing genetic factors have been implicated in the pathogenesis of COPD, including a single nucleotide polymorphism (SNP) in the latent transforming growth factor (TGF)-beta binding protein 4 (Ltbp4)-encoding gene. Consistent with this finding, mice with a null mutation of the short splice variant of Ltbp4 (Ltbp4S) develop pulmonary emphysema that is reminiscent of COPD. Here, we report that the mutational inactivation of the antioxidant protein sestrin 2 (sesn2) partially rescues the emphysema phenotype of Ltbp4S mice and is associated with activation of the TGF-beta and mammalian target of rapamycin (mTOR) signal transduction pathways. The results suggest that sesn2 could be clinically relevant to patients with COPD who might benefit from antagonists of sestrin function.