The Coordinating Role of the Hippo-YAP/TAZ Pathway in Adipose Plasticity and Energy Balance

Adipose tissue not only serves as an energy storage depot but also functions as an endocrine organ. However, the mechanisms coordinating these functions remain elusive. This study unveiled that the transcriptional co-regulators YAP and TAZ maintain metabolic homeostasis by uncoupling adipose mass and leptin levels, and regulate adipocyte plasticity. The findings demonstrated that activating the YAP/TAZ pathway in adipocytes by deleting the upstream regulators LATS1 and LATS2 converted mature adipocytes to adipocyte-like cells without causing metabolic dysfunctions associated with adipose depletion. Notably, the increase in circulating leptin levels due to this phenomenon did not lead to metabolic dysfunctions associated with adipose depletion. Mechanistically, the YAP/TAZ-TEAD signal directly binds to the leptin gene upstream enhancer to regulate leptin expression. Further studies revealed that YAP/TAZ activity is closely associated with leptin regulation and functionally required during fasting and refeeding.

This work was conducted by a research team from KAIST (Korea Advanced Institute of Science and Technology) and published in the journal Nature Metabolism in May 2024. The corresponding authors are Daesik Lim and Jinsuk Suh.

Research Background

Adipose tissue functions as both a metabolic and endocrine organ, primarily by storing energy in the form of triglycerides and secreting adipokines such as leptin to regulate whole-body energy balance. In obesity-related metabolic syndrome, the defective energy storage function leads to spillover of excess energy into peripheral organs, resulting in a cascade of metabolic disorders such as fatty liver, hyperglycemia, and insulin resistance. Conversely, adipose tissue depletion also causes severe diabetic phenotypes. Recent studies have shown that restoring adipose endocrine function through replacement therapy can reverse metabolic dysfunctions associated with adipose depletion. While circulating leptin levels are generally considered proportional to adipose mass, the molecular mechanisms linking adipose mass and leptin expression remain unclear.

The Hippo pathway coordinates organ size and cell type-specific functions in conjunction with tissue-specific transcription factors. YAP and TAZ are the downstream effectors of the Hippo pathway, and their activity is determined by their phosphorylation state. The upstream kinases LATS1 and LATS2 directly phosphorylate YAP/TAZ, leading to their cytoplasmic retention and degradation, while the absence of LATS1/LATS2 allows YAP/TAZ to translocate to the nucleus and act as transcriptional co-activators or co-repressors. In adipocyte biology, TAZ has been shown to inhibit adipogenesis by repressing PPARG, a master regulator of adipogenesis. However, YAP/TAZ has been considered dispensable in mature adipocytes. Interestingly, in diet-induced obese mouse models, TAZ deficiency was found to increase PPARG activity and decrease insulin resistance, further hinting at potential roles for YAP/TAZ in mature adipocytes.

Research Methods

This study employed adipocyte-specific LATS1 and LATS2 knockout (AKO) mice generated by crossing LATS1fl/fl;LATS2fl/fl mice with Adipoq-Cre recombinase-expressing mice. Evaluation showed significantly reduced LATS1 and LATS2 expression and upregulation of YAP/TAZ target genes CCN1 and CCN5 in AKO mice. Furthermore, nuclear localization of YAP and TAZ in the inguinal white adipose tissue (IWAT) of AKO mice confirmed the activation state of YAP/TAZ.

In addition to in vivo experiments, the research team employed adipocyte lineage tracing techniques using Rosa-LSL-tdTomato reporter mice and performed in vitro validations, including using 4-hydroxytamoxifen-treated adipocytes and primary adipocytes. Single-cell RNA sequencing was also employed to analyze mitochondrial signatures and adipocyte-specific knockout models.

Research Findings

Adipose YAP/TAZ activation causes severe adipose depletion

Compared to control mice, the IWAT of AKO mice was markedly smaller and no longer floated in phosphate-buffered saline (PBS). Histological examination revealed a significant reduction in lipid droplet content in the IWAT of AKO mice. Correspondingly, the expression of adipocyte-specific genes was also significantly decreased. To confirm that the observed phenotype was indeed caused by YAP/TAZ activation, the research team generated adipocyte-specific LATS1, LATS2, YAP1, and WWTR1 (TAZ) quadruple knockout mice (Quad AKO). The Quad AKO mice reversed the adipose depletion phenotype observed in AKO mice, demonstrating that YAP/TAZ function is indispensable for the adipose depletion caused by LATS1 and LATS2 deficiency.

PPARG agonist reverses YAP/TAZ-induced adipose depletion

The study found that LATS1/LATS2 deficiency induced the acquisition of progenitor cell markers in adipocytes, but these cells retained the potential for adipocyte re-differentiation. By administering Rosiglitazone, a PPARG agonist, the adipose tissue size and mass were significantly restored, and the expression of adipocyte-specific genes was recovered in adipocyte-specific LATS1/LATS2 knockout mice. This indicated that the adipose depletion caused by adipocyte-specific LATS1/LATS2 knockout could be reversed through the re-differentiation process.

Lipoatrophic IAKO mice are spared from metabolic dysfunctions

Despite significant adipose depletion, IAKO mice did not exhibit typical metabolic dysfunctions associated with lipodystrophy. Furthermore, in vivo metabolic flux tracing using 13C-labeled palmitate and 2H-labeled glycerol revealed significantly increased lipolysis, palmitate turnover, and fatty acid oxidation rates in these mice. Consequently, the increased energy expenditure and fatty acid oxidation protected IAKO mice from metabolic dysfunctions associated with adipose depletion, such as fatty liver and glucose intolerance.

YAP/TAZ activation uncouples leptin levels from adipose mass

Unexpectedly, despite the near-complete depletion of adipose tissue, AKO mice exhibited a 15-fold increase in serum leptin concentrations compared to controls. Analysis of inducible adipocyte-specific LATS1/LATS2 knockout mice revealed that YAP/TAZ directly regulate the transcription of the leptin gene. Furthermore, by generating IAKO ob/ob mice (Adipoq-CreER;Lats1fl/fl;Lats2fl/fl;Lepob/ob) lacking functional leptin gene and analyzing their metabolic phenotypes, the study confirmed the crucial role of leptin in protecting adipose-depleted mice from metabolic dysfunctions.

YAP/TAZ directly regulate leptin transcription

Through the construction of a constitutively active TAZ4SA form and FLAG-tagged ChIP-Seq analysis of YAP, the study demonstrated that YAP/TAZ directly bind to the leptin gene enhancer via TEAD binding sequences. Subsequent Luciferase reporter assays also supported this finding, where the TEAD binding sequences were required for TAZ to upregulate leptin gene expression. Ultimately, the role of YAP/TAZ in leptin gene regulation was further substantiated by their responses under physiological conditions, such as high-fat diet and fasting-refeeding.

Research Conclusions and Significance

This study, by unveiling the dual roles of YAP/TAZ in mature adipocytes – increasing systemic energy expenditure through the YAP/TAZ-TEAD axis (via upregulation of leptin expression) and decreasing adipose tissue mass through the YAP/TAZ-PPARG axis (via repression of PPARG target gene expression) – highlights the importance of the Hippo-YAP/TAZ pathway in adipose tissue function and whole-body energy balance. It provides a theoretical basis for developing therapeutic interventions targeting this pathway to achieve a steady-state reduction in adipose mass.