LETM-domain containing 1 (LETMD1) protects against obesity via enhancing UCP1-independent energy expenditure in human beige adipocytes.

Rationale: Brown and beige adipocytes are specialized fat cells that dissipate energy in the form of heat, and hold therapeutic potential for obesity and metabolic diseases. Although in the classical viewpoint brown and beige adipocytes dissipate energy solely via uncoupling protein 1 (UCP1), emerging evidence suggests the importance of non-canonical UCP1-independent energy expenditure in regulating energy expenditure, especially in human beige adipocytes. Leucine zipper-, EF-hand-containing transmembrane protein 1 domain containing 1 (LETMD1) was recently identified as a key protein in maintaining UCP1 expression and the thermogenic activity of brown adipocytes in animal models. But the exact function of LETMD1 and its mechanism of action in human beige adipocytes are unclear. Methods: We tested the function of LETMD1 in human induced pluripotent stem cell (hiPSC)-derived beige adipocytes in vitro in both wildtype (WT) and UCP1 knockout (KO) background. Furthermore, human beige adipocytes harboring a doxycycline-inducible LETMD1 expression cassette were transplanted to NOD/SCID mice and the function of LETMD1 in human beige adipocytes was evaluated in the in vivo setting. RNA-Seq was conducted in normal and LETMD1-overexpressing human beige adipocytes to examine the genes and pathways regulated by LETMD1. Using a knock-in human iPSC line, a preclinical small molecule compound library was screened for compounds increasing LETMD1 expression in human beige adipocytes. The effects of the compound in inducing LETMD1 and UCP1-independent energy expenditure in beige adipocytes were examined in vitro and in animal models. Results: LETMD1 plays an essential role in engaging energy dissipation, in a manner independent of UCP1, in human beige adipocytes. Transplantation of LETMD1-overexpressing human beige adipocytes improved whole-body metabolism of the recipient mice independent of UCP1. Mechanistically LETMD1 enhances the transcription of PPARGC1A, a key regulator of mitochondrial biogenesis. The expression of genes related to UCP1-independent energy expenditure, including creatine futile cycle, was also stimulated upon LETMD1 overexpression. Using LETMD1 reporter human beige adipocytes, SP-8356 was identified as a compound significantly increasing LETMD1 expression. Oral administration of SP-8356 induced genes related to UCP1-independent energy expenditure in beige adipocytes, and counteracted body weight gain and metabolic disorders in mice. Conclusion: Increased LETMD1 action, either genetically or pharmacologically, enhances the non-canonical UCP1-independent energy expenditure in beige adipocytes.