Depot-specific roles for sphingosine kinase 1 in conversion of white adipose to thermogenic adipose

Background: As of 2023, approximately 100.1 million adults and 14.7 million children in the USA are obese. Many comorbidities develop with obesity, which impairs quality of life and burdens the health care system. Consequently, there is an urgent need for interventions and treatments to reverse obesity and its comorbidities and restore health. Sphingosine Kinase 1 (SphK1), a key enzyme in sphingolipid metabolism, produces sphingosine-1-phosphate (S1P), a bioactive lipid implicated in obesity and metabolic dysfunction. While global deletion of Sphk1 protects against diet-induced obesity, adipocyte-specific SPHK1 deficiency paradoxically promotes weight gain, glucose intolerance, and adipose inflammation. Given the known role of sphingolipids in adipose thermogenesis, we investigated whether Sphk1 regulates adipocyte beiging and mitochondrial function. Methods: We assessed thermogenic responses in SphK1-deficient adipocytes and adipocyte-specific Sphk1 knockout mice under basal and beta3-adrenergic stimulation using CL 316,243. Thermoneutral housing (30 degree C) and room temperature (23 degree C) conditions were used to minimize and assess ambient temperature effects on thermogenesis. Molecular, histological, and bioenergetic analyses were conducted across multiple adipose depots. Results: Beta3-adrenergic stimulation upregulated Sphk1 expression in mature white adipocytes, while SphK1-deficient adipocytes exhibited enhanced Ucp1 expression, indicating a suppressive role for SphK1 in beiging. In vivo, adipocyte-specific Sphk1 knockout mice showed elevated Ucp1 expression in inguinal and gonadal white adipose tissue (iWAT, gWAT), both basally and after CL 316,243 treatment. These changes were accompanied by depot-specific alterations in adipocyte size and increased adiposity, independent of ambient temperature. Despite similar elevation of thermogenic markers, Sphk1 deletion had differential effect on mitochondrial function: iWAT showed increased mitochondrial content but reduced complex IV activity and ATP production, whereas gWAT showed reduced mitochondrial abundance without changes in respiration. Conclusion: Our work suggests that Sphk1 may act as a negative regulator of thermogenic expression and affect mitochondrial function in a depot-specific manner. Loss of Sphk1 enhances beiging but compromises mitochondrial efficiency, revealing a complex role for the SphK1/S1P axis in adipose plasticity and metabolic regulation. These insights may inform future therapeutic strategies targeting sphingolipid pathways for obesity and metabolic disease.