Hypercapnia Accelerates Adipogenesis: A Novel Role of High CO2 in Exacerbating Obesity
Obesity is a major risk factor for the development of obstructive sleep apnea (OSA) and obesity hypoventilation syndrome (OHS), which manifest as intermittent hypercapnia and sustained plus intermittent hypercapnia, respectively. In this study, we investigated whether CO2 affects adipocyte differentiation (adipogenesis) and maturation (hypertrophy). Human visceral or subcutaneous preadipocytes were grown to confluence and then induced to differentiate to adipocytes under hypocapnia, normocapnia, and hypercapnia with or without hypoxia. Adipogenesis was also induced under intermittent or sustained hypercapnia. Differentiated adipocytes were maintained to maturity under normocapnia or hypercapnia. Our main findings are as follows: (1) hypercapnia accelerated adipogenesis in visceral and subcutaneous preadipocytes, whereas hypocapnia inhibited adipogenesis; (2) hypercapnia did not affect adipocyte hypertrophy; (3) hypercapnia-accelerated adipogenesis was independent of extracellular acidosis, oxygen concentration, or either intermittent or sustained exposure to high CO2; and (4) the mechanisms underlying hypercapnia-accelerated adipogenesis involved increased production of cyclic adenosine monophosphate (cAMP) via soluble adenylyl cyclase, leading to the activation of protein kinase A and exchanger protein directly activated by cAMP, which, in turn, activated proadipogenic transcription factors, such as cAMP response element binding protein, CCAAT/enhancer binding protein β, and peroxisome proliferator-activated receptor γ. This study reveals a novel role of high CO2 in promoting adipogenesis, which provides mechanistic clues to a pathoetiological interaction between OSA/OHS and obesity. Our data suggest a vicious cycle of disease progression via the following mechanism: OSA/OHS → hypoventilation → hypercapnia → increased adipogenesis → increased fat mass → exacerbated OSA/OHS.