Obesity- and glucose-dependent differential autophagy marker expression in adipose tissues and adipocytes

Background: Autophagy is an evolutionarily highly conserved process and plays an important role in cellular homeostasis and metabolism. In obesity and obesity-associated insulin resistance, both enhanced and suppressed autophagy have been observed in adipose tissues (AT). Objective and Methods: We report on a panel of 7 autophagy markers critically involved in mammalian cellular autophagy in a cohort of patients with obesity (n = 40; mean BMI 54.8 ± 6.7 kg/m2) versus lean individuals (n = 10; mean BMI = 22.7 ± 1.7 kg/m2), during 3T3-L1 adipocyte differentiation in vitro and in mature 3T3-L1 adipocytes in response to insulin stimulation under physiologic versus high-glucose conditions in vitro. Results: Autophagy markers exhibited a differential expression pattern in subcutaneous AT in patients with obesity as compared to patients with normal weight: LAMP2, ATG5, MAP1LC3B and SIRT1 were reduced in patients with obesity, while LAMP1 and SIRT6 were increased. SIRT1 was reduced in visceral versus subcutaneous AT and in patients with obesity with type 2 diabetes mellitus. ATG5 expression in subcutaneous AT correlated positively with systemic HDL cholesterol levels, and LAMP2 and MAP1LC3B expression in subcutaneous AT correlated positively with systemic pro-inflammatory Meteorin-like protein levels (METRNL). LAMP1, LAMP2, BECLIN1, ATG5 and MAP1LC3B increased during 3T3 adipocyte differentiation in vitro, while SIRT1 expression decreased. LAMP2 and MAP3LC3B increased in response to insulin stimulation in mature adipocytes under physiological glucose conditions, while LAMP2 expression increased under high-glucose conditions. Insulin stimulation reduced SIRT1 expression in mature adipocytes under high glucose conditions. Metformin treatment increased BECLIN1 and ATG5 expression in mature adipocytes. Conclusions: These observations indicate that adipocyte differentiation, glucose and insulin stimulation, pharmacological interventions and local and systemic inflammation associated with obesity differentially impact on individual autophagy markers both on a cellular and systemic level, potentially explaining the inconsistent effects in patients with obesity reported in the literature.