Role of the ABA/LANCL system in the insulin-independent stimulation of cell glucose uptake and energy metabolism in myoblasts and adipocytes through an AMPK-mediated mechanism

Abscisic acid (ABA) is a hormone with a very long evolutionary history, dating back to the earliest living organisms, of which modern (ABA-producing) cyanobacteria are likely the descendants, well before separation of the plant and animal kingdoms, with a conserved role as a signal regulating cell responses to environmental challenge. Nanomolar ABA is also present and active in mammals where it controls the metabolic response to glucose availability by stimulating glucose uptake in skeletal muscle and adipose tissue with an insulin-independent mechanism and increasing brown adipose tissue activity and browning through its receptor LANCL2. Previous data showed that, in LANCL2-/- mice, the genetic ablation of the receptor doesn’t modify fasting glycemia values but results in the reduction of glucose tolerance compared with wild-type siblings. Animals are still responsive to exogenous ABA. The result that the effect of ABA was not completely abrogated by LANCL2 silencing, despite the very high percentage of reduction of protein and mRNA expression, suggests that other ABA receptors might be partly involved in the effect of ABA on muscle. LANCL1, a LANCL2 homolog, is indeed expressed in murine skeletal muscle at levels like those of LANCL2. Thus, I focused my attention on the metabolic function of the ABA/LANCL1 system. Recombinant human LANCL1 protein binds ABA with a Kd in a concentration range that lies between the low and high-affinity ABA binding sites of LANCL2. In rat L6 myoblasts, LANCL receptors similarly stimulate both basal and ABA-triggered fluorescently labeled deoxyglucose analog 2-NBDG uptake, activate mRNA levels and protein expression of the glucose transporters GLUT1 and GLUT4 and the signaling proteins AMPK/PGC-1α/Sirt1, stimulate mitochondrial respiration and the expression of the skeletal muscle (SM) uncoupling proteins sarcolipin and UCP3. Moreover, LANCL2-/- mice overexpress LANCL1 in the SM and respond to chronic ABA treatment (1 µg/kg body weight/day) with an improved glycemia response to glucose load and increased SM transcription of GLUTs, of the AMPK/PGC-1α/Sirt1 pathway and of sarcolipin, UCP3 and NAMPT. LANCL1 behaves as an ABA receptor with a somewhat lower affinity for ABA than LANCL2 but with overlapping effector functions: stimulating glucose uptake and the expression of muscle glucose transporters and mitochondrial uncoupling and respiration via the AMPK/PGC-1α/Sirt1 pathway. Furthermore, since adipose tissue seems to be a direct target of ABA, along with skeletal muscle, I investigated the browning effects of ABA in brown and white human adipocytes. ABA increases AMPK phosphorylation and AMPK and PGC-1α protein levels in pre-adipocytes overexpressing LANCL1 and LANCL2. The expression of genes related to browning, oxidative consumption, glucose transport, mitochondrial biogenesis, respiratory uncoupling and AMPK/PGC-1α/Sirt1 pathway is increased after adipocyte differentiation and further increases upon ABA treatment. Chronic ABA treatment stimulates mitochondrial DNA content in the brown adipose tissue of mice. These results suggest that ABA may be used as an anti-obesity molecule. Concluding, receptor redundancy may have been advantageous in animal evolution, given the role of the ABA/LANCL1-2 system in the insulin-independent stimulation of cell glucose uptake and energy metabolism.