Role of HMGB1 in vascular aging and calcification

BACKGROUND- Vascular calcification (VC) is an age-associated complication of cardiovascular diseases, in which the main cellular event is the trans-differentiation of vascular smooth muscle cells (VSMCs) from a contractile to an osteochondrogenic phenotype that leads to an accumulation of calcium deposits. Senescence facilitates VSMCs osteogenic transition. VC is strongly associated with inflammation, oxidative stress and high level of DNA damage. HMGB1 is a highly conserved non-histone chromatin binding protein involved in transcription, DNA repair, and maintenance of nucleosome structure that can be actively secreted or passively released in the extracellular space acting as an alarmin. HMGB1 is involved in age-associated nuclear defects, cellular senescence and the acquisition of senescence-associated secretory phenotype (SASP). Finally, HMGB1 is implicated in VSMCs proliferation and migration and in osteochondrogenic transformation of human dental pulp stem cells (hDPCs) and valve interstitial cells (VICs). OBJECTIVE- The role of HMGB1 in vascular aging and calcification has been only partially explored. Herein, we investigated HMGB1 behavior and function human aortic smooth muscle cells (HASMCs) senescence and osteochondrogenic trans-differentiation associated to senescence in vitro and vascular aging and VC in vivo. RESULTS- HMGB1 protein expression decreases in aortas of old mice and during replicative senescence of HASMCs along with an increase of p16 expression. HMGB1 downregulation during senescence is mainly due to decrease of its gene expression and not relocation of the protein to the cytosol and in the extracellular space. HMGB1 declines also in the course of HASMCs calcification induced by hyperphosphatemia and in calcified aortas of a rat model of adenine-induced calcification and inversely correlates with calcium content in human abdominal aneurism of aorta (AAA). Silencing of HMGB1 in young but not in old HASMCs induces senescence-like phenotype through inhibition of cell proliferation and blocking the cell cycle in G0/G1 phase and increase of p21 and senescence-associate β-galactosidase (SA-β-gal) expression, in respect to control cells. Notably, HMGB1 down-regulation reduces HASMCs secretion of pro-inflammatory SASPs factors, DNA damage and ROS content both in young and old cells. Finally, silencing of HMGB1 in HASMCs initially impairs cell calcification and SASP factors release but eventually favours calcium deposition and IL6, IL1β and OPN secretion. In accordance, aortas of vitamin D-treated Hmgb1+/- mice exhibit a lower accumulation of calcium in the early phase of calcification while a higher tissue mineralization later on, in respect to Hmgb1+/+ animals. CONCLUSION- Hence, during vascular aging, the reduction of HMGB1 in VSMC induces a senescence-like phenotype that favours DNA damage repair, avoid SASP spreading and limit cell proliferation. However, this response is initially protective but becomes deleterious after a long-term period in response to pro-calcification conditions.