Moderate fluid shear stress regulates heme oxygenase-1 expression to promote autophagy and ECM homeostasis in the nucleus pulposus cells

In vertebrate, the nucleus pulposus (NP), which is an essential component of the intervertebral disk, is constantly impacted by fluid shear stress (FSS); however, molecular mechanism(s) through which FSS modulates the NP homeostasis is poorly understood. Here we show that FSS regulates the extracellular matrix (ECM) homeostasis in NP cells. A moderate dose of FSS (i.e., 12 dyne/cm(2)) increases the sulfated glycosaminoglycan (sGAG) content and protein levels of Col2a1 and Aggrecan and decreases those of matrix metalloproteinase 13 (MMP13) and a disintegrin and metalloproteinase with thrombospondin motif 5 (ADMATS5) in rat NP cells, while a higher dose of FSS (i.e., 24 dyne/cm(2)) displays opposite effects. Results from RNA sequencing analysis, quantitative real-time RT-PCR analysis and western blotting establish that the heme oxygenase-1 (HO-1) is a key downstream mediator of the FSS actions in NP cells. HO-1 knockdown abolishes FSS-induced alterations in ECM protein production and sGAG content in NP cells, which is reversed by HO-1 induction. Furthermore, FSS activates the autophagic pathway by increasing the LC3-II/LC3-I ratio, Beclin-1 protein level, and formation of autophagosome and autolysosome and thereby regulates ECM protein and sGAG production in a HO-1 dependent manner. Finally, we demonstrate that the intraflagellar transport (IFT) 88, a core trafficking protein of primary cilia, is critically involved in the HO-1-mediated autophagy activation and ECM protein and sGAG production in FSS-treated NP cells. Thus, we for the first time demonstrate that FSS plays an important role in maintaining ECM homeostasis through HO-1-dependent activation of autophagy in NP cells.