Glucocorticoid dose determines osteocyte cell fate

Abstract

In response to cellular insult, several pathways can be activated, including necrosis, apoptosis, and autophagy. Because glucocorticoids (GCs) have been shown to induce both osteocyte apoptosis and autophagy, we sought to determine whether osteocyte cell fate in the presence of GCs was dose dependent by performing in vivo and in vitro studies. Male Swiss-Webster mice were treated with slow-release prednisolone pellets at 1.4, 2.8, and 5.6 mg/kg/d for 28 d. An osteocyte cell line, MLO-Y4 cells, was treated with various doses of dexamethasone. We found that GC treatments dose dependently decreased activation of antioxidant-, autophagy-, and antiapoptosis-focused RT-PCR gene pathways in mouse cortical bone. The activation of antioxidant genes was correlated with autophagy gene expression after the GC treatments. The presence of osteocyte autophagy, as detected by immunostaining for LC3, increased ∼50% at the distal femur cortical bone region but not at trabecular bone region at the 1.4 and 2.8 mg/kg/d GC dose levels. The number of apoptotic osteocytes was increased at the cortical bone region by ∼40% initially observed at the 2.8 mg/kg/d dose level. In addition, the presence of the osteocyte autophagy was associated with an increased protein level of cathepsin K in vitro after the GC treatments. In summary, we found that GC treatment dose-dependently decreased antioxidant gene expression, with lower GC doses activating autophagy, whereas a higher dose increased apoptosis. These data suggest that autophagy may provide a mechanism for osteocytes to survive the stress after GC exposure and provide further insight into how GCs alter bone cell fate.

Osteoporosis and autophagy: What is the relationship?

“…61 Low-dose glucocorticoid treatment has also been shown to induce autophagy in osteocytes in response to increased oxidative stress caused by treatment, preserving the viability of these cells. [62][63][64] Studies indicate that autophagy also plays an important role in bone resorption by osteoclasts. 65 It has been reported that proteins involved in the autophagic pathway are important in the regulation of osteoclastogenesis, 66,67 indicating that this process participates both in bone formation and resorption.…”

Autophagy in stem cells

“…146,147 The specific depletion of BECN1 notably reduces both the size and formation efficiency of mammospheres, thus development of autophagy, preserving osteocyte viability depending on glucocorticoid dose. 129,130 Osteoblasts are the precursor of osteocytes and are responsible for bone formation. As osteoblasts actively produce mineralized matrix during bone formation, it is conceivable that autophagy is activated to meet the high-energy demands of active osteoblasts.…”

Dexamethasone reduces ATDC5 chondrocyte cell viability by inducing autophagy

“…Administration of GCs leads to a reduction in bone mineral density (BMD), decreased generation of osteoblasts and osteocytes and prolonged lifespan of osteoclasts, with fractures occurring in 30-50% of patients chronically treated with GCs (1). Recent studies have demonstrated that GCs can induce apoptosis and autophagy in osteocytes (4)(5)(6). The influence of GCs has been rarely reported on chondrocytes, which play a key role in skeletal development.…”