Phosphorylated peptides from Antarctic krill (Euphausia superba) ameliorated osteoporosis by activation of osteogenesis-related MAPKs and PI3K/AKT/GSK-3β pathways in dexamethasone-treated mice

Highlights

• PP-AKP improved osteoporosis induced by dexamethasone.

• PP-AKP promoted osteogenesis with a reduction in the accumulation of fat.

• PP-AKP activated osteogenesis-related MAPKs and PI3K/AKT/GSK-3β pathways.

• This study provides theory basis for high-value utilization of Antarctic krill.

Abstract

In this study, the effects of phosphorylated peptides from Antarctic krill (PP-AKP) on osteoporosis induced by dexamethasone were investigated in vivo. Results showed that PP-AKP significantly improved bone turnover status, reduced bone loss and degeneration of microarchitecture, in addition to accelerated bone formation. Further mechanism investigation revealed that PP-AKP suppressed the mRNA expression of MKP-1 and CB1, which activated the downstream osteogenesis-related MAPKs and PI3K/AKT/GSK-3β signaling pathways through elevation of the expression of the key factors p38, ERK, PI3K, AKT and β-catenin, in addition to osteogenic nuclear transcription factors Runx2 and OSX. Additionally, reduction in number of adipocytes and an increase in trabeculae in the bone marrow cavity, in addition to a decrease in abdominal adipose further verified that PP-AKP augmented bone formation with a comparable reduction in the accumulation of fat. In conclusion, PP-AKP ameliorated osteoporosis via promoting MAPKs and PI3K/AKT/GSK-3β pathways related to bone formation in dexamethasone-treated mice.

Introduction

Osteoporosis (OP), regarded as a systemic skeletal disorder, is characterized by bone loss, impaired bone microarchitecture and thereby results in an increased risk of bone fracture, which seriously affects the quality of life for sufferers of the condition. Osteoporosis is generally divided into two types. Primary OP includes postmenopausal osteoporosis and senile osteoporosis. Glucocorticoid-induced osteoporosis (GIOP) is considered the most common form of secondary OP (Komori, 2015, Zhang et al., 2016). Glucocorticoid is widely used as an effective therapy for alleviating chronic inflammatory and autoimmune diseases. Globally, it is estimated that more than 1% of all adults are treated with glucocorticoids annually (Lash et al., 2009, Overman et al., 2015, Thomas et al., 2013). However, long-term use of glucocorticoids is accompanied by severe side effects, including increased incidence of osteoporosis (Rauch et al., 2010). Approximately 30–50% of glucocorticoid users suffer from osteoporotic fracture (Kenanidis et al., 2015, Rizzoli et al., 2012, Rizzoli and Biver, 2015), which no doubt leads to an increased risk of mortality and heavy economic burden in health-care. Therefore, GIOP requires careful consideration.

Bone homeostasis is maintained by osteoblast-mediated bone formation and osteoclast-mediated bone resorption. A disruption in the number or function of osteoblasts and osteoclasts can destabilize bone homeostasis, eventually leading to bone loss (An et al., 2016). A general consensus has been reached that impaired bone formation mediated by osteoblasts predominantly accounts for GIOP (Bouvard et al., 2013, Canalis and Delany, 2002). Glucocorticoids not only depress osteoblast activity but also decrease the number of osteoblasts by shifting the differentiation of bone marrow stem cells (BMSCs) towards adipocytes rather than osteoblasts (Li et al., 2013, Yun et al., 2009). Currently, drugs which promote osteogenesis, such as teriparatide, are used in the management of GIOP (Bultink et al., 2013, Warriner and Saag, 2013). However, their long-term use presents adverse side-effects and high cost (Bultink et al., 2013). Therefore, there is an urgent need to find a safe and effective bioactive compound to ameliorate the osteoporosis induced by long-term glucocorticoid therapy.

Antarctic krill (Euphausia superba) is a crustacean living in the Antarctic Ocean, the total biomass of which is estimated to be more than 400 million metric tons. Antarctic krill protein contains all the dietary essential amino acids with a nutritional value superior to that of meat or milk proteins (Chen et al., 2009, Wang et al., 2011). Therefore, the development and utilization of Antarctic krill have health and economic significance. Previous studies have reported that phosphorylation of proteins enhances their physiological functions. For example, the phosphorylation of ovalbumin and egg white protein significantly improved their heat stability, emulsifying as well as gelling functions (Li et al., 2004, Wang et al., 2015). In our current study, PP-AKP prepared by our team included two types of phosphate bonds, which enabled enhanced protein function. In a previous study we demonstrated that PP-AKP suppressed bone resorption in ovariectomized rats (Xia & Zhao et al., 2015). However, it is unclear whether PP-AKP is able to promote osteogenesis in vivo. On that basis, we have explored the ameliorative effects of PP-AKP on GIOP and the molecular mechanisms involved in the osteogenesis-related MAPKs and PI3K/AKT/GSK-3β pathways were studied further.