SLPI is a novel bone-coupling factor that controls parathyroid hormone-induced bone formation

Osteoclastic bone resorption and osteoblastic bone formation/replenishment are closely coupled in bone metabolism. Anabolic parathyroid hormone (PTH), which is commonly used for treating osteoporosis, shifts the balance from osteoclastic to osteoblastic, although the mechanisms underlying this phenomenon are unclear. Here we identified a serine protease inhibitor, secretory leukocyte protease inhibitor (SLPI), as a novel coupling factor that is involved in the PTH-mediated shift to the osteoblastic phase. SLPI was highly upregulated in osteoblasts by PTH, and genetic ablation of SLPI severely impaired PTH-induced bone formation. SLPI induction in osteoblasts enhanced osteoblast differentiation intracellularly and was also secreted extracellularly, attracting neighboring osteoclasts to suppress osteoclastic function. Intravital bone imaging revealed that the PTH-mediated association between osteoblasts and osteoclasts was disrupted in the absence of SLPI. Collectively, these results demonstrate that SLPI, a previously unrecognized bone-coupling factor, mediates the communication of osteoblasts with osteoclasts to promote PTH-induced bone formation. Overall design: We evaluated transcripts in osteoblasts by RNA-seq, using an iPTH mouse model (Iida-Klein et al. 2002) treated with iPTH or vehicle for 3 weeks.