Effects of the amide alkaloid piperyline on apoptosis, autophagy, and differentiation of pre-osteoblasts

Background: Amide alkaloidsare typical constituents in plants of the Piperaceae family. Most of the pharmacological properties of Piper nigrum L. are attributed to the major amide alkaloid, piperine. Piperyline (PIPE) is a further amide alkaloid that has been isolated from P. nigrum.

Hypothesis/purpose: This study was performed to examine the biological effects of PIPE on pre-osteoblasts and elucidate the underlying mechanisms.

Study design: We investigated the effects of PIPE in MC3T3E-1 cells, which are widely used for studying osteoblast behavior in in vitro cell systems.

Methods: We evaluated cell viability based on the MTT assay, apoptosis by TUNEL staining, adhesion and migration by cell adhesion and migration assays, and osteoblast differentiation by alkaline phosphatase activity and staining. Western blot and immunocytochemical analyses were used to investigate cell signaling pathways.

Results: We found that at concentrations ranging from 1 to 30 μM, PIPE inhibited cell growth and induced apoptosis in pre-osteoblasts, which was accompanied by the upregulation of apoptotic proteins but downregulation of anti-apoptotic proteins. In contrast, PIPE had no appreciable effect on the autophagy pathway. Nevertheless, PIPE reduced cell adhesion and migration via the inactivation of non-receptor tyrosine kinase (Src)/focal adhesion kinase (FAK) and mitogen-activated protein kinases, and also promoted the downregulation of matrix metalloproteinase 2 and 9 levels. Furthermore, at concentrations of 10 and 30 μM, PIPE suppressed osteoblast differentiation, as indicated by reductions in alkaline phosphatase staining and activity. In addition, PIPE reduced the protein levels of phospho-Smad1/5/8 and runt-related transcription factor 2, and the mRNA levels of osteopontin, alkaline phosphatase, and osteocalcin.

Conclusion: The findings of this study indicate that PIPE has biological effects associated with cell adhesion, migration, proliferation, and osteoblast differentiation, and suggest a potential role for this alkaloid in the treatment of bone diseases.

Keywords: Phytomedicine; Piper nigrum L; Src; osteoblast; piperyline.