Leptin expression by annulus cells in the human intervertebral disc

Abstract

Background context It is now known that leptin acts not only as a metabolic signal related to energy homeostasis but also as an endocrine hormone regulating traditional endocrine systems and neuroendocrine function in various cells. It participates in bone remodeling and acts as a growth factor stimulating proliferation. Expression of leptin and the presence of leptin receptors have not been explored in disc tissue. Purpose To determine (1) whether leptin is produced by cells in the human annulus in vivo, (2) whether annulus cells have leptin receptors in vivo and in vitro, and (3) whether measurable amounts of leptin are produced during three-dimensional culture of human annulus cells. Study design/setting Studies were approved by the human subjects Institutional Review Board. Surgical and donor disc tissue was obtained and assessed by using immunocytochemistry of paraffin-embedded disc tissue. Annulus cells were also cultured from disc specimens and conditioned media assessed for the production of leptin during three-dimensional culture. Patient sample Disc tissue was examined from 7 young subjects and 29 adult subjects. Outcome measures Immunodetection of leptin and leptin receptors in cells of the human annulus; conditioned media was analyzed for production of leptin in vitro by human disc cells. Methods Human annulus tissue and cultured cells were examined by using immunohistochemical methods to identify the presence of leptin and leptin receptors. Human disc cells were assayed for leptin production in three-dimensional culture. Results Immunocytochemistry showed the presence of intracellular leptin and the presence of leptin receptors in some (but not all) annulus cells in the human disc. Production of leptin by annulus cells was further confirmed by assays of conditioned media from three-dimensional annulus cell culture. Conclusions These novel studies identify the presence of a heretofore unrecognized cytokine/hormone and its receptor in human annulus cells. Because of the mitogenic role of leptin in other tissues, the present work points to the importance of future studies to explore whether leptin has a mitogenic function in maintaining disc cell numbers.