Leptin increases proliferation of human steosarcoma cells through activation of PI(3)-K and MAPK pathways.
BACKGROUND
Serum leptin levels are strongly and directly related to fat body mass (FBM). Bone mineral density (BMD) increases with FBM, and obesity has a protective effect against osteoporosis. We have previously demonstrated that leptin therapy has a significant effect in preventing ovariectomy-induced bone loss in rats and leptin also exerts direct osteogenic effects in vitro. To obtain a better understanding of the physiology and pharmacology of leptin in bone metabolism, we evaluated the leptin-induced signal transduction pathways and proliferative response in the human osteosarcoma cell line Saos-2.


METHODS
Saos-2 cell lines were used. Leptin receptor common form (OB-Ra) and long form (OB-Rb) were detected by RT-PCR and immunocytochemistry. PI(3)-K activity was immunoprecipitated using antibodies directed against tyrosine-phosphorylated proteins and IRS-1. The activated form of p42/p44 MAPK was investigated in cytosolic extracts of confluent Saos-2 in response to leptin.


RESULTS
In this study, we tested the hypothesis that leptin might be a mediator linking obesity and bone cell proliferation. We found that Saos-2 cells expressed OB-Ra and OB-Rb. Leptin (10 nmol/L - 2 umol/L) caused a significant increase in the activation of PI (3)-K that was accompanied by an increase in cell proliferation dose dependently based on the [3H]-thymidine incorporation. The specific PI (3)-K inhibitors LY294002 and wortmannin blocked leptin-induced cell proliferation. Interestingly, leptin activated MAPK and the specific MAPK-inhibitor PD98059 blocked DNA synthesis induced by leptin.


CONCLUSIONS
Our data support the hypothesis that leptin may increase bone mass by stimulating osteoblast proliferation through activation of the P1 (3)-K and MAPK signaling pathways.
Burguera
Bartolome
B
Division of Endocrinology, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA. bburguera@hsd.es
Brunetto
Amy
A
Garcia-Ocana
Adolfo
A
Teijeiro
Roseta
R
Esplen
James
J
Thomas
Thierry
T
Couce
Marta E
ME
Zhao
Allan
A
D001842:Bone and Bones / Q000378:metabolism; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D055504:Insulin Receptor Substrate Proteins; D020738:Leptin / Q000378:metabolism* / Q000502:physiology*; D020935:MAP Kinase Signaling System; D012516:Osteosarcoma / Q000378:metabolism* / Q000473:pathology*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D015398:Signal Transduction; D014443:Tyrosine / Q000737:chemistry
D004791:Enzyme Inhibitors; C526292:IRS1 protein, human; D055504:Insulin Receptor Substrate Proteins; C526296:Irs1 protein, rat; D020738:Leptin; D010750:Phosphoproteins; D014443:Tyrosine; D019869:Phosphatidylinositol 3-Kinases