Neferine, an alkaloid ingredient in lotus seed embryo, inhibits proliferation of human osteosarcoma cells by promoting p38 MAPK-mediated p21 stabilization. Identification of natural products that have antitumor activity is invaluable to the chemoprevention and therapy of cancer. The embryos of lotus (Nelumbo nucifera) seeds are consumed in beverage in some parts of the world for their presumed health-benefiting effects. In this report we studied the effects of neferine, a major alkaloid component in lotus embryos, on human osteosarcoma cells and the underlying mechanisms. We found that neferine possessed a potent growth-inhibitory effect on human osteosarcoma cells, but not on non-neoplastic human osteoblast cells. The inhibitory effect of neferine on human osteosarcoma cells was largely attributed to cell cycle arrest at G1. The induction of G1 arrest was p21(WAF1/CIP1)-dependent, but was independent of p53 or RB (retinoblastoma-associated protein). The up-regulation of p21 by neferine was due to an increase in the half-life of p21 protein. We examined four kinases that are known to affect the stabilization of p21, and found that p38 MAPK and JNK were activated by neferine. However, only SB203580 (an inhibitor of p38), but not SP600125 (the inhibitor of JNK), can attenuate the up-regulation of p21 in response to neferine. Furthermore, the p21-stabilizing effect of neferine was abolished when p38 was silenced by RNA interference. Finally, we showed that neferine treatment led to an increased phosphorylation of p21 at Ser130 that was dependent on p38. Our results for the first time showed a direct antitumor effect of neferine, suggesting that consumption of neferine may have cancer-preventive and cancer-therapeutic benefit.Copyright © 2011 Elsevier B.V. All rights reserved.