Gambogic acid mediates transferrin receptor-1 phosphorylation leading to jnk-dependant apoptosis

3399 Transferrin receptor 1 (TfR1) is a ubiquitous type II membrane receptor with 63aa in the N-terminal cytoplasmic region. TfR1 is highly expressed in proliferating normal cells and cancer cells but not quiescent cells. Overexpression of TfR1 in highly proliferating cells and cancer cells is thought to be a result of the increased requirement of iron uptake necessary for cell growth. Recently, our study has shown that total iron content, measured by atomic absorption, is comparable among various types of cells expressing different levels of TfR1. These results suggest that TfR1 may have other functions because TfR1 gene expression could be independent of iron status. In the present study, we have tested a hypothesis that, in addition to its known function of iron transport through transferrin, TfR1 is a signaling molecule that plays a role in cell survival and apoptosis. Using gambogic acid (GA), an apoptosis inducer and recently identified TfR1 ligand, we have found that GA induced tyrosine phosphorylation of TfR1 protein, most likely in theY20 position of the cytoplasmic region, in primary normal human epidermal keratinocytes and breast cancer MDA-MB-231 cells. On the contrary, the iron transport protein transferrin and inorganic iron did not induce tyrosine phosphorylation of TfR1. Furthermore, GA, but not transferrin, activated the JNK and p38 signal pathways and cell apoptosis. The JNK kinase inhibitor SP600125 partially blocked GA-induced apoptosis and caspase-3 and -8 cleavages. Upon GA treatment, Src was activated exerting its anti-apoptotic effect and the Src inhibitor PP2 promoted GA-induce apoptosis and JNK activation. Our results indicate that in addition to iron transport, TfR1 may play a role in regulating cell survival and apoptosis. This work is supported by R21 CA132684.