Abstract 2550: B7-H3 induces paclitaxel resistance in human breast cancer cell lines
Authors: Tekle Christina , Liu Hao , Chen Yih-Wen , Mælandsmo Gunhild M , Fodstad Oystein , Tan Ming 
Published: February 2011

Abstract
B7-H3 is previously known as an immunoregulatory protein with either stimulatory or inhibitory effects on the activation of T-cells. The correlation between high expression of B7-H3 and poor prognosis has been reported in many human tumor types and this has been ascribed to the putative immune-regulatory function of the protein. Previously, we reported results indicating a direct association between B7-H3 expression and metastasis, whereas in this study we examined the role of B7-H3 with respect to paclitaxel resistance in metastatic breast cancer cells as well as the underlying mechanisms. The results show that silencing B7-H3 increased the sensitivity to both MDA-MB-231 and MDA-MB-435 breast cancer cells to paclitaxel. This drug is known to exert its cell killing effect through induction of apoptosis, and our data confirmed that the silencing of B7-H3 sensitized breast cancer cells to paclitaxel-induced apoptosis. The extent of apoptosis was investigated by checking the amount of Annexin-5 stained cells, a marker for early stage apoptosis and for the amount of TUNEL positive cells, which reflects late stage apoptosis. To confirm the apoptotic response observed by the use of flow cytometry, we examined the cleavage of poly (ADP-ribose) polymerase (PARP), another marker of apoptosis, and found that MDA-MB-231 and MDA-MB-435 B7-H3 knockdown cells showed increased PARP cleavage compared to the vector control cells upon treatment with paclitaxel.
We then studied whether the effects of B7-H3 on drug resistance could be related to molecules and signaling pathways known to be involved in the apoptotic response. Interestingly we discovered that the STAT3 pathway, also known to be involved in chemo-resistance, was affected by B7-H3, with markedly decreased phosphorylation of STAT3 Tyr705 in B7-H3 knockdown cells. Furthermore, two members of the IAP family of antiapoptotic proteins, survivin and Mcl-1 which are direct downstream targets of Stat3, were both repressed by the decrease in B7-H3 and Stat3 phosphorylation levels.
The phosphorylation level of JAK2, which is a well-known upstream component of STAT3 signaling, was also significantly decreased in B7-H3 knockdown cells.
Finally, in vivo experiments showed that in nude mice treated with paclitaxel, the growth rate of tumors in the mice injected s.c with MDA-MB-435 B7-H3 knockdown cells was significantly reduced in contrast to the growth rate in mice injected with MDA-MB-435 control cells. Taken together, our data demonstrates that in breast cancer cells B7-H3 expression is associated with antiapoptosis and paclitaxel resistance, at least partly through the STAT3 pathway
The novel findings of this study have important implications for the design of new approaches targeting B7-H3 overexpressing breast cancers.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2550.