Abstract. Signal transducers and activators of transcription 3 (STAT3) signaling is persistently activated in many types of cancer cells, and represents a valid target for anticancer drug design. However, few reports have described the constitutive activation of STAT3 in human sarcoma cells. In this study, we demonstrate that the STAT3 signaling pathway is constitutively activated in human rhabodomyosarcoma cells (RH28, RH30, and RD2). We also investigated the inhibitory effects of two newly developed small molecules, LLL12 and FLLL32, on the STAT3 signaling pathway in human rhabodomyosarcoma cells. Both LLL12 and FLLL32 downregulated STAT3 constitutively and interleukin-6 (IL-6) stimulated phosphorylated STAT3 (p-STAT3). The inhibition of STAT3 via LLL12 and FLLL32 was confirmed by the inhibition of STAT3 DNA binding activity. The downstream targets of STAT3, cyclin D1, Bcl-xL, and survivin were also downregulated by LLL12 and FLLL 32 at both messenger RNA and protein levels. The potency of LLL12 and FLLL32 to inhibit proliferation/viability in human rhabodomyosarcoma cells (RH28, RH30, and RD2) was higher than that of the 5 previously reported Janus kinase 2 (JAK2)/ STAT3 inhibitors (LLL3, WP1066, Stattic, S3I-201, and AG490) and curcumin. Thus, in this study, we investigated the inhibitory effects of two STAT3 inhibitors, LLL12 and FLLL32, on the STAT3 signaling pathway in human rhabodomyosarcoma cells; we also demonstrated their higher potency in inhibiting proliferation on human rhabodomyosarcoma cells as compared to other five JAK2/STAT3 inhibitors and curcumin.
IntroductionRhabdomyosarcoma, a highly malignant mesenchymal tumor, originating from immature striaed muscle, is the most common soft tissue sarcoma in children (1). Patients with rhabodomyosarcoma have a high relapse rate of 30% (1,2). Despite aggressive therapy, incorporating surgery, radiation therapy and dose-intensive chemotherapy, mortality rate of relapsing rhabodomyosarcoma is still 50-80% (2,3). Thus, development of higher potency and less toxicity anticancer drugs is pivotal to improve survival for patients with rhabdomyosarcoma.Signal transducers and activators of transcription 3 (STAT3) signaling is constitutively activated in various types of cancer cells, such as breast cancer, prostate cancer, leukemia and lymphomas (4). Activated STAT3 binds the promoters of the targeted genes to control cell growth and survival. Thus, inhibition of STAT3 signaling pathway seems to be an attractive strategy for anticancer drug design (5,6). For the past few years, several small molecules of the JAK2/STAT3 inhibitors, such as WP1066 (7-9), LLL3 (10) and AG490 (11-13), Stattic (14,15) and S3I-201 (16,17), have been reported with a wide range of antitumor effects. Although persistent activation of STAT3 has been detected in diverse cancer cell lines and tissue, only few reports have described elevated p-STAT3 in rhabdomyosarcoma.