The signal transducer and activator of transcription 3 is an oncogenic protein that is capable of inducing tumours and enhancing cancer proliferation. In its inactive form, it is found predominantly in the cytoplasm. Phosphorylation at Tyr-705 causes it to dimerise and translocate to the nucleus where it binds to specific promoter sequences on its target genes. Blocking signalling to STAT3 by dominant-negative (DN) STAT3 mutant or antisense STAT3 oligonucleotides inhibits cancer cell growth, showing that STAT3 is crucial to the survival and growth of tumour cells (Kaptein et al, 1996
; Aoki et al, 2003
; Calvin et al, 2003
). Constitutive STAT3 signalling participates in oncogenesis by stimulating cell proliferation, mediating immune evasion, promoting angiogenesis, and resistance to apoptosis induced by conventional therapies (Shen et al, 2001
; Buettner et al, 2002
; Real et al, 2002
; Wang et al, 2004
). There have been several strategies to inhibit the STAT3 pathway as a therapeutic approach in treating cancers, such as by using small molecules and DNA decoys (Lui et al, 2007
). In addition, other inhibitors of STAT3 have also been reported including Cucurbitacin Q, which inhibits STAT3 phosphorylation (Sun et al, 2005
), E804 that inhibits both Src and STAT3 (Nam et al, 2005
); Stattic (Schust et al, 2006
) and S3I-201 (Siddiquee et al, 2007
), which inhibit STAT3 dimerisation, and SD-1029, which inhibits STAT3 nuclear translocation (Duan et al, 2006
One difficulty with targeting proteins directly is that the targeted proteins may often also control other vital pathways sometimes leading to severe unwanted effects. LLL-3, a structural analogue of STA-21 with a smaller molecular weight and the added benefit of being easier to synthesise, may be a more suitable agent for targeting cancer cells with constitutively activated STAT3 pathway.
In this study, we evaluated the inhibitory efficacy of LLL-3 in human GBM cells. Our results show that, LLL-3 inhibits STAT3 activities, inhibits cell viability, and induces apoptosis in U87, U251, and U373 glioblastoma cells. We further show that LLL-3 inhibits tumour growth and inhibits tumour invasiveness as well as prolongs survival in an intracranial U87 glioblastoma xenograft mouse model. These results suggest that inhibition of STAT3 pathway may be an effective therapeutic approach for treating human glioblastoma.
Given that the STAT3 pathway may be necessary for renewal in some embryonic stem cells (Niwa et al, 1998
), there is legitimate concern that indiscriminate inhibition of the STAT3 pathway may lead to side effects of some stem cells. However, as STAT3 phosphorylation in normal cells is transient, LLL-3 would likely have little effect on the proliferation of normal cells as suggested by its effect on normal bladder smooth muscle cells. Furthermore, studies using normal mouse fibroblasts showed that disrupting STAT3 signalling has much less profound effect in normal human and murine cells (Catlett-Falcone et al, 1999
; Niu et al, 1999
; Bowman et al, 2001
; Burke et al, 2001
; Song et al, 2005
), suggesting that blocking STAT3 signalling may not be grossly toxic. Based on our findings, LLL-3 appears to be a potential therapeutic agent for human glioblastoma cells and possibly other tumours that have constitutively active STAT3. In addition to its apoptotic effects it may potentially render cancers more susceptible to other cytotoxic agents. It should be of interest to further explore the possibility of using LLL-3 as a potential agent for human glioblastoma treatment.