Recently, we have shown that inhibition of endogenous AMPK by expression of the dominant negative mutant of AMPK α1 subunit or its shRNA augments malignant behaviors of prostate cancer cells, whereas activation of AMPK by AICAR or introducing LKB1 to cancer cells containing loss-of-functional mutations causes changes in an opposite direction. The effects of LKB1/AMPK might be mediated by concerted actions of various downstream targets (Zhou et al., 2009
). Among them, we have identified two novel targets, LITAF and TNFSF15. In the present study, we have for the first time demonstrated that activation of AMPK stimulates expression of LITAF, which in turns upregulates TNFSF15, and that knockdown of LITAF accelerates prostate cancer cell proliferation and tumor development. Our data also show that TNFSF15 remarkably inhibits the growth of prostate cancer cells and angiogenesis and induces morphological transition from malignant to benign tumors.
AMPK has been shown to play important roles in mediating the tumor suppressive function of LKB1. To understand the underlying mechanisms, our exciting but challenging task is to identify new targets downstream of AMPK in addition to those well characterized, such as mTOR, FASN, p53, p27 and FOX3a (Luo et al.2010). In this study, we have found that AMPK regulates the transcription of LITAF, which then binds to a specific sequence in the promoter region of TNFSF15 and regulates its transcription. Currently, we do not know how AMPK regulates the transcription of LITAF. It is possible that AMPK indirectly exerts such an effect through regulation of an unidentified transcription factor. However, our data do not support that this occurs through p53. Therefore, it will be our interest to identify the transcription factor that mediates this effect.
Existing data support that LITAF is a transcription factor involved in regulating production of proinflammatory cytokines in response to LPS (Tang et al., 2005
; Tang et al., 2006
). An additional function of LITAF relates to sorting and degradation of plasma membrane proteins by lysosome and late endosome (Niemann et al., 2006
). Mutations of LITAF/SIMPLE have been suggested to alter this second function, which may play a role in the pathogenesis of CMT1C (Beauvais et al., 2006
; Bennett et al., 2004
; Latour et al., 2006
). Thus far, no direct evidence indicates a role of LITAF in tumorigenesis. However, several recent studies suggest that there is such a link. For instance, homozygous deletion the LITAF
gene and promoter hypermethylation are reported in some types of lymphomas (Mestre-Escorihuela et al., 2007
). Secondly, LITAF was found to associate with WW domain oxidoreductase (WWOX) (Ludes-Meyers et al., 2004
). The latter has been found altered in multiple types of cancer and its ectopic expression inhibits xenograft tumor growth of breast cancer cells (Chang et al.2010
). Therefore, it will be interesting to assess the direct effect of LITAF on tumorigenesis and elucidate the underlying mechanisms.
Although LITAF is known to upregulate transcription of TNFα, little information has been documented on its role in regulating other TNF superfamily members. A possible link between LITAF and TL1A has been suggested by the findings that their expressions are increased in macrophages of inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis (Picornell et al., 2007
; Stucchi et al., 2006
; Young and Tovey, 2006
). Interestingly, one study has so far reported that bacterially expressed chicken LITAF or LITAF expressed, secreted and purified from COS7 cells, stimulates transcription of TNFSF15 in chicken macrophages (Hong et al., 2006
). This report is coincidently in line with our findings where we placed LITAF as an upstream modulator of TNFSF15 in response to AMPK activation.
A clinical investigation has indicated that high levels of TNFSF15 are associated with increased survival rate of breast cancer patients (Parr et al., 2006
). Numerous studies have reported that exogenous application of different isoforms of recombinant TNFSF15 to culture media or to animals inhibits tumor cell growth and tumor development and angiogenesis (Haridas et al., 1999
; Hou et al., 2005
; Pan et al., 2004
; Yue et al., 1999
; Zhai et al., 1999a
; Zhai et al., 1999b
). Our present study demonstrates that bovine aorta endothelial cells are more sensitive to TNFSF15 than LNCaP cells, which is in accord with previous reports that TNFSF15 exerts anti-tumor effects mainly through inhibition of angiogenesis (Sethi et al., 2009
). Consistently, our animal study showed that the treatment of xenograft tumors with TNFSF15 not only inhibits angiogenesis, but also induces morphological changes characteristic of benign tumors.
In the end, we would emphasize that our present study has for the first time demonstrated that AMPK upregulates LITAF, which in turn increases the expression of TNFSF15. The regulation occurs at the transcriptional level. Collectively, our data suggest that LITAF and TNFSF15 may function as tumor suppressors. Therefore, we propose a model illustrated in . LITAF serves as one of downstream targets of AMPK to inhibit cancer cell growth, possibly by multiple mechanisms, one of which is TNFSF15 that potently inhibits angiogenesis. As for other functions of LITAF, such as those related to lysomal/late endosome trafficking and degradation of membrane proteins, it is worthwhile to explore if they participate in tumorigenesis. Overall, our work points to a new direction in investigating the correlation of tumor progression and prognosis with expression levels and activity of AMPK, LITAF and TNFSF15 and the mechanism by which they are involved in tumorigenesis. Likewise, our data also suggest that targeting AMPK pathway would represent a promising approach for prevention and treatment of cancer, as it acts on both tumor cells and angiogenesis.
Model of LITAF regulation of tumorigenesis