Tang Y, Kitisin K, Jogunoori W, Li C, Deng CX, Mueller SC, et al. Progenitor/stem cells give rise to liver cancer due to aberrant TGF-beta and IL-6 signaling. Proc Natl Acad Sci U S A 2008;105:2445–2450. Copyright 2008 National Academy of Sciences, U.S.A. (Reprinted by Permission)
Cancer stem cells (CSCs) are critical for the initiation, propagation, and treatment resistance of multiple cancers. Yet functional interactions between specific signaling pathways in solid organ “cancer stem cells,” such as those of the liver, remain elusive. We report that in regenerating human liver, two to four cells per 30,000–50,000 cells express stem cell proteins Stat3, Oct4, and Nanog, along with the prodifferentiation proteins TGF-beta-receptor type II (TBRII) and embryonic liver fodrin (ELF). Examination of human hepatocellular cancer (HCC) reveals cells that label with stem cell markers that have unexpectedly lost TBRII and ELF. elf(+/−) mice spontaneously develop HCC; expression analysis of these tumors highlighted the marked activation of the genes involved in the IL-6 signaling pathway, including IL-6 and Stat3, suggesting that HCC could arise from an IL-6-driven transformed stem cell with inactivated TGF-beta signaling. Similarly, suppression of IL-6 signaling, through the generation of mouse knockouts involving a positive regulator of IL-6, Inter-alpha-trypsin inhibitor-heavy chain-4 (ITIH4), resulted in reduction in HCC in elf(+/−) mice. This study reveals an unexpected functional link between IL-6, a major stem cell signaling pathway, and the TGF-beta signaling pathway in the modulation of mammalian HCC, a lethal cancer of the foregut. These experiments suggest an important therapeutic role for targeting IL-6 in HCCs lacking a functional TGF-beta pathway.
The classical paradigm for the development of hepatocellular carcinoma (HCC) suggests that abnormal liver cell proliferation induces carcinogenesis.1 Telomere shortening is one mechanism thought to lead to reduced hepatocyte proliferation in end-stage cirrhosis resulting in cancer induction by loss of replicative competition.1 Additionally, it is clear that an altered microenvironment promoting tumor cell proliferation in cirrhosis is necessary. However, it is now apparent that a new paradigm of HCC pathogenesis is emerging in which hepatic stem cell populations give rise to tumor cells (Fig. 1).
The notion that many forms of human malignancy arise from stem cell populations is not novel.2 Stem cell populations are an obvious potential source of malignant progeny given their self-renewal capacity, plasticity, and cell turnover in injury and regeneration.2 However, experimental evidence suggesting that a number of malignancies develop from stem cells has only recently been published.2, 3 Although the demonstration of clonality within a malignancy suggests a single cell [that is, cancer stem cell (CSC)] origin, demonstrating a direct progression from a stem cell to malignancy is extremely difficult. Furthermore, much of the experimental evidence presented to date may also be interpreted as demonstrating that many malignancies have a phenotype similar to that of stem cells even if their origin is not from a stem cell population. A recent article by Tang et al.4 suggests that CSCs give rise to HCC involving changes in interleukin-6 (IL-6)/transforming growth factor beta (TGF-β) signaling. These investigators have demonstrated that within the regenerating human liver, there are small clusters of stem cells at a frequency of ∼1:10,000. These stem cells express the embryonic markers, octamer 4 (Oct4) and Nanog as well as TGF-β signaling proteins, TGF-β receptor type II (TBRII) and embryonic liver fodrin (ELF), a protein crucial for the propagation of TGF-β signaling. Importantly, these probable stem cells express hepatocyte (albumin) and bile duct (cytokeratin 19) markers. Furthermore, when 9 of 10 HCC specimens were examined, the Oct4-positive cells did not express TBRII or ELF. Therefore, Tang and colleagues hypothesized that stem cells involved in regeneration express TGF-β signaling proteins and when these are lost the stem cells have an enhanced potential to become HCC CSCs. Previous work by this laboratory has shown that HCC spontaneously develops in elf+/− mice at an incidence rate of approximately 40%.5 Tissue from these elf−/+ mice, when studied by microarray analysis, demonstrated markedly increased expression of IL-6/signal transducer and activator of transcription 3 (Stat3), Wnt, and cyclin-dependent kinase 4 signaling pathways. Additionally, Tang and colleagues studied human HCC and showed that it had increased Stat3 and inter-alpha-trypsin inhibitor-heavy chain-4 (ITIH4) expression. Therefore, to investigate the IL-6 role in HCC development, these investigators used itih4 (IL-6 regulatory protein) -deficient mice in which IL-6/Stat3 signaling was decreased. However, as the itih4−/− mice were normal, these mice were crossed with elf+/− to generate animals with significant suppression of IL-6 signaling. Interestingly, despite the ELF deficiency, these mice did not show a high incidence of HCC (approximately 4%) suggesting an interaction between TGF-β and IL-6 signaling in HCC development. Therefore, HCC development from Oct4+/Nanog+ CSCs appears to require diminished TGF-β signaling in combination with active IL-6 signaling.
Although the data from this group are intriguing, they need to be interpreted with some caution. First, the demonstrated stem cell population is at a much lower frequency than that widely attributed to hepatic stem cell/progenitor populations.6 Clearly, this may represent a subpopulation of cells. Furthermore, even in regenerating livers, there were no Oct4-positive TGF-β signaling–deficient cells, and this brings into doubt the causative link. It is unclear if CSCs develop because of a loss of TGF-β signaling proteins or if these proteins are lost only once CSCs have arisen. Additionally, knockout animals develop multiple compensatory pathways, and it is unknown if these are the true oncogenic pathways or if the engineered gene defect directly accounts for the HCC.
The importance of the work resides in the potential identification of a CSC population reliant on TGF-β and IL-6 signaling. Strategies aimed at this CSC population and in particular the modulation of IL-6 signaling may be novel and important approaches to both preventing and treating HCC. Further work needs to establish if these cells are representative of a subgroup of resident stem cells, if IL-6 blockade changes the rate of HCC formation, and, most importantly, if progressive liver injury is causative of this phenotype or an epiphenomenon that simply develops with the onset of HCC. In conclusion, HCC does appear to arise from CSCs, and we now have some understanding of the pathobiological pathways underlying this process.