The specific role of the Alox5 gene in regulating the function of LSCs but not normal HSCs shows a new mechanism for how cancer and normal stem cells distinctly self-renew and differentiate, and provides a potential novel strategy for curative therapy of CML induced by BCR-ABL. Although Alox5 and its related pathways were identified in LSCs of CML in this study, they may play a critical role in regulating LSCs of myeloid leukemias induced by other types of oncogenes. This idea is supported by our preliminary finding that the development of Tel-PDGFR-beta induced myeloid leukemia was also largely prevented in the absence of the Alox5 gene (data not shown). In addition, similar phenomenon that a gene specifically regulates cancer stem cells but not normal stem cells may likely be found in other types of cancers.
The mechanism for the specific role of the Alox5
gene in regulating function of LSCs but not normal HSCs remains to be further elucidated. We showed the differential regulation of the beta-catenin gene in wild type and Alox5
deficient LSCs, partially explaining the specific inhibitory effect of Alox5
deficiency on LSCs but not on normal HSCs. It is reasonable to think that these two types of stem cells utilize different pathways for self-renewal and differentiation, and Alox5
represents a critical difference in these stem cells. It is likely that BCR-ABL stimulates transcription of Alox5
to jointly form a unique pathway critical for LSC function. Because the transcriptional activation of Alox5
by BCR-ABL is not inhibited by the BCR-ABL kinase inhibitor imatinib, this at least partially explains why imatinib does not inhibit LSCs in CML mice 23
. It is hopeful that targeting Alox5
in combination with a BCR-ABL inhibitor that eliminates BCR-ABL protein, such as a heat shock protein 90 inhibitor 40
, would lead to a better control of CML. 5-LO inhibitor and imatinib could also be a good combination, with one inhibiting LSCs and another eliminating differentiated leukemia cells.
We show that Alox5 deficiency causes the depletion of LSCs in CML mice, and we believe that three biological changes (blockade of differentiation, asymmetric cell division and apoptosis) contribute to the eradication of these stem cells. At present, it is difficult to know which of the three biological changes plays a major role in depleting LSCs, and further mechanistic studies are required to answer this question.
No studies have shown a role of Alox5
in regulation of LSCs. Our findings in this study first demonstrate that Alox5
is a critical regulator of LSCs in CML, providing an in vivo
system for investigating underlying molecular mechanisms. Human CML microarray studies have shown that Alox5
is differentially expressed in CD34+
CML cells 41,42
, suggesting a role of Alox5
in human CML stem cells. An in vitro
study also supports the role of Alox5
in CML, and in this study the treatment of CML blast cells in culture with 5-LO inhibitors reduced cell proliferation 43
, although genetic approach is required to rule out any off-target effects. Our study shows complete eradication of myeloid leukemia in mice by removing and inhibiting 5-LO, prompting us to test this novel therapeutic strategy in human CML patients in the future. Alox5
function has been linked to many important signaling pathways such as p53 26
, NF-kB 31
, and PI3K 31
. PI3K is regulated by Pten, which plays a critical role in AML stem cells in mice 16
. Thus, inhibition of Alox5
function may hold a promise for treating other types of malignant diseases. In addition, the impairment of LSCs in CML development by inhibiting Alox5
function also suggests that Alox5
may activate unknown pathways, and investigation of this Alox5
network is critical to understanding the mechanisms by which LSCs survive, self-renew, and differentiate. Besides LSCs in CML, specific signaling networks are likely found in other cancer stem cells, relative to their normal stem cell counterparts.