In this study, we demonstrated that low concentration of GSK3β inhibitor, BIO, suppressed the proliferation of FLT3-ITD mutated cells by elevating cyclin D2 expression, reducing p21 expression, inhibiting DNA polymerase and eventually causing G1 cell cycle arrest. BIO treatment also promoted the apoptosis of representative FLT3-mutated, MV4-11 cells, through regulating the expression of caspase3 and inhibited tumor growth in vivo. Metabolomics analysis indicated that BIO regulated glycolysis, as indicated by the increased levels of pyruvate and ATP. Therefore, our findings suggest that BIO could be a potential drug for treatment of AML by regulating cancer cell growth and metastasis.
Since GSK3 is a key kinase associated with the regulation of multiple biological process including proliferation, apoptosis and differentiation of leukemia cells, its inhibitors have been considered as potential therapeutic agents for AML [22, 23]. Leukemia with FLT3-ITD mutations accounts for a rare form of AML with high rates of relapse and drug resistance. Several studies have focused on the interactive mechanisms between GSK3 and FLT3-ITD. It is initially found that Linifanib, an inhibitor of constitutive activation of FLT3, suppressed phosphorylation of GSK3β in FLT3-ITD mutant cells. Unexpectedly, inhibition of GSK3 enhanced Linifanib-induced apoptosis of ITD mutant cells, suggesting the association between GSK3 and FLT3 [24]. In contrast, Woolley et al. reported that GSK3 and FLT3 had opposing functions. In their study, PKC412, an inhibitor of FLT3, decreased p22phox, the small membrane-bound component of the Nox complex acting in the regulation of reactive oxygen species (ROS), an effect that was reversed by GSK3 inhibition [25]. Another study showed that sorafenib, a FLT3-ITD inhibitor, suppressed the activation of checkpoint kinase Chk1 to trigger DNA-damage and augmented the apoptosis of MV4-11 cells that was induced by etoposide treatment, meanwhile inactivation of GSK3 recovered durative Chk1 activation and notably abbreviated etoposide-induced apoptosis where it seems like GSK3 played opposite roles in regulating the activation of Chk1 and apoptosis of leukemia cells [26]. In addition, a genome-wide CRISPR screen revealed that reduction of GSK3 enhanced drug resistance to ACC20, a selective inhibitor of FLT3, in AML patients, suggesting that inhibition of GSK3 may reduce the efficiency of clinical treatment with FLT3 inhibitors [27]. Although the above studies investigated the potential interactive and/or synergistic association between GSK3 and FLT3, it was unclear whether GSK3 inhibitors specifically regulated the biological function of leukemia cells with FLT3-ITD and not those with other mutations.
BIO is a selective inhibitor of GSK3β that was initially demonstrated to suppress proliferation of human leukemia TF-1, HL-60, K562, and U937 cells [28]. Afterwards, experimental results showed that BIO promotes the proliferation of E26 transforming sequence-related gene (ERG)-induced K562 cells [29], indicating that GSK3 plays a role in regulating leukemia cell viability. Moreover, Wang et al. reported that BIO significantly inhibited all-trans retionic acid (ATRA)-induced apoptosis of acute promyelocytic leukemia HL60 cells [30]. Although the above studies demonstrated that BIO plays a role in regulating the growth of leukemia cells, the underlying mechanisms as well as its effects on different types of cell were unclear. Interestingly, we found that BIO selectively suppressed the proliferation of cells with FLT3-ITD mutations such as MV4-11 and MOLM13 cells, but did not affect the proliferation of leukemia cell lines harboring other types of mutations. Subsequent experiments reveled that BIO inhibited cell proliferation by arresting cells in the G1 phase. There is need to determine if BIO inhibits GSK3 function by inhibiting interaction between GSK3 and FLT3 or by affecting downstream targets of GSK3 that are associated with FLT3 function.
We then demonstrated that BIO induced apoptosis of MV4-11 by regulating caspase3, AKT (S437) and p53 pathways without changing other 16 intracellular proteins involved in apoptotic signaling pathways. However, these unaffected proteins were reported to be involved in the apoptosis of MV4-11 cells in other studies e.g. loss of ataxia telangiectasia mutated (ATM) mitigated the effect of miR-100 exhaustion on cell viability and apoptosis in AML cells [31], provirus integrating site moloney murine leukemia virus 3 (PIM3) overexpression enhanced AML cell proliferation and inhibited instinctive apoptosis by phosphorylating BAD (pBAD) at Ser112 [32], and Polyphyllin I affected the apoptosis of AML cell lines by regulating expression of phosphorylated-JNK [33]. Future studies are required to evaluate the precise mechanism underlying the effect of BIO on the apoptotic process and to identify downstream target proteins in the pathway triggering attenuated cell viability of AML cells with FLT3 mutation.
In addition to apoptosis, in vitro metabolomics profile analysis and metabolite detection showed that BIO also regulates cell viability by inhibiting the production of two key metabolites, pyruvate and ATP that play significant roles in the glycolysis of AML cells. Irregular metabolism of cancer cells characterized by excessive glycolysis has been shown to promote malignant proliferation and metastasis to aggravate cancer development. Increased ATP and pyruvate levels were observed in acute myeloid leukemia cells and transgenic ALL mouse model, concomitantly with high glucose uptake, glycolytic capacity and reserve, and were associated with aberrant cell proliferation resulting from uncontrolled cell cycle [34, 35]. Here, we found that the decreased levels of ATP and pyruvate following BIO treatment were partly accompanied by suppressed expression of ENO1, implying a potential strategy to restore basic energy metabolism against highly proliferative properties in AML cells with FLT3-ITD. However, further studies are required to identify the specific mechanisms underlying BIO inhibition of the inordinate glycolysis during AML development.
Collectively, we demonstrated that BIO, an effective GSK3β inhibitor, suppresses cell proliferation by arresting cells in the G1 phase in vitro and tumor growth in vivo, and induces cell apoptosis through caspase3 pathway in MV4-11 cells with FLT3 mutation. The metabolomics profiling analysis also reveals that BIO suppresses tumors by inhibiting ATP and pyruvate accumulation. These findings indicate that BIO could be an attractive treatment agent for AML with FLT3 mutations.