Lineage-Specific Functions of LKB1 in CML and B Lymphoid Blast Crisis
Authors: Chan Lai N , Geng Huimin , Muschen Markus 
Published: October 2019

Abstract
Abstract 34

Background:
The serine-threonine liver kinase B1 (LKB1, also called STK11) acts as negative regulator of aerobic glycoloysis, a metabolic pathway that is typically used in cancer cells (commonly referred to as ‘Warburg effect’). LKB1, together with AMPKs, shifts the metabolic balance from aerobic glycolysis to oxidative phosphorylation and thereby reverses the metabolic program of cancer cells and functions as tumor suppressor. Recently, it has been shown that LKB1 plays a critical role in the maintenance of quiescence and metabolic homeostasis of hematopoietic stem cells (HSCs).

Hypothesis:
In the present study, we focused on the roles of LKB1 in BCR-ABL1-driven leukemias including CML and B lymphoid blast crisis/Ph+ ALL (LBC). While LKB1 is widely seen as tumor suppressor in solid tumors, we found that high expression levels of LKB1 at diagnosis predict poor clinical outcome in patients with high risk acute lymphoblastic leukemia (n=207; COG P9906 trial; p=0.0204). In addition, high levels of LKB1 expression correlate with positive minimal residual disease (MRD+, p=0.0323) status in patients. These findings were unexpected and seem to contradict the common notion of LKB1 as a tumor suppressor.

Results:
To study the function of LKB1 in CML and B lymphoid blast crisis/Ph+ ALL (LBC), we developed a mouse model for inducible ablation of Lkb1 in BCR-ABL1-transformed hematopoietic stem and progenitor cells (CML-like) and B cell progenitors (LBC). To this end, Lkb1-fl/fl bone marrow hematopoietic stem and progenitor cells and B cell precursor cells were transformed with BCR-ABL1 and transduced with tamoxifen-inducible Cre. Unexpectedly, Cre-mediated deletion of Lkb1 had opposite effects in CML and LBC. While Lkb1-deletion in CML results in an initial proliferative burst of the leukemia cells, the vast majority of B cell lineage LBC cells undergo rapid cell cycle arrest. These findings are consistent with changes of cell cycle checkpoint proteins in response to Lkb1 deletion in CML and B cell lineage LBC. While Lkb1 deletion in CML cells results in downregulation of Arf, p53 and p27 levels, Lkb1 deletion in B lineage LBC cells resulted in upregulation of Arf and p27. In addition, Lkb1 deletion in CML resulted in inactivation of AMPK, a known substrate of LKB1, as well as enhanced activation of mTORC1. By contrast, while deletion of Lkb1 in B cell lineage LBC cells resulted in inactivation of AMPK as shown by reduced phosphorylation of AMPKα T172, there was reduction in mTORC1 activity based on diminished levels of phospho-p70 S6 kinase and S6 following LKB1 deletion.
The effects of LKB1 on sensitivity of BCR-ABL1 CML and B lineage LBC cells to Imatinib were also examined. Lkb1-deficient ALL cells became more sensitive to Imatinib treatment. On the other hand, initial Lkb1 deletion rendered CML cells more resistant to Imatinib treatment. When primary patient-derived Ph+ ALL cells (n = 3) were treated with Imatinib, upregulation of phospho-LKB1 (S428) was observed. Finally, LKB1 was also shown to regulate energy homeostasis in CML and B cell lineage LBC in different manners, as measured by monitoring ATP and lactate production.

Conclusions:
Here we show that Lkb1 plays critical roles in mediating proliferation and cell growth in BCR-ABL1-driven leukemias. While LKB1 is widely seen as a tumor suppressor that limits aerobic glycolysis in cancer cells according to the Warburg effect, our findings demonstrate that LKB1 has lineage-specific functions in BCR-ABL1 driven leukemias. While LKB1 function in CML resembles its tumor suppressor function in solid tumors, LKB1 is critical for survival and proliferation on B cell lineage CML blast crisis and Ph+ ALL. The finding of a divergent role of Lkb1 in CML and B cell lineage LBC/Ph+ ALL is relevant because small molecule inhibitors of AMPK and mTORC1 are currently under development for the treatment of BCR-ABL1-driven leukemias.

Disclosures:
No relevant conflicts of interest to declare.