Leukemia is a form of cancer that affects the blood and bone marrow of human beings, resulting in an increased abundance of white blood cells. A specific type of leukemia, acute myeloid leukemia (AML) occurs as a result of myeloid cells, immature cells that differentiate into blood-producing cells other than that of white blood cells. In the case of AML, aggressive leukemic blast cells quickly proliferate in the bone marrow and blood (Minzel et al., 2018).

To prevent leukemic cell accumulation, researchers created oral drugs used on mice models that inhibited the functions of casein kinase 1-alpha (CK1α), a kinase that phosphorylates molecules and promotes tumor progression, and cyclin-dependent kinases (CDK) 7 and 9, transcriptional kinases that promote RNA transcription. The researchers also hypothesized that ablation (scarring) of the CK1α can activate p53, a protein which recognizes whether a cell should induce DNA repair or apoptosis (Samarasinghe, 2013).

CK1α found in cells can operate in various ways including forming a complex with Beta-catenin (β-catenin), a semi-cancerous protein that regulates gene transcription and cellular communication. When the complex is formed, CK1α phosphorylates the β-catenin, allowing for degradation. However, when β-catenin is not in complex with CK1α, it allows for the proliferation of β-catenin in the cell and translocating into the nucleus where it promotes uncontrolled gene expression, allowing for leukemic cell build up (Cruciat, 2014). When there is an abundance of CK1α in a cell, it suppresses the action of

On the other hand, the researchers observed that CK1α inhibition by the target drugs activated p53. In (Figure 1A), the researchers displayed a chart showing the number of cells that existed after knocking out/inhibiting CK1α. When there is an abundance of CK1α in a cell, it suppresses the action of programmed cellular death, thus allowing for the division of leukemic cells to continue. The chart displayed low cell numbers because the inhibition of CK1α allows for leukemic cell to go into cellular arrest by p53. The oral inhibiting drugs A51 and A86 created by the researchers were shown to promote a significant amount of apoptosis of leukemic cells in the mouse models that were used (Figure 2A). This chart presented all of the newly developed drugs, their concentrations, and their apoptotic percentage in the cells of leukemic mice after 18 hours (Figure 2A). The apoptosis of the leukemic cells is due to a cascade of events from signaling molecules that aid p53 in both recognition and cellular arrest.

The inhibitory drugs lead to this cascade of events. γH2AX, a histone protein that recognizes and fixes double-stranded DNA damage, increases the activity of p53 by deciding if the DNA is damaged to an extent that must result in cellular arrest (Fragkose et al., 2009). Once cellular arrest by p53 is initiated, a pro-apoptotic hormone, Bax is released and stimulates the release of cytochrome c, protein, from the mitochondria. Cytochrome C and Apaf-1, which is an apoptotic protease activating factor, form a complex which activates caspase-9. Caspases are proteins that degrade other proteins, which are activated by cellular signaling. Caspase-9 then binds to and activates caspase-3, which is the executioner protein that results in apoptosis (Samarasinghe, 2013).

The researchers also observed that A51 and A86 decreased phosphorylation activity of the cyclin-dependent kinases (CDK 7 and 9). Both CDK 7 and 9 release proteins including MDM2, MCL1, and MYC, which suppress the activities of p53, preventing cellular apoptosis (Minzel et al., 2018). The inhibitory drugs targeted the kinases of the leukemic cells and reduced the signaling of phosphorylation which resulted in the halt of transcriptional elongation of the mRNA. This halt prevents the release of the p53 suppressing proteins, allowing for p53 to properly perform its cellular function of apoptosis of leukemic cells.

(Figure 4F) presents the mean value variations of expression in the p53 suppressor proteins MDM2, MCL1, and MYC after being treated with either A51 or A86 for 4 hours. Both MYC and MCL1 had high binding affinities for A51 and A86 due to the extremely low expression of approximately 0.2. MDM2 had a slightly lower binding affinity with the inhibitory drugs due to an expression of approximately 0.3.

In class, we’ve learned about cellular signaling pathways and how it triggers a cascade of events using proteins. This comes into effect for leukemic cells because CK1α signals β-catenin for phosphorylation to either promote degradation or to inhibit its function, resulting in uncontrolled gene expression of leukemic cells. However, when CK1α is inhibited, it allows for p53, the tumor suppressing protein, to signal the pro-apoptotic protein, Bax, which continues to signal other proteins leading to cellular death. The researchers noticed the apoptotic phenomenon in their mice after treating them with the co-targeting inhibitory drugs. Observing the mice over the basis of hours or weeks, the researchers performed genome-wide analysis’ recording the newly inhibited phosphorylation signals which the target molecules, CDK7/9 and CK1α, were no longer producing. These results correlated with the researchers’ goals by somewhat curing AML in the experimental mice. However, I am not sure if we should begin clinical testing on humans because we would have to measure the amounts of inhibitory drugs to administer to humans due to the possible toxicity effects. Observing how cellular signaling provides a parade of events, negative or positive, goes on to show how intracellular mechanisms work together to respond to a stimulus, repair damage, or in this case, apoptosis.

 

 

Minzel, W., Venkatachalam, A., Fink, A., Hung, E., Brachya, G., Burstain, I., Shaham, M., Rivlin, A., Omer, I., Zinger, A., Elias, S., Winter, E., Erdman, P., Sullivan, R., Fung, L., Mercurio, F., Li, D., Vacca, J., Kaushansky, N., Shlush, L., Oren, M., Levine, R., Pikarsky, E., Snir-Alkalay, I. and Ben-Neriah, Y. (2018). Small Molecules Co-targeting CKIα and the Transcriptional Kinases CDK7/9 Control AML in Preclinical Models. Cell 175, 171-185.

Samarasinghe, B. (2013). Hallmarks of Cancer 3: Evading Apoptosis

Cruciat, C. -M. (2014). Casein kinase 1 and Wnt/β-catenin signaling. Current Opinion in Cell Biology 31, 46-55.

Fragkos, M., Jurvansuu, J., and Beard, P. (2009). H2AX Is Required for Cell Cycle Arrest via the p53/p21 Pathway. Molecular and Cellular Biology 29, 2828.