Of the various different functions that proteins perform in a cell, a crucial one is the recognition and transmission of certain "signals," collectively referred to as signal transduction.

Receptors (proteins) on the cell surface recognise certain molecules and then initiate a chain of biochemical events inside the cell.

These biochemical events are responsible for cellular activities such as multiplication, survival, etc.

Needless to say, any perturbation of this "biochemical signalling" can be extremely detrimental to the cell, even leading to cancer in some cases.

But, it is possible to target defective biochemical signalling pathways within a cell to treat cancer, provided the underlying mechanisms are studied thoroughly.

This is exactly what a group of scientists from Japan set out to do in their study, which was published in Cell Communication and Signaling.

This research group - whose study was supported by Japan Agency for Medical Research and Development (AMED) was headed by Assoc. Prof Yuuki Obata from Tokyo University of Science (also National Cancer Center), and consists of Prof Isamu Shiina (Tokyo University of Science), Prof Ryo Abe (Tokyo University of Science and Teikyo University), Dr Toshirou Nishida (National Cancer Center), and Prof Koji Okamoto (National Cancer Center).

A certain signalling receptor protein called KIT tyrosine kinase functions in the growth and survival of different types of cells, including haematopoietic cells (the progenitors of all blood cells), mast cells (a type of immune cells), and interstitial cells of Cajal (electrical pacemakers in gastrointestinal tract).

Active mutations of this protein have been identified in several cancers, such as mast cell leukaemia (MCL), gastrointestinal stromal tumour (GIST), and acute myeloid leukaemia (AML).

In MCL, the mutations D816V (human) and D814Y (mouse) are frequently found; here, the mutated KIT protein "mislocalises" in a cellular compartment called the "endolysosome" (EL).

In GIST, mutated KIT accumulates in and conducts cancer-specific signalling from the Golgi, the site in a cell where macromolecules are produced, modified, and packaged, especially proteins.

Active KIT mutations have been found in about 10 percent of core binding factor AML (CBF-AML) patients; these are also associated with poor prognosis in AML.

However, it remains unclear whether KIT transduces signals from intracellular compartments in AML.

The research group from Japan aimed to answer this question by using a newly synthesised compound called M-COPA (along with other existing ones) that targets intracellular transport.

According to them, this also represents an attractive strategy to combat cancer. Prof Shiina candidly said: "We wanted to investigate the anticancer effect of the new anticancer drug lead compound M-COPA synthesised at our university against haematological cancers (leukaemia, lymphoma, etc.)."

Apart from D816V, another major active KIT mutation in AML is N822K.

D816V has been characterised extensively, but the signalling platforms and mechanisms of N822K are relatively unknown.

Also, before this study, it was unclear how the mutated KIT and where the downstream signalling molecules are activated.

The scientists investigated the relationship between the localisation of KITN822K (KIT protein carrying the N822K mutation) and its activation in an AML cell line, Kasumi-1.

The scientists found that in AML cells, KITN822K mislocalised to and accumulated in the EL.

Newly produced KIT in the endoplasmic reticulum (ER) travels to the cell membrane via the Golgi and then relocates to EL.

However, immunofluorescence experiments (those that use antibodies against the mutated KIT, tagged with fluorescent dyes for identification) showed that KIT was activated in the Golgi.

KIT activation on the Golgi was also found in other leukaemia cells that have the receptor mutation.

Next, Prof Shiina and colleagues found that in the Golgi in AML cells, KITN822K also activates downstream signalling proteins called AKT, ERK, and STAT5.

They did this by using specific compounds that target intracellular transport of proteins: brefeldin A (BFA), 2-methylcoprophilinamide (M-COPA) (inhibitors of transport from ER to the Golgi), and monensin (inhibitor of Golgi export).

They found that in cells treated with BFA or M-COPA, KIT was retained in the ER.

This also decreased the auto-phosphorylation of KIT and thereby its downstream signalling.

Suppression of Golgi export of KIT using monensin did not suppress the KIT signals, which told the scientists that mutated KIT carries out cancer signalling specifically at the Golgi.

So, what are the future applications of this study?

Small molecule TKIs (tyrosine kinase inhibitors) and antibodies against RTKs (receptor tyrosine kinases) have been developed to suppress cancer proliferative signalling using mechanisms similar to the ones described above.

According to Prof Shiina and the group, this study reveals that the novel compound M-COPA can be used to block transport of KIT from the ER to the Golgi (where it is activated and carries out downstream oncogenic signalling).

The scientists say that the compound M-COPA has applications such as treatment of patients with AML, improved prognosis for these patients, and improvement in the quality of life of these patients.

Prof Shiina concludes by stating, "Currently, the synthesis of various M-COPA analogs is progressing every day at our university, and their inhibitory effects against haematological cancers and solid cancers (stomach cancer, lung cancer, ovarian cancer, etc.) are being verified."

Source: Tokyo University of Science