Loyola doctors focus on STAT3 signaling

Science may be key to figuring out therapy for Gleevec-resistant GIST

By Jerry Call
Life Raft Science Coordinator

Doctors at the Loyola University Medical Center are helping to define signaling that occurs downstream of KIT in GIST patients. Since some GISTs are resistant to Gleevec, understanding downstream signaling might provide additional effective therapies for GIST patients.

It is likely that a combination of different drugs may someday prove to be more effective than a single drug in treating GIST. We are starting to see some of these “cocktail” mixtures move into clinical trials.

A team of doctors — Gladell Paner, Simone Silberman, Grace Hartman, Kenneth Micetich, Gerard Aranha and Serhan Alkan — recently published an article about GIST signaling in the journal “Anticancer Research.” This article provides an analysis of the JAK2/STAT3 pathway in GIST.

The Loyola team studied tumors from 11 c-kit positive GIST patients. They found that STAT3, a protein downstream from KIT, was constitutively activated in all 11 cases. To further study the effects of STAT3 activation, the team generated two primary GIST cell lines from tumors with exon 11 mutations. They then exposed these two cell lines to several inhibitors including inhibitors of c-kit (Gleevec), JAK2 (AG490), MAPK (PD98059) and PI-3K (LY294002).

The results:
•Gleevec (c-kit) -greater than 95 percent inhibition of cellular proliferation.
•AG490 (JAK2)-82 percent inhibition of proliferation with evidence of apoptosis (cell death). •PD98059 (MAPK)-partial inhibition of proliferation, but no evidence of apoptosis.
•LY294002 (PI-3K)-a slight increase in absorbance compared to control cells, indicating no inhibition, and rounding of the cells, but no apoptotic bodies.

The results for Gleevec are not surprising; it remains the gold standard for treating GIST. What was somewhat surprising, to this reporter, was how effective AG490 was. While STAT1 and STAT3 have been identified as being part of the KIT pathway (see slide 15 from a presentation by Dr. Jonathon Fletcher given at the London GIST conference, at http://liferaftgroup. org:8185/medical/docs/london_conference/Fletcher/Fletcher_files/frame.htm), this paper provides some quantification of how important the STAT3 pathway may be in GIST.

In their paper, the Loyola researchers report: “Since JAK2, the main upstream regulator of STAT3, has been shown to be associated with c-kit, it is most plausible to consider that STAT3 activation may be a consequence of JAK2 activation. However, further confirmation of this hypothesis is warranted and that STAT3 activation through other kinases must also be excluded.”

Dr. Serhan Alkan, associate professor of pathology at Loyola, says that AG490 is not ready for use in patients. It has not even been tested in animals at this time, but identifying likely drug targets are an important first step.

Alkan’s research focus is in the field of hematological cancers (Hodgkin’s disease, non-Hodgkin’s lymphoma, multiple myeloma, and leukemia). Because c-kit and STAT3 is important is some of these cancers, Alkan was interested in evaluating these pathways in GIST, since KIT signaling is so prominent in GIST.

This is another indication of the disproportionate interest in GIST (compared to its frequency of occurrence) due to its interesting biology. Since GIST cell lines are not readily available, Alkan created two GIST primary cell lines. These are different than immortal types of cell lines, in that they are only viable for a limited number of cell divisions. This allowed the Loyola team to perform these experiments.

According to Alkan, lack of access to GIST cell lines is the largest obstacle to more GIST research of this kind. Alkan goes on to add, “No functional or physiological questions can be answered from frozen cells, for more comprehensive work you need cell lines.” While there may not be any currently available drugs targeting either JAK2 or STAT3, the Loyola team also found that bcl-2 was expressed in all 11 of the GISTs they studied. Bcl-2 inhibits apoptosis and therefore prevents tumors from dying. They found that there was a marked reduction in Bcl-2 expression in the GIST cells following treatment with AG490 compared to control (untreated) GIST cells.

Clinical trials of Gentasense, a drug that targets Bcl-2 will soon start at Dana-Farber. In these phase I trials, Gentasense will be given in combination with Gleevec.

The work of the Loyola team provides further rational for the use of drugs that target Bcl-2 in GIST, and drugs that target JAK2 or STAT3 when they become available. These drugs would be an addition to the current primary strategy of inhibiting KIT (or PDGFRA in some cases). Inhibiting KIT signaling would still be the most important part of therapy, but in some cases inhibition of one of these secondary targets may add to the therapy.

It should be noted that the downstream targets discussed here are not the only targets being explored in GIST and they may not even turn out to be the most important targets. Other important downstream targets include the AKT/mTOR pathway and PKC. Other important downstream pathways that have been shown to be important in KIT signaling in non-GIST cells, pathways such as PLCgamma, are conspicuously absent in the literature on GIST (although PKC is typically in the PLCgamma pathway in non-GIST c-kit pathways). It remains to be seen if they play any part in GIST.