Dr. Christopher Counter (Duke Associate Professor Pharmacology, Associate Professor Radiation Oncology) - Oncogene addiction in Pancreatic cancer
Below is a summary of Dr. Counters talk. I have left out a fair amount of the experimental details; all results come from in vitro animal models.
EGF couples with GEF which then binds to Ras-GDP -> Ras->GTP (ras is membrane bound)
Ras-GTP interacts with RalGEF, Raf, PI3K
GAP then inactivates Ras-GTP -> Ras-GDP
KRas is mutated in 90% of pancreatic cancer, to the constitutively active form
in about 1/3 of human cancers this is also mutated, and predicts for resistance to EGFR inhibitors (specifically cetuximab in colon)
Ras oncogene addiction - Chin, Nature 2000: in a mouse model - turning off ras activation led to involution of the tumor.
Thinking of Tumor Initiation and Maintenance, and how Ras fits in:
Created a similar mouse model to Chin 2000 (mouse with a constitutive ras switch)
tumor is initiated in a mouse with ras active.
ras is then inactivated and then investigations focused on the three secondary pathways downstream from ras:
· Ral-GEF only activation led to involution
· Raf only activation led to involution
· PI3K only activation led to tumor maintenance
Therefore for maintenance, it seems that PI3K is necessary for tumor maintenance.
Tumor microenvironment seems to activate a fair amount of paracrine signaling to maintain the tumor.
Ras induces secretion of IL-6
IL-6 is also found in pancreatic patients serum (along with IL-8)
if IL-6 is inhibited, tumor generated in xenograft model where much smaller and slower growing. there was also a decrease in the amount of CD31 staining (an endothelial stain, and a surrogate for angiogenesis).
IL-6 knockout mice also were resistant to Ras-mediated tumorgenesis (papilloma model)
IL-6 monoclonal antibodies are available and are shown in mouse model to delay tumor growth. in 10 human pancreatic lines tested, 2 had a response.
IL-6 inhibition also seems to reduce cachexia.
AKT is a downstream target from PI3K, and similar to the above seems to be a key component of tumor maintenance.
AKT works on a large number of targets, including eNOS (endothelial nitric oxide synthase), which also seems to be a key component of tumor maintenance.
Phosporylation of eNOS is performed on a specific serine (S1177), which seems to be necessary for tumor maintenance.
eNOS knockout model: resistant to ras mediated tumorigenesis.
L-NAME inhibits eNOS, which was developed for another clinical application. In xenograft model, L-NAME led to tumor growth delay. in 9 cell lines, activity was seen in 7 (in contrast to the IL-6 mab intervention). (L-NAME developed for septic shock treatment by GSK)
eNOS nitrosylates Wild type Ras and activates it
N, K, and H are all subtypes of Ras, K being the most important for human tumors
eNOS (activated by KRas) then activates NRas and HRas via nitrosylation.
HRas nitrosylation seems to be required for tumor growth.
Farnesyl transferase inhibitors were developed to target Ras. To be active Ras needs to get to the plasma membrane, most often by gaining franesyl groups. FTIs inhibited the ability of Ras to get to the membrane, but it turns out that KRas is able to escape this by a number of other methods. FTIs do however work well on HRas. Perhaps some combination with the previously mentioned investigations would be fruitful.
Obviously a lot of this is ‘in the pipeline’ work, but it may well be key to improving the overall dismal results with current standard of care treatment.