B.E. Cooper Union for the Advancement of Science, 2007
4th Year Medical Student
Graduate Program: Molecular & Cellular Pharmacology
Title: The PI3K p110a-Rac1 signaling pathway is essential
for Kras-induced pancreatic tumorigenesis.
ES Carpenter, CY Wu, LM Ballou, HC Crawford, RZ Lin.
drug targets are urgently needed to develop primary and adjuvant therapies
for pancreatic ductal adenocarcinoma (PDA). Almost all PDAs are caused
by mutations in the Kras gene. Pharmacological inhibition of Kras has
so far proven unsuccessful. Fortunately, mutant Kras activates several
molecules more easily targeted with pharmacological inhibitors, including
the focus of our studies, phosphatidylinositol 3-kinase (PI3K). Our
studies utilize the pancreas-specific Ptf1a-Cre;LSL-KrasG12D (abbreviated
as KC) mouse model that faithfully reproduces human pancreatic tumorigenesis.
Using the cre-lox system, we generated conditional knockout mice for
PI3K catalytic isoforms p110a and p110ß. We found that genetic
ablation of p110a, but not p110ß, in the mouse pancreas completely
abrogated the development of pancreatic tumors in KC mice without causing
detectable harmful effects. Furthermore, we generated a mouse strain
expressing a constitutively active p110a mutant in the pancreas and
found that activation of PI3K alone is sufficient to induce pancreatic
tumor formation. Previous reports indicate that Kras can activate PI3K
through direct binding to the Ras-binding domain of p110a. Surprisingly,
replacement of pancreatic p110a with mutant p110a incapable of binding
to Kras did not protect KC mice against tumorigenesis, indicating an
indirect mechanism by which oncogenic Kras signals to p110a to induce
pancreatic tumor formation. Interestingly, KC pancreata exhibited markedly
elevated p110a expression, suggesting an alternative mechanism by which
Kras activates PI3K in our model. We also investigated signaling pathways
downstream of PI3K in KC mice. Unexpectedly, Akt, a known downstream
effector of PI3K, did not show increased activation in the KC pancreas,
and loss of p110a did not decrease its activation level. Upon exploring
alternate PI3K effectors, we found that the KC pancreas exhibited a
marked increase in activation of Rac1, a small G protein involved in
actin cytoskeleton rearrangement. More importantly, ablation of p110a,
but not p110ß, completely blocked this Rac1 activation, a finding
consistent with a previous study showing that ablation of Rac1 blocks
oncogenic Kras-induced pancreatic tumorigenesis. In summary, we found
that Kras requires and activates the p110a-Rac1 pathway to induce pancreatic
tumorigenesis through an indirect mechanism and not through direct binding
to p110a. Further studies are needed to determine if pancreatic tumor
maintenance requires the continual presence of p110a signaling in order
to assess the viability of targeting this enzyme in treating PDA.
(pre-MSTP publications indicated with an *)
CA, Grüner BM; Takeuchi KK, Lubeseder-Martellato C, Teichmann N,
Mazur PK, DelGiorno KE, Halbrook CJ, Carpenter ES,
Hall JC, Pal D, Briel T, Herner A, Trajkovic-Arsic M, Sipos B, Liou
G-Y, Storz P, Murray NR, Threadgill DW, Sibilia M, Washington MK, Wilson
CL, Schmid RM, Raines EW, Crawford HC and Siveke JT. (2012). EGF Receptor
is Required for KRAS-induced Pancreatic Tumorigenesis. Cancer
Cell. In press.
L, Carpenter ES, Wu AP, Alston N, DelGiorno KE, Crawford
HC. (2010). Pancreas-specific ablation of beta1 integrin induces tissue
degeneration by disrupting acinar cell polarity. Gastroenterology.
138(7):2531-40, 2540.e1-4. Epub 2010 Feb 23. PMID:20188101.