Abstract A51: The role of ribosomal protein s6 in the development of pancreatic cancer precursor lesions
Authors: Khatib Areej A.S , Dor Yuval , Khalilah Abed 
Published: June 2012

Background: Carcinoma of the pancreas kills nearly all of the 230,000 people diagnosed with the disease annually worldwide. This high mortality results from poor detection, metastatic disease at the time of detection, and lack of specific or effective therapy.

Early precursor lesions termed pancreatic intraepithelial neoplasia (PanIN) are thought to be the initial step in pancreatic ductal carcinoma. The PI3K signaling pathway is a central transducer of information from extracellular signals to the cell, impacting diverse cellular function such as replication, survival, migration, protein synthesis, and more. Ribosomal protein s6 (rps6) is a component of the protein translation machinery, identified originally in the regenerating liver. It is phosphorylated at specific residues by an upstream kinase, s6K, which itself is activated by mTOR and the PI3K-AKT pathway. No functional link is currently known between rps6 phosphorylation and pancreatic cancer.

Methods: Pellets soaked with 7,12-dimethylbenz(a)anthracene(DMBA) were implanted in the body of wild-type mouse pancreas (n-12) and in the pancreas of mutant mice lacking rps6 phosphorylation sites (rps6ki/ki) (n=11). Three to five months later, the mice were scarified and pathological analysis and scoring for PanIN lesions were performed.

To examine in a crude way if mTOR signaling is important for the development of DMBA-induced PanIN lesions, we administered rapamycin to DMBA-treated mice, and have determined the effect on PanIN score. We then examined the role of rps6 activation in a genetic model of PanIN lesions. Using extensive breeding, we generated cohorts of pdx1-Cre; LSL-KRAS G12D; rps6 ki/ki mice, in order to examine the effect of rps6 phosphorylation on the development of PanIN lesions, in comparison to LSL-KRAS wild rps6, after 3–5 months the mice were sacrificed and the same analysis and scoring were performed. In both models, rps6 pattern of staining was evaluated in all groups.

Results: In the DMBA model, we observed the development of mouse PanIN lesions at all levels (PanIN 1–3) in all wild-type mice examined. Rapamycin-treated mice had a significant reduction in the score of PanIN lesions, supporting the hypothesis that mTOR signaling downstream of PI3K, participated in the formation of PanIN lesions.

In mutant mice, strikingly, PanIN lesions observed were greatly reduced. These results suggest that rps6 phosphorylation is an essential event in the development of pancreatic cancer, in the DMBA carcinogenesis model.

In the KRAS model, mice developed PanIN lesions, as previously described, KRAS mice lacking rps6 phosphorylation developed fewer PanIN lesions that had lower score, similar to DMBA carcinogenesis model. This shows that development of PanIN lesions driven by mutant KRAS requires at least partially phosphorylation of rps6.

The expressive pattern of rps6 in normal mice revealed that rps6 phosphorylation is limited to a subset of islet cells and to a few acinar cells surrounding the islets, with minimal or no activity in ducts. In PanIN lesions generated by implantation of DMBA in the pancreas, as well in Pdx1-Cre; LSL-KRAS G12D mice, strong phosphorylation of rps6 is observed in malignant epithelial cells. This suggests that oncogenic signaling in the pancreas, and specifically KRAS signaling, activates PI3K signaling, including S6 kinase which phosphorylates rps6.

Conclusion: rps6 is activated (phosphorylated) in mouse models of PanIN lesions and pancreatic cancer, and development of PanIN lesions requires at least partial phosphorylation of rps6.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr A51.