Prediction of lung cancer metastasis relies on post-resection assessment of tumor histology, which is a severe limitation since only a minority of lung cancer patients are diagnosed with resectable disease. Therefore, characterization of metastasis-predicting biomarkers in pre-resection small biopsy specimens is urgently needed. Here we report a biomarker consisting of the phosphorylation of the retinoblastoma protein (Rb) on serine 249 combined with elevated p39 expression. This biomarker correlates with epithelial-to-mesenchymal transition traits in non-small cell lung carcinoma (NSCLC) cells. Immunohistochemistry staining of NSCLC tumor microarrays showed that strong phospho-Rb S249 staining positively correlated with tumor grade specifically in the squamous cell carcinoma (SCC) subtype. Strong immunoreactivity for p39 positively correlated with tumor stage, lymph node invasion, and distant metastases, also in SCC. Linear regression analyses showed that the combined scoring for phospho-Rb S249, p39 and E-cadherin in SCC is even more accurate at predicting tumor staging, relative to each score individually. We propose that combined immunohistochemistry staining of NSCLC samples for Rb phosphorylation on S249, p39, and E-cadherin protein expression could aid in the assessment of tumor staging and metastatic potential when tested in small primary tumor biopsies. The intense staining for phospho-Rb S249 that we observed in high grade SCC could also aid in the precise sub-classification of poorly differentiated SCCs.

Lung cancer is characterized by its poor prognosis, aggressiveness, and proclivity for early metastasis. Non-small cell lung cancer (NSCLC) it’s the most common type of lung cancer with a five-year survival rate of 18%. Most predictors of metastasis and recurrence of NSCLC rely on post-resection evaluation of tumor histology, which is a severe limitation since only 15% of the patients are diagnosed with resectable disease. Hence, there is a need to characterize biomarkers with metastasis-predicting value in pre-resection small biopsy specimens. In addition to the proclivity for metastasis, another feature of lung cancers is the inactivation of the retinoblastoma protein (Rb). The retinoblastoma protein (Rb) is a tumor suppressor inactivated due to hyper-phosphorylation in most human cancers, including NSCLC. Our preliminary data relates the Rb S249/T821 phosphorylation signature to an epithelial-to-mesenchymal transition (EMT), a trait strongly associated to metastasis. We found that cells that have hyperphosphorylated Rb in residues S249/T821 express EMT markers such as decreased expression of E-cadherin and integrin α5 as well as increased expression of N-cadherin, Vimentin, and p-FAK. We propose to focus on the cell adhesion and migration-related kinase Cdk5 with its activator p39 as the kinase responsible for engendering the Rb S249/T821 phosphorylation signature. Our hypothesis is that Rb phosphorylation in S249 and T821 due to CDK5 activity can have prognostic value by being associated with a metastatic phenotype and EMT. Studies performed in a tissue microarray in a population of NSCLC adenocarcinoma that had undergone EMT found an up-regulation in p39 and that its expression is correlated with metastasis. Knockdown of CDK5 induced a decrease in phosphorylation of Rb S249/T821 with a decrease in E-cadherin expression. These data suggest that CDK5-dependent phosphorylation of Rb can affect cell adhesion by regulating E-cadherin. Introducing p39 into Rb hypo-phosphorylated cell line induced an increase in phosphorylation of Rb residues. Furthermore, the CDK5-p39-Rb axis might point to the explanation and prediction of the metastatic phenotype in lung cancer. Citation Format: Jaileene Perez-Morales, Darielys Mejias-Morales, Bryan Torres-Collazo, Harry Negron-Pagan, Pedro Santiago-Cardona. A Rb phosphorylation code associated with lung cancer metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2845. doi:10.1158/1538-7445.AM2017-2845

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