Question

1. Mr. Ragnor, a 66 year old male, was referred to the department of respiratory diseases at Mount Sighania Hospital with a dry cough accompanied with blood in phlegm over a month. The previous medical history of Mr. Ragnor is uneventful, with the exception of a 40 pack/year history of smoking. Physical examination revealed a rough respiratory murmur in each lung. Laboratory findings were within normal limits, with the exception of prostate special antigen (PSA) at 5.35 ng/ml. Chest CT images revealed an irregular soft-tissue mass with a 9.6 cm maximum diameter, internal uniformity density in the middle-low right lobe close to pulmonary hilum, and a nodular high-density shadow with a 2.5 cm maximum diameter that was a lobulated lesion in the left upper lobe. The patient was positive for metastasis to the mediastinal lymph node. Fiber bronchoscopic biopsy revealed squamous cell carcinoma in the left lobe and SCLC in the right lobe. Additionally, the pharmacogenomic testing revealed mutations in RAS because of which the RAS signaling is ON all the time. Which of the 2 intracellular signaling pathways activated by RAS may promote cell growth and proliferation? Please draw the downstream signaling events involved in the pathways.

Answer 1

Ras is up regulated in response to receptor tyrosine kinase signaling in response to ligand interaction. The binding of RTK to ligand, causes receptor dimerization and auto-phosphorylation of tyrosine residues present on intracellular domain of the receptor. These phosphotyrosine residues will allow recruitment and docking of adapter proteins that have SH2 or PHB domain. Adapter protein such as Grb will cause recruitment of SOS, a guanosine exchange factor. SOS is the GEF for Ras. It causes the exchange of GTP for GDP on Ras protein

Ras is in the activated form in cancer such as squamous cell carcinoma. Thus, it is always in GTP bound state. The two intracellular pathways that are activated by Ras GTP are the PI3K-AKT-mTOr pathway and the ERK-MAPK pathway. Image of signaling pathways is included below.

1. ERK-MAPK pathway: Ras-GTP will activate Raf, a serine/threonine kinase by homo or heterodimerization and phosphorylation by MAP3K. Raf will activate MEK1 (MAP2K1), a mitogen activated protein kinase kinase 1 by phosphorylation. MEK1 will further phosphorylate mitogen activated protein 2 kinase (MAP2K). MAP2K will then activate ERK1/2 by phosphorylation. ERK1/2 will then translocate to nucleus where it bind to target genes such as c-myc, c-jun, ETS or c-fos. The increased expression of these genes leads to cell proliferation.

2. PI3K-Akt-mTOR pathway: Ras GTP activates Phosphoinositol 3 kinase (P3K) which converts phosphatidylinositol (4,5)-bisphosphate (PIP2), present in cell membrane into phosphatidylinositol (3,4,5)-trisphosphate (PIP3). PIP2 will then activate Akt (protein kinase B by phosphorylation at serine residues. Phospho-AKT will activate mTORC1 or mTORC2 (mammalian target of rapamycin C1 or C2). This leads to phosphorylation of ribosomal protein S6K and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1). These genes will regulate transcription/translation and thereby cause increased cell proliferation. Akt also activates cyclin D1 to increase cell proliferation.