Julius O. Ebinu

RasGRP, a guanyl nucleotide-releasing protein for the small guanosine triphosphatase Ras, was characterized. Besides the catalytic domain, RasGRP has an atypical pair of "EF hands" that bind calcium and a diacylglycerol (DAG)-binding domain. RasGRP activated Ras and caused transformation in fibroblasts. A DAG analog caused sustained activation of Ras-Erk signaling and changes in cell morphology. Signaling was associated with partitioning of RasGRP protein into the membrane fraction. Sustained ligand-induced signaling and membrane partitioning were absent when the DAG-binding domain was deleted. RasGRP is expressed in the nervous system, where it may couple changes in DAG and possibly calcium concentrations to Ras activation.

Stimulation of the T-cell receptor (TCR) alters a number of intracellular signaling pathways including one that involves protein tyrosine kinases, phospholipase C-gamma1 (PLC-gamma1), diacylglycerol (DAG), and calcium messengers. By a divergent pathway, TCR-stimulated protein tyrosine kinase activity is thought to result independently in recruitment of the Ras activator Sos to the plasma membrane, leading to Ras activation. Here we show that RasGRP, a Ras activator that contains calcium-binding EF hands and a DAG-binding domain, is expressed in T cells. A PLC-gamma1 inhibitor diminished activation of Ras following TCR stimulation. Membranes from TCR-stimulated Jurkat T cells exhibited increased RasGRP and increased Ras-guanyl nucleotide association activity that was inhibited by antibodies directed against RasGRP. Overexpression of RasGRP in T cells enhanced TCR-Ras-Erk signaling and augmented interleukin-2 secretion in response to calcium ionophore plus DAG analogues phorbol ester myristate or bryostatin-1. Thus, RasGRP links TCR and PLC-gamma1 to Ras-Erk signaling, a pathway amenable to pharmacologic manipulation.

Renal cell carcinoma (RCC) and melanoma brain metastases have traditionally been considered radioresistant lesions when treated with conventional radiotherapeutic modalities. Radiosurgery provides high-dose radiation to a defined target volume with steep fall off in dose at lesion margins. Recent evidence suggests that stereotactic radiosurgery (SRS) is effective in improving local control and overall survival for a number of tumor subtypes including RCC and melanoma brain metastases. The purpose of this study was to compare the response rate to SRS between RCC and melanoma patients and to identify predictors of response to SRS for these 2 specific subtypes of brain metastases. We retrospectively reviewed a prospectively maintained database of all brain metastases treated with Gamma Knife SRS at the University Health Network (Toronto, Ontario) between October 2007 and June 2010, studying RCC and melanoma patients. Demographics, treatment history and dosimetry data were collected; and MRIs were reviewed for treatment response. Log rank, Cox proportional hazard ratio and Kaplan-Meier survival analysis using SPSS were performed. A total of 103 brain metastases patients (41 RCC; 62 melanoma) were included in the study. The median age, Karnofsky performance status score and Eastern Cooperative Oncology Group performance score was 52 years (range 27-81), 90 (range 70-100) and 1 (range 0-2), respectively. Thirty-four lesions received adjuvant chemotherapy and 56 received pre-SRS whole brain radiation therapy. The median follow-up, prescription dose, Radiation Therapy Oncology Group conformity index, target volume and number of shots was 6 months (range 1-41 months), 21 Gy (range 15-25 Gy), 1.93 (range 1.04-9.76), 0.4 cm3 (range 0.005-13.36 cm3) and 2 (range 1-22), respectively. Smaller tumor volume (P=0.007) and RCC pathology (P=0.04) were found to be positive predictors of response. Actuarial local control rate for RCC and melanoma combined was 89% at 6 months, 84% at 12 months, 76% at 18 months and 61% at 24 months. Local control at 12 months was 91 and 75% for RCC and melanoma, respectively. SRS is a valuable treatment option for local control of RCC and melanoma brain metastases. Smaller tumor volume and RCC pathology, predictors of response, suggest distinct differences in tumor biology and the extent of radioresponse between RCC and melanoma.