A novel assay for the measurement of Raf-1 kinase activity.

Raf-1 is a serine-threonine protein kinase that functions as a central component of the mitogen-activated protein kinase signal transduction pathway. Raf-1 activity is currently assayed in vitro by either measuring 32P incorporation into MEK, Raf-1's only characterized substrate, or by using the phosphorylated MEK to initiate a coupled assay culminating in the phosphorylation of myelin basic protein by MAP kinase. These assays are plagued by a potential lack of specificity in the case of the former, and the time consuming and error-prone nature of the later indirect assay. In this report, we demonstrate a novel single step assay for Raf-1 kinase activity based on phosphorylation of recombinant MEK-1, detected using an activation-specific MEK antibody that recognizes MEK only when specifically phosphorylated by Raf-1 on Ser 217 and Ser 221. The assay readily detected stem cell factor-mediated Raf-1 activation. MEK phosphorylation by immunoprecipitated Raf-1 plateaued at 10 min following initiation of the kinase reaction and was completely dependent on the inclusion of Raf-1. There was a linear correlation between the degree of MEK phosphorylation and the amount of Raf-1 protein immunoprecipitated. In addition to detecting growth factor-mediated activation, the assay was also able to detect paclitaxel-mediated Raf-1 activation. This assay is rapid, sensitive, and specific and therefore is a marked improvement over currently utilized techniques.

Src family kinase activity is required for Kit-mediated mitogen-activated protein (MAP) kinase activation, however loss of functional retinoblastoma protein makes MAP kinase activation unnecessary for growth of small cell lung cancer cells.

Small cell lung cancer (SCLC) is characterized by multiple genetic alterations that include inactivation of the retinoblastoma protein (Rb), the establishment of several autocrine loops including that induced by coexpression of stem cell factor (SCF) and Kit, and the ectopic expression and activation of Src family kinases. Previous studies have shown that Lck associates with, and becomes activated by, Kit after SCF stimulation of SCLC cells. In the present study, we have demonstrated that PP1, a pharmacological inhibitor of Src kinases, blocked SCF-mediated activation of mitogen-activated protein (MAP) kinase, but it also inhibited Kit activation. However, MAP kinase activation was more sensitive than Kit activation to the effects of PP1. Overexpression of Lck reduced the sensitivity of MAP kinase activation to PP1 without altering the sensitivity of Kit activation, which suggested a role for Lck in SCF-mediated MAP kinase activation. Inducible expression of a dominant negative Lck inhibited MAP kinase activation in a dose-dependent manner, which confirmed that Src family kinase activity is required for SCF-induced MAP kinase activation. The growth of cells that expressed dominant negative Lck was unaffected, however, despite the inhibition of MAP kinase. Growth was also unaffected by the inhibition of the MAP kinase pathway using PD 98059, but sensitivity to the MAP/extracellular signal-regulated kinase kinase inhibitor could be partially restored by expression of wild-type Rb. Therefore, MAP kinase activation seems to be dispensable for the growth of SCLC only in the absence of Rb expression. These data suggest that the SCF/Kit autocrine loop, through activation of Lck and subsequently MAP kinase, and the mutational inactivation of Rb contribute to the loss of G1-S phase checkpoint regulation during the pathogenesis of SCLC. Furthermore, the data demonstrate that, in established SCLC cell lines, proliferative signal transduction initiated by Kit is mediated by pathways other than the classic MAP kinase pathway.