Protein Kinase A-independent Ras Protein Activation Cooperates with Rap1 Protein to Mediate Activation of the Extracellular Signal-regulated Kinases (ERK) by cAMP.

Cyclic adenosine monophosphate (cAMP) is an important mediator of hormonal stimulation of cell growth and differentiation through its activation of the extracellular signal-regulated kinase (ERK) cascade. Two small G proteins, Ras and Rap1, have been proposed to mediate this activation, with either Ras or Rap1 acting in distinct cell types. Using Hek293 cells, we show that both Ras and Rap1 are required for cAMP signaling to ERKs. The roles of Ras and Rap1 were distinguished by their mechanism of activation, dependence on the cAMP-dependent protein kinase (PKA), and the magnitude and kinetics of their effects on ERKs. Ras was required for the early portion of ERK activation by cAMP and was activated independently of PKA. Ras activation required the Ras/Rap guanine nucleotide exchange factor (GEF) PDZ-GEF1. Importantly, this action of PDZ-GEF1 was disrupted by mutation within its putative cyclic nucleotide-binding domain within PDZ-GEF1. Compared with Ras, Rap1 activation of ERKs was of longer duration. Rap1 activation was dependent on PKA and required Src family kinases and the Rap1 exchanger C3G. This is the first report of a mechanism for the cooperative actions of Ras and Rap1 in cAMP activation of ERKs. One physiological role for the sustained activation of ERKs is the transcription and stabilization of a range of transcription factors, including c-FOS. We show that the induction of c-FOS by cAMP required both the early and sustained phases of ERK activation, requiring Ras and Rap1, as well as for each of the Raf isoforms, B-Raf and C-Raf.

Analysis of Drug Resistance Using Kinome-Wide Functional Screens.

The clinical success of tyrosine kinase inhibitors specific for BCR-ABL-, EGFR-, ALK-, and ROS1-driven cancers continues to spur the quest to match specific oncogene-defined tumor types with an appropriate molecularly targeted therapy. Unfortunately, responses to these agents are not durable with intrinsic or acquired resistance limiting benefit. Additionally, efforts to identify the appropriate targets of new drugs have focused on nonfunctional assays such as large-scale sequencing for somatic mutations or analysis of gene copy number. Acknowledging both the problem of resistance and the shortcomings of the current methods for detecting appropriate drug targets, much interest has been focused on RNAi-based screens. These screens utilize a library of shRNAs targeting the whole genome or a subset of genes and provide a high-throughput and unbiased means to functionally assess genes impacting various aspects of tumor biology, especially proliferation and survival. The function of genes can be measured in the context of a specific drug treatment, termed a synthetic lethal screen, or genes may be assessed for their individual dependency, termed an essential gene screen. Here, we describe a method for performing both of these types of screens using a kinome-targeted shRNA library in human cancer cell lines.