Principles of K-Ras effector organization and the role of oncogenic K-Ras in cancer initiation through G1 cell cycle deregulation

Abstract

Illustrated here is the critical role of oncogenic KRAS in the initiation of cancer through deregulation of the G1 cell cycle, and elements and scenarios taking place under physiological conditions and in KRAS-driven cancer. Raf, PI3K and RalGDS are major K-Ras effectors. They bind at the same Ras site. What decides the cell selection among them? This temporal and spatial decision is critical since in some cellular context the outcome of their signaling pathways may oppose each other. Key among them is the concentration of calcium/calmodulin, negative feedback loops, where a downstream member of the pathway inhibits its upstream activator and cross-inhibition, where inhibition entails blocking another pathway. These three elements, in addition to spatial restrictions by K-Ras-membrane interactions, are not independent; they integrate to provide blueprints for cell decisions. Importantly, elucidation of signaling requires not only K-Ras binary interactions; but the structures and dynamics of its multiprotein complexes.

Keywords:

• K-Ras
• KRAS4B
• K-Ras4B
• lung cancer
• pancreatic cancer
• calmodulin
• Ras isoforms
• cell cycle
• Ras effectors
• PI3K
• Raf
• RalGDS

Key issues

• The three major pathways required for cancer development initiate by binding of Raf, PI3K and RalGDS at the same K-Ras surface and their affinities to K-Ras vary. Here we ask: how their selective activation is decided and what consequences will this cell decision have in cancer initiation?

• Effector concentration can be considered not only in terms of how many molecules are in a certain volume of solution which reflects synthesis, localization and degradation, but also in terms of the availability of the respective binding site surface which may be occupied by another effector.

• The emerging picture suggests that in addition to the orientation of Ras with respect to membrane microdomains, key factors deciding selective activation include calcium and calmodulin concentration, negative feedback and cross-inhibition.

• High Ca²⁺/calmodulin concentration will temporally down-regulate Raf’s activation and MAPK signaling; at the same time it is required for full activation and signaling via the PI3Kα/Akt pathway. At low calmodulin/Ca²⁺ levels the high-affinity Raf dominates, with activation of the Ras/Raf/MEK/ERK pathway; at high levels, PI3Kα/Akt pathway does.

• PI3Kα’s affinity to K-Ras4B is considerably lower than RalGDS, which also binds at the same Ras surface and its signaling can also be enhanced by calmodulin/Ca²⁺. Two proteins can keep RalGDS in check under conditions of high calmodulin/Ca²⁺ concentration: Rap1 and RASSF/RapL.

• We provide an overview of the deregulation of the G1 cell cycle by Ras effector pathways focusing on the passage through the restriction point, a bistable pRb-E2F switch. The fact that the G1 cell cycle progression can be regulated by two distinct phases of signaling nicely corresponds to the early G1 ‘growth factor-dependent’ and the late G1 ‘cell growth’ checkpoints.

• We show that major oncogenic Ras signaling pathways deregulate core players that control the restriction point in the G1 cell cycle. We note that YAP1 is a driver of compensation for the loss of K-Ras signaling in K-Ras-dependent cancers, and further illustrate the cross-talk between oncogenic Ras effector pathways, for example, Myc activation and FoxO inactivation.

Disclaimer

The content of this publication does not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products or organizations imply endorsement by the US Government.

Financial & competing interests disclosure

This project was funded by federal funds from the Frederick National Laboratory for Cancer Research, NIH, under contract HHSN261200800001E. This research was supported (in part) by the Intramural Research Program of NIH, Frederick National Lab, Center for Cancer Research. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.