An NF-κB pathway–mediated positive feedback loop amplifies Ras activity to pathological levels in mice

Abstract

Genetic mutations that give rise to active mutant forms of Ras are oncogenic and found in several types of tumor. However, such mutations are not clear biomarkers for disease, since they are frequently detected in healthy individuals. Instead, it has become clear that elevated levels of Ras activity are critical for Ras-induced tumorigenesis. However, the mechanisms underlying the production of pathological levels of Ras activity are unclear. Here, we show that in the presence of oncogenic Ras, inflammatory stimuli initiate a positive feedback loop involving NF-κB that further amplifies Ras activity to pathological levels. Stimulation of Ras signaling by typical inflammatory stimuli was transient and had no long-term sequelae in wild-type mice. In contrast, these stimuli generated prolonged Ras signaling and led to chronic inflammation and precancerous pancreatic lesions (PanINs) in mice expressing physiological levels of oncogenic K-Ras. These effects of inflammatory stimuli were disrupted by deletion of inhibitor of NF-κB kinase 2 (IKK2) or inhibition of Cox-2. Likewise, expression of active IKK2 or Cox-2 or treatment with LPS generated chronic inflammation and PanINs only in mice expressing oncogenic K-Ras. The data support the hypothesis that in the presence of oncogenic Ras, inflammatory stimuli trigger an NF-κB-mediated positive feedback mechanism involving Cox-2 that amplifies Ras activity to pathological levels. Because a large proportion of the adult human population possesses Ras mutations in tissues including colon, pancreas, and lung, disruption of this positive feedback loop may be an important strategy for cancer prevention.

Emerging therapies for pancreatic ductal carcinoma

“…[8] K-Ras activation from inflammation in mouse models has been shown to cause pancreatic lesions. [9] A separate study showed that mice with inducible K-Ras mutations initiated carcinogenesis by inhibiting the repair of the pancreas. [10] This same study showed that inactivation of K-Ras led to tumor regression.…”

Targeting cytokine networks in KRAS-driven tumorigenesis

“…For example, IL-1 bridges NF-kB pathway activation with KRAS signaling in pancreatic cancer and facilitates its oncogenicity [11]. Even transient introduction of an inflammatory stimulus in oncogenic KRAS expressing pancreatic acini leads to positive feedback and NF-kB-dependent neoplasia [12]. The direct requirement for NF-kB signaling in KRAS-driven tumorigenesis has also been solidified by multiple recent studies in oncogenic Kras lung genetically engineered mouse models [13,14].…”

“…IL-6 and STAT3 signaling also stimulate the expression of IKKe, unique among family members as an inducible IKK [17], which reinforces NF-kB activation and cytokine signaling downstream of KRAS. Oncogenic KRAS is further tipped into an active state by multiple factors in this inflammatory microenvironment by virtue of the defective catalysis of GTP hydrolysis [1,12]. Thus, the dynamics of such a system engender feedforward signaling that can propagate and sustain cytokine activation, further contributing to the tumorigenic potential of oncogenic KRAS and fueling its aggressive behavior.…”

“…In particular, the addition of cytokine network targeting to current approaches that focus on inhibiting mitogenic pathways could result in synergistic action and minimize overlapping toxicity. Furthermore, disrupting the production of the very stimulants that also directly fuel oncogenic KRAS activation may have an even greater impact on prevention [12]. Thus, while inhibiting KRAS itself and optimizing further the inhibition of well-established pathways such as RAF/ MEK and PI3K/AKT remains a laudable goal, attacking in parallel the components that drive cytokine signaling holds great promise and may ultimately contribute to therapeutic success.…”

Pancreatic oncogenic signaling cascades converge at Protein Kinase D1

“…Moreover, mutant Kras is not constitutively-active per se, 1 and requires additional stimulation to initiate a feed forward loop for enhanced pathological level of activity. 2 In addition, a cooperation of signaling pathways activated by mutant Kras with signaling pathways initiated by epidermal growth factor receptor (EGFR)-activated wildtype Kras is needed for the development of PDA. 3 Given that Kras is difficult to target, an ideal strategy would be to target an enzyme that serves as a "bottleneck," converging both pathways.…”