KRAS as a Modulator of the Inflammatory Tumor Microenvironment: Therapeutic Implications

Abstract

KRAS mutations are one of the most frequent oncogenic mutations of all human cancers, being more prevalent in pancreatic, colorectal, and lung cancers. Intensive efforts have been encouraged in order to understand the effect of KRAS mutations, not only on tumor cells but also on the dynamic network composed by the tumor microenvironment (TME). The relevance of the TME in cancer biology has been increasing due to its impact on the modulation of cancer cell activities, which can dictate the success of tumor progression. Here, we aimed to clarify the pro- and anti-inflammatory role of KRAS mutations over the TME, detailing the context and the signaling pathways involved. In this review, we expect to open new avenues for investigating the potential of KRAS mutations on inflammatory TME modulation, opening a different vision of therapeutic combined approaches to overcome KRAS-associated therapy inefficacy and resistance in cancer.

Keywords: KRAS mutations; colorectal cancer; inflammation; lung cancer; pancreatic cancer; therapy; tumor microenvironment.

Figure 1

KRAS mutations in pancreatic, colorectal, and lung cancer. KRAS mutations are one of the earliest major events in pancreatic cancer, found in over 95% of cases. Among KRAS mutations, G12D and G12V are the most frequent alterations, followed by G12R. In colorectal cancer, KRAS mutations are present in about 52% of cases. Oncogene KRAS activating mutations G13D, G12D, and G12V are the most frequent in this type of cancer. In lung cancer, activating KRAS mutations are found in over 30% of cases and are one of the most prevalent mutations associated with tobacco exposure. Among those, G12C and G12V mutations are the most associated with patients who smoke, whereas G12D is mainly found in never-smokers.

Figure 2

KRAS as a crucial TME modulator. The TME is composed, not only of tumor cells, but also several non-tumor stromal elements such as immune cells, fibroblasts, adipocytes, endothelial cells, neurons, osteoblasts, osteoclasts, and ECM components. This dynamic, challenging microenvironment modulates and can be modulated by several factors, namely, KRAS mutations. Several studies have reported that KRAS mutations can drive the secretion of anti-inflammatory cytokines, such as IL-10, TGF-β, and GM-CSF, with the ability to sustain an immunosuppressive TME and to promote tumor progression. Other studies have also demonstrated that KRAS mutations may interfere with the secretion of pro-inflammatory cytokines, with an anti-tumor effect, such as ICAM-1, TNF-α, and IL-18. Thus, KRAS seems to act as a modulator of both an anti-inflammatory and a pro-inflammatory TME.

Figure 3

KRAS as a pro-inflammatory tumor microenvironment modulator. Several studies reported the association of KRAS with pro-inflammatory cytokines and chemokines, such as ICAM-1 and IL-18. Normal acinar cells transfected with oncogenic mutant KRAS are described to express high levels of ICAM-1, which is then secreted into its soluble form. The sICAM-1 acts as a chemoattractant for pro-inflammatory macrophages and stimulates them to produce MMP-9 that allow ECM degradation, as well as pro-inflammatory chemokines, such as TNF-α that can drive transdifferentiation signaling. KRAS mutations can also modulate the TME by impairing IL-18 secretion, blocking its immune-stimulatory function and, thus, contributing to evasion of the local immune system during tumor development.

Figure 4

KRAS as an anti-inflammatory tumor microenvironment modulator. Several reports emphasize that KRAS mutations may sustain an anti-inflammatory microenvironment through the secretion of several inflammatory chemokines and cytokines, such as TGF-β, IL-10, and IL-6. In fact, cells harboring KRAS^G12D mutations secrete high levels of these anti-inflammatory mediators that inhibit T cell activation, suppress cytotoxic CD8⁺ T cell-mediated tumor killing, and convert pro-inflammatory CD4⁺ T cells to anti-inflammatory Tregs. Moreover, KRAS mutations were also described to induce the downregulation of MHC class I molecules and the upregulation of PD-L1, reducing the ability of CD8⁺ cytotoxic T cells to recognize and kill cancer cells. Additionally, KRAS mutations may drive an anti-inflammatory and pro-tumor immune suppressive microenvironment mediated through IL-6 secretion. Notably, when IL-6 was blocked, a reduction of anti-inflammatory macrophage gene expression, such as Arg, FIZZ1, Mg1, and Mrc1, and a reduction of the immunosuppressive cytokines TGF-β and IL-10 were observed. Moreover, it has also been described that IL-6 induces higher levels of T cell exhaustion markers, such as PD-1, CTLA-4, and TIM-3. Furthermore, KRAS mutations effects can also be mediated through exosomes containing KRAS^G12D. These exosomes can be taken via an AGER-dependent mechanism—a multiligand receptor—by macrophages, modulating their differentiation into a pro-tumor/anti-inflammatory phenotype.

Figure 5

Strategies to target KRAS mutations. Several therapies have been developed to target KRAS, namely, KRAS plasma membrane association inhibitors, KRAS synthetic lethal interactors, KRAS downstream signaling pathways blockade, KRAS-mediated inflammation, and immunotherapy. One of the most promising strategies is the novel KRAS synthetic lethal interactors that specifically target the cysteine in the mutated KRAS^G12C through covalent irreversible binding and favor KRAS-GDP state over GTP. These alterations impair RAF binding and the activation of the signaling pathway, decreasing cell viability and increasing apoptosis of those cells harboring KRAS^G12C mutations.

Figure 6

Combined therapeutic approach in KRAS mutated cancers: immunotherapy and MEK inhibitors. Immunotherapy targeting immune checkpoint molecules, such as PD-1, PD-L1, and CTLA-4, has been demonstrated to be one of the most hopeful cancer treatments, with positive results in KRAS mutated cancers. Surprisingly, the combined therapies of MEK inhibitors with antibodies targeting PD-1, PD-L1, or CTLA-4 exert higher anti-tumor effects than monotherapies. In a murine KRAS-mutant colorectal cancer model, the MEK inhibitor selumetinib attenuated anti-CTLA-4-mediated T cell activation and infiltration into tumors and blocked monocytes differentiation into anti-inflammatory macrophages. Thus, MEK inhibition, specifically selumetinib, brings beneficial effects to the TME in the context of CTLA-4 blockade, and, more importantly, this combination of MEK inhibitors with CTLA-4 blocking antibodies re-educates the TME from an immunosuppressive to an immune alert status, expanding therapeutic intervention.