Metabolic products can lead to crucial biological function alterations, by regulating cellular enzymes. Itaconate is probably the best example of how a metabolic process can be diverted to generate an immunomodulator effect in macrophages. The endogenous metabolite itaconate has been reported to regulate macrophage function. Through inflammatory stimuli, such as bacterial endotoxin, the enzyme aconitate decarboxylase 1 (ACOD1), also called immune response gene 1 (IRG1), is activated and promotes the production of itaconate from the mitochondrial tricarboxylic acid cycle by decarboxylating cis-aconitate. Itaconate has been reported to have multiple immunoregulatory and redox effects. In addition, reports have described its antibacterial and protumor effects.
The involved mechanism in these effects includes the inhibition of aerobic glycolysis by targeting glyceraldehyde-3-phosphate dehydrogenase and aldolase A, inhibition of succinate dehydrogenase and blockade of IκB-ζ translation. Moreover, other redox-sensitive targets of itaconate are transcription factors. For example, the antioxidant nuclear E2-related factor 2 (Nrf-2) is activated by itaconate by alkylation of its inhibitor Keap-1, in a manner similar to dimethyl-fumarate (used to treat multiple sclerosis; trade name Tecfidera). The electrophilic nature of these substances allow to covalently modify free protein cysteines in these targets, leading to their activation or inhibition. STAT-3 is another transcription factor affected by itaconate and its derivatives.
4-octyl itaconate (4-OCIT), a cell-permeable (i.e. hydrophobic) derivative, is more electrophilic than the sister derivative and directly alkylates cysteine residues of its upstream kinase JAK1, which leads to its inactivation. It also alkylates the cysteines of the transcription factor NF-kB, which regulates the production of countless inflammatory factors, and deactivates it. This opposes the phosphorylation of its inhibitor IkB, which is phosphorylated by the IKK kinases, causing its degradation by the proteasome. As a direct consequence, there is a strong suppression of cyclo-oxigenase 2 (COX-2) at both mRNA and protein levels. This means a sharp drop in prostanoid production (i.e. PGE2), meaning as a consequence a reduction in inflammation and pain.
4-OCIT inhibited NLRP3 inflammasome activation, but not AIM2 or NLRC4: the compound affects the interaction between NLRP3 and NEK7 kinase, by alkylating cysteine residue 548 of NLRP3. Furthermore, it inhibits NLRP3-dependent IL-1β release from blood white cells isolated from cryopyrin-associated periodic syndrome (CAPS) patients, and reduced inflammation in an in vivo model of urate-induced peritonitis. This means that, similarly to dimethyl fumarate, itaconate derivatives can find use in a more selective management of conditions such as multiple sclerosis, rheumatoid arthritis, systemic lupus and also systemic auto-inflammatory diseases, for which it is known that there are not many therapeutic options.
Regarding this last point, very recently published research discovered that 4-OCIT also affects the functions of the intracellular sensor STING. The stimulator of interferon genes (STING) signaling platform functions as a hub in macrophage immunity and has a deep impact on the prognosis of sepsis. The drug alkylates cysteine residues 65, 71, 88 and 147 of STING, thereby inhibiting its phosphorylation and the downstream signaling. It also inhibits cGAS-STING-related antiviral immune responses and autoimmune inflammation. However, it seems that endogenous itaconate does not affect cGAS-STING activation, indicating that 4-OCIT and itaconate work differently. Considering that the cGAS-STING axis is almost selectively involved in the pathogenesis of systemic lupus, its modulation by 4-OCIT could be a more selective therapy than the current durgs.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
Scientific references
Xu L, Cai J, Li C et al. Mol Med. 2023; 29(1):58.
Diskin C et al. J Immunol. 2023; 211(6):1032-41.
Blanco LP et al. Arthritis Rheumatol. 2022; 74(12):1971.
Runtsch MC et al. Cell Metab. 2022; 34(3):487-501.
Diskin C et al. J Immunol. 2021; 207(10):2561-69.
Hooftman A et al. Cell Metab. 2020; 32(3):468-78.
Liao ST et al. Nature Commun. 2019; 10(1):5091.
Mills EL et al. Nature. 2018; 556(7699):113-117.
Liao ST et al. Nat Commun. 2019; 10(1):5091.