TNIP1 Regulates Cutibacterium acnes-Induced Innate Immune Functions in Epidermal Keratinocytes

Abstract

Human skin cells recognize the presence of the skin microbiome through pathogen recognition receptors. Epidermal keratinocytes are known to activate toll-like receptors (TLRs) 2 and 4 in response to the commensal Cutibacterium acnes (C. acnes, formerly known as Propionibacterium acnes) bacterium and subsequently to induce innate immune and inflammatory events. These events may lead to the appearance of macroscopic inflammatory acne lesions in puberty: comedos, papules and, pustules. Healthy skin does not exhibit inflammation or skin lesions, even in the continuous presence of the same microbes. As the molecular mechanism for this duality is still unclear, we aimed to identify factors and mechanisms that control the innate immune response to C. acnes in keratinocytes using a human immortalized keratinocyte cell line, HPV-KER, normal human keratinocytes (NHEK) and an organotypic skin model (OSM). TNIP1, a negative regulator of the NF-κB signaling pathway, was found to be expressed in HPV-KER cells, and its expression was rapidly induced in response to C. acnes treatment, which was confirmed in NHEK cells and OSMs. Expression changes were not dependent on the C. acnes strain. However, we found that the extent of expression was dependent on C. acnes dose. Bacterial-induced changes in TNIP1 expression were regulated by signaling pathways involving NF-κB, p38, MAPKK and JNK. Experimental modification of TNIP1 levels affected constitutive and C. acnes-induced NF-κB promoter activities and subsequent inflammatory cytokine and chemokine mRNA and protein levels. These results suggest an important role for this negative regulator in the control of bacterially induced TLR signaling pathways in keratinocytes. We showed that all-trans retinoic acid (ATRA) induced elevated TNIP1 expression in HPV-KER cells and also in OSMs, where TNIP1 levels increased throughout the epidermis. ATRA also reduced constitutive and bacterium-induced levels of TNFα, CCL5 and TLR2, while simultaneously increasing CXCL8 and TLR4 expression. Based on these findings, we propose that ATRA may exhibit dual effects in acne therapy by both affecting the expression of the negative regulator TNIP1 and attenuating TLR2-induced inflammation. Overall, TNIP1, as a possible regulator of C. acnes-induced innate immune and inflammatory events in keratinocytes, may play important roles in the maintenance of epidermal homeostasis.

Cutibacterium acnes regulates the epidermal barrier properties of HPV-KER human immortalized keratinocyte cultures

“…In our experiments, the extent of barrier changes in keratinocyte cultures appeared to be strain- and dose-dependent: faster growing C. acnes strains (889 and ATCC 11828) exhibited more pronounced changes. These results confirm our earlier findings, suggesting that, in parallel with bacterial growth, the pathogenic potential of C. acnes increases 37 , 52 . This conclusion contradicts findings that the amount of the bacterium does not differ in control and acne skin 53 , 54 .…”

“…Overall, through the induction of mostly TLR-dependent innate immune and inflammatory changes and the production of bioactive molecules, the C. acnes bacterium may induce innate immune and inflammation activation and autophagy, as well as altering the differentiation state of skin cells and epidermal barrier functions 19 – 21 , 52 .…”

Involvement of Pattern Recognition Receptors in the Direct Influence of Bacterial Components and Standard Antiacne Compounds on Human Sebaceous Gland Cells

“…In contrast to our findings, Liu et al [25] reported that atRA was able to inhibit cytokine release from monocytes by downregulating TLR2 and CD14 expression and function, while TLR4-induced cytokine release was not affected. On the other hand, atRA reduced TLR2 expression in human keratinocytes while simultaneously increased the levels of IL-8 and TLR4 [48]. Evidently, the mode of retinoid-regulated gene expression depends on cell type with exertion of an anti-inflammatory effect via two pathways, one specifically affecting TLR2 and CD14 expression and another TLR-independent, the latter probably is common with human sebocytes.…”

A Janus-Faced Bacterium: Host-Beneficial and -Detrimental Roles of Cutibacterium acnes

“…Less well-understood is which C. acnes components, e.g., genomic DNA, surface/secreted proteins, peptidoglycan and cell wall polysaccharide, are actually sensed and how the pro-inflammatory activity of C. acnes is controlled/dampened on normal skin. C. acnes-exposed keratinocytes may attenuate TLR-induced inflammation by negative regulatory circuits involving proteins such as TNIP1 and TNFAIP3 and microRNAs such as miR-146a, that can downregulate the production of inflammatory cytokines and chemokines (Erdei et al, 2018(Erdei et al, , 2021Zeng et al, 2019). In addition, besides keeping bacteria in physical distance to immunocompetent cells, e.g., within infundibula of sebaceous follicles, another strategy to limited responses might be to eliminate invasive C. acnes.…”