The Role of Proprotein Convertases in the Regulation of the Function of Immune Cells in the Oncoimmune Response

Abstract

Proprotein convertases (PC) are a family of 9 serine proteases involved in the processing of cellular pro-proteins. They trigger the activation, inactivation or functional changes of many hormones, neuropeptides, growth factors and receptors. Therefore, these enzymes are essential for cellular homeostasis in health and disease. Nine PC subtilisin/kexin genes (PCSK1 to PCSK9) encoding for PC1/3, PC2, furin, PC4, PC5/6, PACE4, PC7, SKI-1/S1P and PCSK9 are known. The expression of PC1/3, PC2, PC5/6, Furin and PC7 in lymphoid organs such as lymph nodes, thymus and spleen has suggested a role for these enzymes in immunity. In fact, knock-out of Furin in T cells was associated with high secretion of pro-inflammatory cytokines and autoantibody production in mice. This suggested a key role for this enzyme in immune tolerance. Moreover, Furin through its proteolytic activity, regulates the suppressive functions of Treg and thus prevents chronic inflammation and autoimmune diseases. In macrophages, Furin is also involved in the regulation of their inflammatory phenotype. Similarly, PC1/3 inhibition combined with TLR4 stimulation triggers the activation of the NF-κB signaling pathway with an increased secretion of pro-inflammatory cytokines. Factors secreted by PC1/3 KD macrophages stimulated with LPS exert a chemoattractive effect on naive auxiliary T lymphocytes (Th0) and anti-tumoral activities. The link between TLR and PCs is thus very important in inflammatory response regulation. Furin regulates TL7 and TLR8 processing and trafficking whereas PC1/3 controls TLR4 and TLR9 trafficking. Since PC1/3 and Furin are key regulators of both the innate and adaptive immune responses their inhibition may play a major role in oncoimmune therapy. The role of PCs in the oncoimmune response and therapeutic strategies based on PCs inhibition are proposed in the present review.

Keywords: furin; immunothearpy; macrophages; microbiota; oncoimmune; proprotein convertase; proprotein convertase 1/3.

Figure 1

Proenkephalin and proprotein convertases in immune cells. (A) Scheme of Proenkephalin cleavage by PCs in macrophages, (B) Expression of proenkephalin in spleen with or without LPS treatment in time course, inset photograph representing the expression of proenkephalin in spleen ‘marginal zone after 12h LPS treatment, (C) Table of the immune action of the enkephalins on immune cells, (D) identification of PC1/3 expression in rat spleen in time course after LPS treatment.

Figure 2

Comparison of PC1/3 KO mice phenotype with WT and the size of their spleen.

Figure 3

Proposed model depicting the impact of PC1/3 knockdown on the activation of TLR4. Yellow droplets in the cell membrane symbolize that KD cells exhibited modifications of their lipid profile. (1) After LPS treatment, TLR4 is internalized in the endosomes to trigger the MYD88-independentsignaling resulting in the activation of IRF-3. (2) As a result, the level of TLR4 at the plasma membrane decreases. (3) In the absence of PC1/3, TLR4 is more quickly re-expressed at the cell surface as symbolized by the clock pictogram, i.e,. 3 h after LPS treatment. These receptors may translocate from endosomal recycling compartment (ERC) where they are stored or from the Golgi apparatus after their synthesis and maturation. (4) TLR4 receptor can be synthesized from a pre-existing pool of messengers. This may support the de novo expression of the receptor at the cell surface or the replenishment ofTLR4 stock in ERC if the receptor translocated from this compartment. (5) The re-expression of TLR4 at the cell surface after 3 h of LPS challenge in PC1/3 KD macrophages leading to a stronger activation of the pro-inflammatory NFKB pathway while the anti-inflammatory STAT3 pathway is downregulated. (6) As a consequence, PC1/3 KD macrophages stimulated with LPS secrete more pro-inflammatory cytokines and antitumor factors. After treatment of PC1/3 KD cells with LPS during 6 h, TLR4 is again internalized, and its amount at the cell surface decreased (2). (7) The increase of MYD88 level observed in PC1/3 KD macrophages after 6 h of LPS treatment may compensate the decrease of TLR4 at the cell surface and maintain the activation of the signaling pathway. (8) A clear correlation between protein synthesis and gene expression in KD cells treated with LPS cannot be made. Therefore, another mechanism, such as augmentation of protein half-life or stability may be responsible for this increase. (9) PC1/3 KD cells resist to the inhibitory effect of IL-10 and clearly show a pro-inflammatory response.

Figure 4

Schematic depicting how PC1/3 knockdown impacts TLR4 and 9 activation in macrophages.

Figure 5

Schematic representation of reprogrammed macrophages after PC inhibition in tumor microenvironment.