B7-33 Potential Actions on Inflammation and Fibrosis

B7-33 is an experimental peptide designed as a simplified analogue of the natural hormone relaxin-2 (H2 relaxin). In physiological systems, relaxin family peptides such as H1, H2, and H3 relaxins, as well as several insulin-like peptides (INSL3–6) are thought to interact with specific G-protein-coupled receptors (RXFP1–4) to regulate complex processes such as fibrosis, inflammation, and tissue remodeling. Among these, H2 relaxin has attracted scientific interest because of its potential to reduce fibrotic tissue formation and promote vascular relaxation. However, its complex structure makes it unstable in biological systems and difficult to produce for research purposes.

To overcome these limitations, researchers from the team of Hossain et al. developed B7-33 as a single-chain derivative of the H2 relaxin B-chain that hypothetically maintains the ability to interact with the RXFP1 receptor, aka the primary target of native relaxin signaling, while at the same time it is water-soluble and less prone to degradation.⁽¹⁾ Cellular studies in fibroblasts, which are the cells responsible for producing fibrotic scar tissue, suggests that B7-33 exhibits comparable potential to H2 relaxin in activating ERK1/2 signaling and upregulating matrix metalloproteinases (most notably MMP-2) that may help degrade excess collagen and ultimately combat fibrosis. By providing a simplified and stable scaffold, B7-33 may allow scientists to study and prevent the molecular mechanisms of fibrosis, where damaged functional cells are replaced by rigid, non-functional structures.

 

Research

B7-33 and Fibrosis Reduction

In the context of fibrosis reduction, research by Bhuiyan et al. suggests that B7-33 may stimulate collagen breakdown indirectly by increasing the activity of the collagen-degrading enzyme MMP-2 in fibroblast cultures.⁽²⁾ Specifically, B7-33 appeared to elevate MMP-2 detected by gelatin zymography and densitometry, with potential broadly similar to equimolar H2-relaxin. Specifically, Bhuiyan commented that “B7-33 may engage this ERK1/2 pathway to drive the downstream increase in MMP-2.” ERK1/2 are posited as intracellular “switch” enzymes in the MAPK pathway that, when activated (phosphorylated), pass signals from surface receptors to the nucleus to change gene expression. Furthermore, the response of MMP-2 levels by B7-33 introduction was apparently abolished by an RXFP1 antagonist and by an AT2 receptor antagonist, suggesting that B7-33 may act through RXFP1–AT2R heterodimers to engage downstream signaling, culminating in higher MMP-2 levels and, by extension, potential pro-degradative pressure on collagen in vitro. In addition B7-33 signaling may be associated with lowering tissue inhibitors of metalloproteinase-1 (TIMP-1) levels, which might further tip the MMP/TIMP balance toward matrix turnover.⁽²⁾

Further research by Alam et al. in RXFP1-positive myofibroblasts also suggests that B7-33 may rapidly activate ERK1/2, which in turn may connect to a downstream relaxin-like antifibrotic signaling.⁽³⁾ In post-MI research models and isoproterenol-induced cardiomyopathy research models, B7-33 was suggested to reduce left-ventricular collagen burden and stiffness, similarly to H2-relaxin. Specifically, the researchers commented that the peptide may have “equivalently reduced interstitial LV collagen deposition by ∼40 %.” B7-33 may have also seemed to down-modulate pro-fibrotic drivers (myofibroblast accumulation and TGF-β1 expression) and to lessen cardiomyocyte hypertrophy by about 70–75%. Anti-inflammatory potential indicated by 65–75% reduced macrophage infiltration and apparent restoration of vascular densities toward control levels were observed in parallel, which may secondarily favor anti-fibrotic remodeling.

B7-33 and Nociceptive Signaling

Research by Abboud et al. suggests that activation of the relaxin receptor RXFP1 with H2-relaxin or its peptidomimetic B7-33 may transiently reduce nociceptive signaling in lab models.⁽⁴⁾ These changes were observed in models of inflammation but not in sham controls, suggesting that RXFP1-linked antinociception may emerge under sensitized conditions such as by inflammation rather than at baseline. Specifically, the researchers tested both mechanical and thermal sensitivity, revealing that the mechanical nociception reduction may have been RXFP1-dependent. By contrast, the thermal nociception was not antagonized possibly due to the limited RXFP3 activation by B7-33. Researchers suggest that via RXFP1 activation, B7-33 may rebalance local excitatory–inhibitory processing in certain nerve circuits that influence nociceptive transmission, which might plausibly account for the rapid, state-dependent antinociception observed in the experimental laboratory models. Mechanistically, the immediate onset and short duration of the peptide’s potential may point to fast GPCR signaling (eg, cAMP/PKA or MAPK cascades) rather than slower transcription-dependent action. Overall, from a bench standpoint, the data suggest that B7-33 and H2-relaxin may acutely modulate nociceptive processing via RXFP1-rich nodes tied to descending control, with efficacy that appears context-dependent (inflamed vs. naïve), transient, and possibly modality-specific. Further work would be needed to localize the critical nuclei, resolve RXFP1-selective versus off-target contributions, and map the intracellular pathways that couple RXFP1 to mechanical versus thermal antinociception.

B7-33 and Vasodilation

Marshall et al. suggested that B7-33 may support vessel relaxation in mesenteric artery models, particularly in response to bradykinin.⁽⁵⁾ Instead of primarily enhancing the mechanism commonly seen with many vasodilators which is nitric oxide (NO) signaling, B7-33 seemed to shift the response toward endothelium-derived hyperpolarization (EDH). This may involve activation of small and intermediate calcium-activated potassium channels (KCa2.x and KCa3.1), which often play a dominant role in regulating resistance-vessel tone. In the ex vivo setup, the researchers exposed mesenteric arteries to trophoblast-conditioned medium rich in anti-angiogenic factors such as sFlt-1 and sEng, which are linked to inducing endothelial dysfunction. When B7-33 was co-incubated at low-nanomolar concentrations, it appeared to prevent or lessen the usual decline in acetylcholine sensitivity. This observation may indicate that B7-33 may help maintain or restore endothelial signaling under oxidative or anti-angiogenic stress, possibly by rebalancing vasodilator pathways (EDH, NO, and PGI₂) relative to vasoconstrictor influences (e.g., constrictor prostanoids, endothelin-1, reactive oxygen species). The precise molecular processes remain uncertain, but given that B7-33 interacts with the relaxin receptor RXFP1, it might engage mechanisms involving cAMP/PKA signaling, PI3K/Akt–eNOS cross-talk, or modulation of endothelial KCa channel activity. Overall, these findings suggest that B7-33 may possess vasoprotective potential in preclinical settings, though this remains to be confirmed through further mechanistic investigation.

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References:

1. Hossain, M. A., Kocan, M., Yao, S. T., Royce, S. G., Nair, V. B., Siwek, C., Patil, N. A., Harrison, I. P., Rosengren, K. J., Selemidis, S., Summers, R. J., Wade, J. D., Bathgate, R. A. D., & Samuel, C. S. (2016). A single-chain derivative of the relaxin hormone is a functionally selective agonist of the G protein-coupled receptor, RXFP1. Chemical science, 7(6), 3805–3819. https://doi.org/10.1039/c5sc04754d
2. Bhuiyan S, Shen M, Chelvaretnam S, Tan AY, Ho G, Hossain MA, Widdop RE, Samuel CS. Assessment of renal fibrosis and anti-fibrotic agents using a novel diagnostic and stain-free second-harmonic generation platform. FASEB J. 2021 May;35(5):e21595. doi: 10.1096/fj.202002053RRR. PMID: 33908676.
3. Alam F, Gaspari TA, Kemp-Harper BK, Low E, Aw A, Ferens D, Spizzo I, Jefferis AM, Praveen P, Widdop RE, Bathgate RAD, Hossain MA, Samuel CS. The single-chain relaxin mimetic, B7-33, maintains the cardioprotective effects of relaxin and more rapidly reduces left ventricular fibrosis compared to perindopril in an experimental model of cardiomyopathy. Biomed Pharmacother. 2023 Apr;160:114370. doi: 10.1016/j.biopha.2023.114370. Epub 2023 Feb 6. PMID: 36753958.
4. Abboud C, Brochoire L, Drouet A, Hossain MA, Hleihel W, Gundlach AL, Landry M. Analgesic effect of central relaxin receptor activation on persistent inflammatory pain in mice: behavioral and neurochemical data. Pain Rep. 2021 Jun 16;6(2):e937. doi: 10.1097/PR9.0000000000000937. PMID: 34159282; PMCID: PMC8213244.
5. Marshall SA, O’Sullivan K, Ng HH, Bathgate RAD, Parry LJ, Hossain MA, Leo CH. B7-33 replicates the vasoprotective functions of human relaxin-2 (serelaxin). Eur J Pharmacol. 2017 Jul 15;807:190-197. doi: 10.1016/j.ejphar.2017.05.005. Epub 2017 May 3. PMID: 28478069.