**10. CNP during inflammatory disease activity**

came after the description of two novel mutations that resulted in gain of function in humans. The C-type natriuretic peptide (CNP), encoded by NPPC gene, is located on chromosome 2q37.1. Two independent studies have described three patients with a Marfan-like phenotype presenting a de novo balanced translocation involving the same chromosomal region 2q37.1 and overexpression of NPPC [70]. One study reported on two partially overlapping interstitial 2q37 deletions. These two patients showed opposite phenotypes characterized by short stature and skeletal overgrowth, respectively. The patient with short stature presented a 2q37 deletion causing the loss of one copy of the NPPC gene with normal CNP plasma concentration. The deletion identified in the patient with a Marfan-like phenotype interrupted the DIS3L2 gene without involving the NPPC gene. In addition, a strongly elevated CNP plasma concentration was found in this patient with Marfanoid features and a tall stature [71, 72].

CNP was first isolated from porcine brain and was expected to be a neuropeptide [73], but the physiological significance of the CNP/GC-B system has been established in the vascular and skeletal systems [53, 58, 71, 72, 74–78]. It was reported that CNP and GC-B are expressed in the central [79–83] and peripheral nervous systems [84, 85]. It has been shown that the hypothalamus is an important center to control food intake and energy expenditure [86]. CNP mRNA was detected in the rat hypothalamus indicating this peptide's role in numerous

**Figure 3.** CNPcol2a1TG mice (on the right) are tall and slender as compared to the wild type littermates (on the left).

**9. CNP roles in energy metabolism**

56 Newest Updates in Rheumatology

Initial reports about CNP's effect on inflammatory disease activity came from osteoarthritis disease models. Since both nitric oxide and CNP are the two main activators of cGMP in cartilage they were checked for their role in development of osteoarthritis. The production of nitric oxide (NO) regulates host defense and inflammation. Nitric oxide has vasodilation, cytotoxicity, and it has a role in cytokine-dependent tissue injury. NO effects have been blamed in the tissue injury of a variety of rheumatologic conditions including systemic lupus erythematosus, rheumatoid arthritis, and osteoarthritis. Pro-inflammatory effects of nitric oxide include vasodilation, edema, cytotoxicity, and the mediation of cytokine-dependent processes that can lead to tissue destruction. In contrast to NO effect in the cartilage, NO secretion from vascular wall is protective against neutrophil adhesion and related vascular injury. Thus, nitric oxide has been found as a clear role player in osteoarthritis pathogenesis but not in microvasculature injury [88, 89]. Although CNP is known to induce articular chondrocyte hypertrophy, it was never blamed to be involved in osteoarthritis pathogenesis, while it was considered to contribute to the disease progression. On the other hand, we were able to show in an inflammatory arthritis murine model that cartilage overexpression of CNP increased the chondrocyte number, matrix synthesis, and maintained a thicker hypertrophic cartilage in the growth plate and in the joints and thus was protective against the cartilage degenerative effects of inflammatory arthritis. The mouse model we used was for a K/BxN rheumatoid arthritis mouse model that developed severe inflammatory arthritis which was evident from first synovitis, pannus formation, and then secondary cartilage deterioration [28, 90]. Our transgenic mouse under Col2a1 promoter overexpressed CNP in the cartilage tissue mainly in the growth plate and in the joint cartilage. CNP transgenic mice developed thick growth plates with enlarged chondrocytes and wider growth plates with proliferating chondrocytes that produced a rich matrix. Furthermore, when CNP overexpressing mice was crossed with K/BxN mouse and developed systemic arthritis, we observed that cartilage matrix integrity and cartilage structure was protected against the inflammation. CNP transgenic mice did not develop severe complications of arthritis (**Figure 4**) [27, 91].

Another evidence for CNP's systemic anti-inflammatory effect was reported in a rat model of hemorrhagic shock and resuscitation. CNP infusion to this model lowered the myeloperoxidase activity and decreased the expression of TNF-α, IL-6, and IL-1β in the kidneys. CNP treatment suppressed oxidative stress, ameliorated the inflammatory response, and caused acute kidney injury [92]. Investigators suggested that they demonstrated CNP infusion's inhibitory effect on the generation of reactive oxygen species (ROS) and pro-inflammatory cytokines after hemorrhagic shock induction and subsequently suppressed the activation, recruitment, and adherence of neutrophils in the kidney. Neutrophil recruitment is one of the fundamental pathways in hemorrhagic shock. It is suggested that neutrophil recruitment is also delayed after CNP treatment. In an earlier study, Chen et al. showed that CNP treatment

transgenic mice showed a decrease in fat weight and adipocyte hypertrophy and increases in fatty acid β-oxidation, lipolysis-related gene expression, and energy expenditure during high fat diet (HFD)-induced obesity. Furthermore it seems like CNP overexpression diminished the inflammatory activity in adipogenic tissue. Adipogenic cell CNP transgenic mice were reported to have significantly decreased gene expression of TNF-α, interleukin-6 (IL-6), monocyte chemoattractant protein-1 (MCP-1), and F4/80 in mature adipocytes [98]. Adipogenic CNP transgenic mice also developed better glucose tolerance and insulin sensitivity, which were found to be associated with enhanced insulin-stimulated Akt phosphorylation. It was suggested that CNP overexpression in adipocytes protects against adipocyte hypertrophy, excess lipid metabolism, inflammation, and decreased insulin sensitivity during HFD-induced obesity. An earlier report also suggested that overexpression of endothelialspecific CNP overexpression protects against visceral adipose tissue hypertrophy, systemic inflammation, and insulin resistance during the development of obesity due to the feeding of a high-fat diet (HFD) [99]. Overall current knowledge suggests that natriuretic peptides are new pathways controlling human adipose tissue lipolysis operating via a cGMP-dependent

Longitudinal Growth in Rheumatologic Conditions: Current and Emerging Treatments...

http://dx.doi.org/10.5772/intechopen.75879

59

One of the intriguing studies is one that examined serum levels of 53 patients with Behçet's disease and showed that all patients with active disease had lower levels of CNP indicating its suppression by inflammatory disease activity [100]. Thus, there is evidence that during chronic inflammatory diseases CNP serum levels might be suppressed which can impact the skeletal growth if the

Use of CNP and its analogues in achondroplasias: gain-of-function mutations in the FGFR3 gene result in achondroplasia. Achondroplasia is known as the most common form of dwarfism. In patients with achondroplasia there is impaired proliferation and differentiation of the chondrocytes in the growth plate cartilage that causes stunted longitudinal growth due to endochondral growth suppression and skull abnormalities due to membranous ossification disruption. In achondroplasia, FGFR3 mutations induce increased phosphorylation of the tyrosine kinase receptor FGFR3 and increase the mitogen-activated protein kinase (MAPK). It is known that C-type natriuretic peptide (CNP) suppresses FGFR3 downstream signaling by inhibiting the pathway of mitogen-activated protein kinase (MAPK) in vivo and in vitro. Mice overexpressing CNP rescues FGFR3 gain of mutation-related dwarfism. Exogenous administration of CNP has been challenging since it is rapidly cleared and degraded in vivo through receptor-mediated and proteolytic pathways such as proteolyticneutral endopeptidase degradation. Therefore, multiple variants of CNP molecules have been tested for their efficacy. Recently, a variant of CNP called BMN111, neutral endopeptidase-resistant CNP analog, showed significant ability to stimulate signaling downstream of the CNP receptor, natriuretic peptide receptor B. Initial trial of continuous delivery of CNP through intravenous (IV) infusion in the form of BMN111 in 2014 showed normalization of

individuals affected have open growth plates and ongoing longitudinal growth process.

pathway.

dwarfism [101].

**12. Current clinical use of CNP**

**Figure 4.** K/BxN+CNPcol2a1TG double transgenic mice do not develop growth retardation or severe complications of arthritis.

effectively attenuates lipopolysaccharide (LPS)-induced endothelial activation by eliminating intracellular ROS production, inhibiting the NF-κB and MAPK p38 signaling pathways and activating the PI3 K/Akt/HO-1 pathway in human umbilical vein endothelial cells (HUVECs), [93] suggesting an anti-inflammatory effect for CNP.

Another study that showed CNP's effect in reducing the LPS-induced lung injury suggested that mechanism of action might involve downregulation of inflammatory cytokine expression in lung parenchyma and again downregulation of neutrophil migration in the lungs [94].

Finally, evidence for CNP and its derivate anti-inflammatory treatment potential were shown in a wounded cartilage explant model in steers. In this study, wounded explants were cultured with 0 or 10 ng/mL IL-1β and/or microcapsules loaded with or without CNP for a period of 48 h. The presence of CNP microcapsules had a concentration-dependent effect with significant inhibition of NO release in response to IL-1β at 2000 (p < 0.01), 10,000 (p < 0.01), and 50,000 microcapsules/well (p < 0.001) [45]. Others suggested that the effect of CNP on preventing the inflammatory effects of IL-1β in chondrocytes depends on local protein concentration. While low concentrations (pM) were shown to promote a proliferative response, high concentrations (μM) lead to anabolic effects such as matrix synthesis in chondrocytes [95, 96].

Acute inflammation and the inflammatory mediators seems to suppress the activity of CNP in growing organisms [97]. It is possible that in chronic inflammatory diseases, serum NT-proCNP levels are also low and contribute to the growth arrest during active disease in children.
