Advances in the Canine Cranial Cruciate Ligament, 2nd edition

Role of Nitric Oxide Production and Matrix Protease Activity in Cruciate Ligament Degeneration 83

Figure 10.2 The catabolic actions of NO and matrix metalloproteinases (MMPs) on CrCL matrix homeostasis.

Role of NO and reactive oxygen
species in the joint

NO is believed to be mainly a catabolic player in
joint physiology (see Figure 10.2). NO is a pre-
dominant mediator in the progression of OA by
inhibiting proteoglycan and collagen synthesis,
and promoting activation of metalloproteinases
or chondrocyte apoptosis which could further
contribute to ECM reduction in OA (Taskiran
et al. 1994; Goggset al. 2003; Nakagawaet al.
2010). In pathological situations such as OA,
additional reactive molecules, so-called reactive
oxygen species (ROS), are abnormally produced
and contribute directly or indirectly to cartilage
degradation, MMP activation and cell death
induction (Lepetsos & Papavassiliou 2016). ROS
are free radicals containing oxygen molecules
including hydroxyl radical, hydrogen perox-
ide, superoxide anion and nitric oxide, and
are generated mainly by the mitochondria.
Peroxynitrite, a redox derivative of NO, has
been implicated in pro-inflammatory and pro-
apoptotic effects by triggering the oxidative

damage of cellular proteins, oxidizing lipids,

and DNA in human articular cartilage (Pacher

et al. 2007). Chondrocyte death is regarded as a

major factor in OA pathogenesis. NO has been

reported to be the primary inducer of chon-

drocyte apoptosis mediated by a caspase- and

mitochondria-dependent pathway (Maneiro

et al. 2005), while other studies have shown

that endogenous NO alone does not induce

cell death; rather, the generation of additional

ROS is also required (Del Carlo & Loeser 2002;

Liang et al. 2014). Studies of human chon-

drocytes showed that peroxynitrite induced

apoptosis via a caspase-independent pathway

(Whitemanet al. 2004). NO-induced apoptosis

of chondrocytes depends on crosstalk between

the iNOS and cyclooxygenase 2 (COX-2)

systems, which is linked to the activation of

mitogen-activated protein kinase kinase 1 and

2 (MAP2K1/2) and p38 mitogen-activated

protein kinase pathways. Apoptosis has also

been detected in healthy and ruptured canine

CrCLs, with a moderate but significant corre-

lation to NO (Gygeret al. 2007). Analyzing the