Rankl/OPG system is altered in hepatic osteodystrophy

ABSTRACT

The coexistence of liver disease and metabolic bone disease has been recognized for many years and is now the subject of increasing attention. Hepatic Osteodystrophy has been established in patients with cholestatic liver disease, but new research suggests that it is prevalent in patients with other chronic liver diseases. Its etiology is complex and multifactorial. The Receptor activator of nuclear factor Kb ligand [RANKL] plays a role in the differentiation and activation of bone resorbing osteoclasts by binding to its high affinity receptor [RANK] located on the surface of osteoclasts. This effect is counterbalanced by osteoprotegren [OPG], which acts as a decoy receptor competing with RANKL for RANK. In this study we aim to evaluate OPG/RANKL system in cirrhotic patients with backache. This study includes 50 subjects suffering backache, divided into 4 groups as follows Group I10 subjects with normal bone mineral density [BMD] as control, Group II 10 patients with pathological BMD but who are otherwise healthy, Group III 15 patients with cirrhosis and normal BMD, Group IV 15 patients with cirrhosis and pathological BMD. All patients underwent clinical examination, routine liver function tests, alkaline phosphatase, total calcium, serum OPG, serum RANKL, added to BMD estimation. The lowest BMD values were estimated at the lumber spine, then femoral neck, and lastly lower end of radius. There was a significant decrease in OPG in osteopenic non cirrhotic patients compared to the control group, while it was significantly higher than controls in both osteopenic and non osteopenic patients of the cirrhotic groups. SRANKL was significantly higher in non cirrhotic patients with pathological BMD compared to the control group, but lower than controls in cirrhotic groups both with normal and pathological BMD, with a significant difference in cirrhotics with pathological BMD, and a non significant difference in those with normal BMD compared to controls. Serum OPG was negatively correlated to serum calcium, albumin, and INR, but positively correlated to bone alkaline phosphatase, and AST in cirrhotic patients of both groups. OPG/RANKL system plays a role in the pathogenesis of hepatic Osteodystrophy. In cirrhotic patients, low BMD has a tendency to affect axial bone earlier, which is similar to postmenopausal osteoporosis. However in cirrhosis there are higher OPG and lower sRANKL levels which are opposite to postmenopausal osteoporosis. This difference indicates that either OPG/RANKL system is working in a different way in cirrhosis, which might be due to an increased RANK/RANKL affinity which is not measurable, and consumes part of total RANKL leaving a smaller amount of measurable soluble RANKL to be assessed, which would explain its lower level in serum despite increased osteoporotic changes in bone, or there are other factors associated with this process to make their mechanism of action different than in postmenopausal osteoporosis