1,25(OH)2D3, the active form of vitamin D, has an antiproliferative and antifibrotic effect on hepatic stellate cells. Our aim was to investigate the potential of 1,25(OH)2D3 to inhibit the development of liver fibrosis and to ameliorate established fibrosis in vivo. The antifibrotic effect of 1,25(OH)2D3 was investigated in a thioacetamide (TAA) model (as a preventive treatment and as a remedial treatment) and in a bile duct ligation model. In the preventive model, rats received simultaneously intraperitoneum injection of TAA and/or 1,25(OH)2D3 for 10 wk. In the remedial model, rats were treated with TAA for 10 wk and then received 1,25(OH)2D3 or saline for 8 wk. Fibrotic score was determined by Masson staining. Collagen I, α-smooth muscle actin (α-SMA), tissue inhibitor of metalloproteinase-1 (TIMP1), platelet-derived growth factor (PDGF), and transforming growth factor-β (TGF-β) expression were measured by Western blot analysis and real-time PCR. Hypercalemia was detected by chemistry measurements. Preventive treatment of 1,25(OH)2D3 significantly suppressed liver fibrosis both macroscopically and microscopically and significantly lowered the fibrotic score of the TAA + 1,25(OH)2D3 group compared with the TAA group. 1,25(OH)2D3 significantly inhibited expression of PDGF and TGF-β by ∼50% and suppressed the expression of collagen Iα1, TIMP1, and α-SMA by approximately three-, two-, and threefold, respectively. In contrast, 1,25(OH)2D3 was inefficient in amelioration of established liver fibrosis. Administration of 1,25(OH)2D3 to bile duct ligation rats led to a high mortality rate probably caused by hypercalcemia. We conclude that 1,25(OH)2D3 may be considered as a potential preventive treatment in an in vivo model but failed to ameliorate established cirrhosis.

Secondary hyperparathyroidism is characterized by increased serum parathyroid hormone (PTH) level and parathyroid cell proliferation. However, the molecular pathways mediating the increased parathyroid cell proliferation remain undefined. Here, we found that the mTOR pathway was activated in the parathyroid of rats with secondary hyperparathyroidism induced by either chronic hypocalcemia or uremia, which was measured by increased phosphorylation of ribosomal protein S6 (rpS6), a downstream target of the mTOR pathway. This activation correlated with increased parathyroid cell proliferation. Inhibition of mTOR complex 1 by rapamycin decreased or prevented parathyroid cell proliferation in secondary hyperparathyroidism rats and in vitro in uremic rat parathyroid glands in organ culture. Knockin rpS6 p2/2 mice, in which rpS6 cannot be phosphorylated because of substitution of all five phosphorylatable serines with alanines, had impaired PTH secretion after experimental uremia-or folic acid-induced AKI. Uremic rpS6 p2/2 mice had no increase in parathyroid cell proliferation compared with a marked increase in uremic wild-type mice. These results underscore the importance of mTOR activation and rpS6 phosphorylation for the pathogenesis of secondary hyperparathyroidism and indicate that mTORC1 is a significant regulator of parathyroid cell proliferation through rpS6. Secondary hyperparathyroidism (SHP) is a major complication of CKD and characterized by increases in parathyroid hormone (PTH) expression and parathyroid cell proliferation. 1-5 Decreased expression of the calcium, vitamin D, and fibroblast growth factor-23 receptors contributes to the increased parathyroid proliferation in uremia. [6][7][8][9][10][11] Expression of TGF-a and its receptor, EGF receptor (EGFR), is increased in uremic rats and patients. 12-14 TGF-a activation of EGFR decreases the CCAAT/ enhancer-binding protein-b liver-enriched activator protein:liver-enriched inhibitory protein, inducing parathyroid growth in uremia. A dominant negative EGFR gene expressed specifically in the parathyroid glands prevented the activation of endogenous EGFR and the increase in parathyroid gland enlargement and serum PTH. 15,16 Parathyroid cell proliferation is increased in transgenic mice overexpressing cyclin D1 only in the parathyroid. 17 The mammalian target of rapamycin (mTOR) integrates signaling pathways to regulate cell growth