Triiodothyronine modulates interleukin-6 synthesis in osteoblasts: inhibitions in protein kinase A and C pathways

An unusual case of hyperthyroidism associated with jaundice and hypercalcaemia

A 51-year-old woman with rheumatoid arthritis presented with a 3 month history of painless jaundice and significant weight loss and constipation. Laboratory values were consistent with hyperthyroidism, cholestatic jaundice and parathyroid hormone-independent hypercalcaemia. Three months after beginning of methimazole, euthyroidism was achieved and serum adjusted calcium, total and direct bilirubin levels were normal.

Adenylyl cyclase-cAMP system inhibits thyroid hormone-stimulated osteocalcin synthesis in osteoblasts

It is generally recognized that thyroid hormone modulates osteoblast cell function. We have previously shown that triiodothyronine (T(3)) activates p38 mitogen-activated protein (MAP) kinase, resulting in the synthesis of osteocalcin in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the effect of the adenylyl cyclase-cAMP system on thyroid hormone-stimulated osteocalcin synthesis in these cells. Dibutyryl-cAMP (DBcAMP) reduced the osteocalcin synthesis stimulated by T(3). Forskolin and cholera toxin suppressed the osteocalcin synthesis while dideoxyforskolin, a forskolin derivative that does not activate adenylyl cyclase, had little effect on the synthesis. KT5720, a selective inhibitor of protein kinase A, reversed the inhibitory effect of forskolin or DBcAMP. DBcAMP and forskolin markedly reduced the phosphorylation of p38 MAP stimulated by T(3). Pituitary adenylate cyclase-activating polypeptide (PACAP) significantly inhibited the T(3)-stimulated osteocalcin synthesis. These results strongly suggest that the adenylyl cyclase-cAMP system has an inhibitory role in thyroid hormone-stimulated osteocalcin synthesis via suppression of p38 MAP kinase activation in osteoblasts.

Activation of p38 mitogen-activated protein kinase mediates thyroid hormone-stimulated osteocalcin synthesis in osteoblasts

It is well known that thyroid hormone modulates osteoblast cell function. We have previously shown that triiodothyronine (T(3)) activates p44/p42 mitogen-activated protein (MAP) kinase, which limits T(3)-induced alkaline phosphatase activity in osteoblast-like MC3T3-E1 cells. In the present study, we investigated whether p44/p42 MAP kinase or p38 MAP kinase is involved in the thyroid hormone-stimulated osteocalcin synthesis in these cells. T(3) markedly induced the phosphorylation of p38 MAP kinase in addition to p44/p42 MAP kinase. PD98059 and U0126, inhibitors of the upstream kinase that activates p44/p42 MAP kinase, had little effect on the T(3)-induced synthesis of osteocalcin. On the contrary, the T(3)-induced osteocalcin synthesis was significantly reduced by SB203580 and PD169316, inhibitors of p38 MAP kinase. SB203580, PD169316 or PD98059 suppressed the T(3)-phosphorylation of myelin basic protein. T(3)-induced osteocalcin synthesis was significantly reduced by SB203580 or PD169316 also in primary cultured mouse osteoblasts. These results strongly suggest that p38 MAP kinase but not p44/p42 MAP kinase takes part in the thyroid hormone-stimulated osteocalcin synthesis in osteoblasts.

Contrasting effects of triiodothyronine on heat shock protein 27 induction and vascular endothelial growth factor synthesis stimulated by TGF-beta in osteoblasts

In osteoblast-like MC3T3-E1 cells, we recently reported that transforming growth factor-beta (TGF-beta) stimulates the induction of heat shock protein 27 (HSP27). In the present study, we investigated the effects of triiodothyronine (T(3)) on the TGF-beta-stimulated induction of HSP27 and synthesis of vascular endothelial growth factor (VEGF) in these cells. T(3) by itself had little effect on the level of HSP27, however, it significantly reduced the TGF-beta-stimulated HSP27 accumulation in a dose-dependent manner in the range between 1 pM and 100 nM. The TGF-beta-stimulated increase in the level of mRNA for HSP27 was also attenuated by T(3). On the other hand, T(3), which alone stimulated the release of VEGF, more than additively stimulated the TGF-beta-induced VEGF release. T(3) enhanced the TGF-beta-induced increase in the levels of mRNA for VEGF. These results strongly suggest that T(3) has contrasting effects on HSP27 induction and VEGF synthesis induced by TGF-beta in osteoblasts.

Activation of p44/p42 mitogen-activated protein kinase limits triiodothyronine-stimulated alkaline phosphatase activity in osteoblasts

It has been shown that thyroid hormone stimulates the activity of alkaline phosphatase, a marker of mature osteoblast phenotype, in osteoblasts. In the present study, we investigated whether p44/p42 mitogen-activated protein (MAP) kinase is involved in the thyroid hormone-stimulated alkaline phosphatase activity in osteoblast-like MC3T3-E1 cells. Triiodothyronine (T(3)) markedly induced the phosphorylation of p44/p42 MAP kinase. PD98059 and U0126, inhibitors of the upstream kinase that activates p44/p42 MAP kinase, significantly enhanced the T(3)-induced alkaline phosphatase activity in a dose-dependent manner. The phosphorylation of p44/p42 MAP kinase induced by T(3) was reduced by U0126. These results strongly suggest that p44/p42 MAP kinase takes part in the thyroid hormone-stimulated alkaline phosphatase activity in osteoblasts and that p44/p42 MAP kinase plays an inhibitory role in the thyroid hormone-effect.