The relationship between the transport of thyroid hormones and that of amino acids was examined by measuring the uptake of amino acids that are characteristic substrates of systems L, A, and N, and the effect of 3,3',5-triiodo-L-thyronine (T3) on this uptake, in cultured astrocytes. Tryptophan and leucine uptakes were rapid, Na(+)-independent, and efficiently inhibited by T3 (half-inhibition at approximately 2 microM). Two Na(+)-independent L-like systems (L1 and L2), common to leucine and aromatic amino acids, were characterized kinetically. System L2 had a low affinity for leucine and tryptophan (Km = 0.3-0.9 mM). The high-affinity system L1 (Km approximately 10 microM for both amino acids) was competitively inhibited by T3 with a Ki of 2-3 microM (close to the T3 transport Km). Several T3 analogues inhibited system L1 and the T3 transport system similarly. Glutamine uptake and alpha-(methylamino)isobutyric acid uptake were, respectively, two and 200 times lower than tryptophan and leucine uptakes. T3 had little effect on the uptakes of glutamine and alpha -(methylamino)isobutyric acid. The results indicate that the T3 transport system and system L1 are related.

The p38 family of MAP kinases (p38-MAPKs) is involved in regulating the proliferation, survival, and migration of various cancer cells. The present study has investigated the expression, subcellular localization, phosphorylation, and activity of p38-MAPKs in normal and tumoural human thyroid tissues and in thyroid cell lines. The expression and nucleo-cytosolic compartmentalization of the alpha-isoform of p38-MAPKs (p38alpha-MAPK) were studied by western blotting in the WRO and B-CPAP cell lines, which are derived from human follicular and papillary thyroid carcinomas, respectively, and in the non-transformed rat thyroid cell lines FRTL-5 and PCCL3. Immunohistochemistry was used to study the expression and subcellular localization of p38alpha-MAPK, and of the phosphorylated forms of p38-MAPKs (P-p38-MAPKs) in human toxic adenomas (TAs), follicular adenomas (FAs), papillary thyroid carcinomas (PTCs), and follicular thyroid carcinomas (FTCs). The activity of p38-MAPKs in PTCs and FTCs was revealed by immunohistochemical detection of their typical phosphorylated substrate, MAPK-activated protein kinase 2/3 (MK2/3). p38alpha-MAPK was expressed in all cell lines and this expression was restricted to the cytosolic compartment. p38 MAPK activity was involved in regulating DNA synthesis in B-CPAP cells. p38alpha-MAPK and P-p38-MAPKs were strongly expressed in PTC and FTC cells, although only in the cytoplasm, whereas they were only very weakly expressed in FA cells, and absent in adjacent normal tissues. They were also expressed at a high level in TAs, but they were found in both nucleus and cytoplasm. Finally, phospho-MK2/3 immunostaining followed very similar patterns to those of p38alpha-MAPK and P-p38-MAPKs in PTCs and FTCs. Taken together, these results show for the first time that the p38-MAPK signalling cascade is functional in two types of differentiated carcinoma of the thyroid. The observation that p38-MAPK hyper-expression occurs in FTC, but not in FA, may provide an additional diagnostic tool for malignancy in some thyroid nodules.