Caprine homologue of rodent 5'-AMP-activated protein kinase subunit and yeast SNF4/CAT3 is down-regulated by thyroid hormone

We isolated a cDNA, B12, that was down-regulated by thyroid hormone (TH) in the goat cerebellum, using a polymerase chain reaction (PCR)-based subtractive hybridization and differential screening procedure. Northern blot analysis of RNA from cerebellum of T-4-treated and untreated hypothyroid goats confirmed that clone B12 was TH-regulated with an average reduction in expression of 21% after 4 days of T-4 supplementation. Other tissues from a T-4-treated and an untreated hypothyroid goat also revealed down-regulation of B12, with the highest reduction in expression found in the thyroid gland (38%). Steady-state levels of the approximately 1.8 kb B12 mRNA were higher in brain than in peripheral tissues. in situ hybridization showed that B12 mRNA in the brain is mainly present in various layers of the cerebellum and the cerebral cortex, the hippocampus, and the olfactory tubercle and is predominantly expressed in neurons. Sequence analysis of the caprine B12 cDNA clone, and the murine homologue, revealed 61% similarity to SNF4/CAT3, a regulator involved in the transcriptional control of glucose-repressible genes in yeast, and 99% identity to a rat 5'-AMP-activated protein kinase subunit, which is involved in the regulation of fatty acid, glycogen and isoprenoid metabolism. In view of these homologies, B12 might encode a regulator involved in distinct metabolic pathways and therefore, TH might also affect gene expression indirectly by down-regulation of regulators like B12.