Abstract
The demonstration that TRa1 mRNA encodes a nuclear thyroid hormone receptor and two proteinsimported into mitochondria with molecular masses of 43 and 28 kDa has brought new clues to betterunderstand the pleiotropic influence of iodinated hormones. If p28 activity remains unknown, p43 bindsto T3 responsive elements occurring in the organelle genome, and, in the T3 presence, stimulatesmitochondrial transcription and the subsequent synthesis of mitochondrial encoded proteins. This influenceincreases mitochondrial activity and through changes in the mitochondrial/nuclear cross talkaffects important nuclear target genes regulating cell proliferation and differentiation, oncogenesis, orapoptosis. In addition, this pathway influences muscle metabolic and contractile phenotype, as well asglycaemia regulation. Interestingly, according to the process considered, p43 exerts opposite or cooperativeeffects with the well-known T3 pathway, thus allowing a fine tuning of the physiological influenceof this hormone.