Two splice variants of the hypoxia-inducible factor HIF-1 alpha as potential dimerization partners of ARNT2 in neurons

The hypoxia-inducible factor (HIF-1 alpha), a basic helix-loop-helix transc ription factor, is known to heterodimerize with ARNT1, a nuclear translocat or, to trigger the overexpression in many cells of genes involved in resist ance to hypoxia. Although HIF-1 alpha and ARNT1 are both expressed in brain , their cellular localization and function therein are unknown. Here, using in situ hybridization and immunocytochemistry, we show that HIF-1 alpha is expressed in normoxic cerebral neurons together with not only ARNT1 but al so ARNT2, a cerebral translocator homologous to ARNT1 but displaying, unlik e ARNT1, a selective neuronal expression. In contrast, other potential part ners of the translocators, i.e. the aryl hydrocarbon receptor (AHR) and the single-minded protein 2 (SIM2), are not expressed in the adult brain. We a lso identify two splice variants of HIF-1 alpha in brain, one of which dime rizes with ARNT2 even more avidly than with ARNT1. The resulting heterodime r, in contrast with the HIF-1 alpha/ARNT1 complex, does not recognize the H IF-1-binding site of the hypoxia-induced erythropoietin (Epo) gene, suggest ing that it controls transcription of a distinct set of genes. We therefore propose that HIF-1 alpha and ARNT2 function as preferential dimerization p artners in neurons to control specific responses, some of which may not be triggered by hypoxia. In support of this proposal, in nonhypoxic PC12 cells constitutively coexpressing HIF-1 alpha, ARNT1 and ARNT2, downregulation o f either HIF-1 alpha or ARNT2, obtained with selective antisense nucleotide s, resulted in inhibition of [H-3]thymidine incorporation.