Opposite regulation of thrombospondin-1 and corticotropin-induced secreted protein/thrombospondin-2 expression by adrenocorticotropic hormone in adrenocortical cells

Abstract

Corticotropin-induced secreted protein (CISP) is a trimeric glycoprotein secreted by primary cultures of bovine adrenortical cells in response to adrenocorticotropic hormone (ACTH). This protein was recently purified in our laboratory, and its N-terminal amino-acid sequence revealed a significant similarity with thrombospondin-2 (TSP2). We report here the nucleotide sequence of a 386 bp RT-PCR fragment specific for CISP. The deduced protein sequence shares 84% identity with the N-terminal portion of mature human TSP2, suggesting that CISP is its bovine counterpart. Northern analysis of adrenocortical cell RNA using the above cDNA fragment as a probe revealed a 6.0 kb CISP/TSP2 mRNA whose abundance was increased nearly fivefold following a 24 h cell treatment with 10(-7) M ACTH. Under the same conditions, the expression of TSP1 mRNA was reduced by tenfold. The protein levels of TSP1 and CISP/TSP2 varied accordingly with their respective mRNA levels, as shown by immunoprecipitation and immunofluorescence experiments. Taken together, these data show that ACTH induces a dramatic shift in the pattern of adrenocortical cell thrombospondin expression from TSP1 to CISP/TSP2. This observation suggests that these two members of the thrombospondin family exert distinct biological functions in the adrenal cortex. This hypothesis is further supported by the observation that anti-CISP antibodies inhibit the maintenance of the morphological changes of bovine adrenocortical cells induced by ACTH, whereas anti-TSP1 antibodies do not.