Glucocorticoid receptor structure and function in an adrenocorticotropin-secreting small cell lung cancer

Abstract

ACTH secretion by tumors of nonpituitary origin is characteristically resistant to negative feedback regulation by glucocorticoids. One possible mechanism for the phenomenon could be a structural defect in the intracellular glucocorticoid receptor (GR). We studied the GR in DMS-79 cells derived from a human ACTH-secreting small cell lung cancer. Compared with control cells, DMS-79 cells were found to have greatly diminished GR ligandbinding activity and immunoreactive 94-kilodalton (kDa) GR content. Northern blot analysis revealed expression of GR transcripts that appeared to be slightly larger than those in control cells. A DMS-79 cell GR cDNA was cloned by reverse transcription/ polymerase chain reaction amplification of mRNA using primers specific for full-length normal GR. The derived sequence of this full-length GR differed from the reported sequence by a single altered codon (G to A; Asn to Ser at codon 363) outside the steroid-binding domain. This N363S DMS-79 GR functioned normally to activate a target gene [mouse mammary tumor virus-chloramphenicol acetyl transferase (MMTV-CAT)] in transient transfection experiments in COS cells. Evidence for expression of a second type of GR mRNA was obtained by screening a DMS-79 cell cDNA library. This GR cDNA contained normal GR sequence up to nucleotide 2155, corresponding exactly to the end of exon 7 in the normal GR gene. The sequence appended to the GR sequences was not matched by any known sequence in DNA databases and included an in-frame termination codon after only 6 bases. The predicted truncated GR protein product (GRA) has a mol wt of 73,740 and lacks most of the ligand-binding domain. Transient transfection of the GRA form into COS cells did not reveal any dominant negative effect on the function of a cotransfected normal GR. GR signaling in DMS-79 cells could be restored by transient transfection with a normal GR cDNA. These data indicate that at least one GR allele is intact in DMS-79, that DMS-79 cells may produce the predominant truncated form of GR by aberrant splicing, and that the resultant diminution of normal GR expression may account for the clinically observed glucocorticoid-resistant state.