The Gene Encoding Human SCGB 2A1 is under Indirect Androgen Control Operating through an Sp Family Binding Site in Prostate Cells
The secretoglobins belong to a minority of proteins with unclear physiological function. In the past research in the field was dominated by work on the family founder member uteroglobin and its orthologs. This was mainly due to the fact that some 20 to 30 years ago uteroglobin served as a first mode...
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|Summary:||The secretoglobins belong to a minority of proteins with unclear physiological function. In the past research in the field was dominated by work on the family founder member uteroglobin and its orthologs. This was mainly due to the fact that some 20 to 30 years ago uteroglobin served as a first model for a steroid hormone binding protein, as the steroid hormone receptors had not been cloned yet. Later on it served as a model gene to study gene regulation by steroid hormone receptors. Many new members of the secretoglobin family were identified in the past few years but remained poorly investigated in terms of gene regulation.
Studying one of the new family members was thought to be promising in finding new clues concerning physiological function of the secretoglobins in general. Many family members are expressed in the genital tract of males and females where they might play a role in reproduction. One of the new members, lipophilin C (= lacryglobin, mammaglobin B, secretoglobin SCGB 2A1) was first shown to be expressed by the lacrimal glands in the form of a heterodimer with lipophilin A, another secretoglobin that is present in tears. Due to functional similarities of lacrimal and prostate glands it was not surprising to find SCGB 2A1 also being expressed in the prostate. Because expression was found to be under androgen control, gene regulation was investigated using the androgen responsive human prostate cancer cell line LNCaP. Relevant regulatory regions were identified in the genes chromatin by mapping DNase I hypersensitive sites before and after androgen induction. Only one prominent hypersensitive site appeared in the proximal promoter after 6 hours of androgen induction indicating an indirect response. Sequence analysis did not reveal any canonical or non-canonical androgen response element(s). DNase I footprinting identified two factors binding to the proximal promoter region that were found to be the ubiquitous transcription factors nuclear factor I (NF1) and CAAT-box transcription factor NF-Y. Analysis of promoter deletions using luciferase reporter constructs showed that 136 bp of the promoter still containing the NF-Y binding site are sufficient to confer androgen responsiveness to the reporter gene or the heterogenous HSV-tk promoter. Analyses of promoter constructs with mutations in the NF1 and NF-Y binding sites demonstrated that both factors significantly contribute to the basal activity, and that NF1 helps to mediate the androgen response. Eventually androgen responsiveness could be traced down to a dimeric inverted repeat GC box immediately upstream of the TATA box that is a binding site for Sp family transcription factors.
Therefore, androgen responsiveness in the SCGB 2A1 gene is mediated by an indirect mechanism that requires the androgen receptor but operates through a binding site for ubiquitous transcription factors of the Sp family, notably the well known factors Sp1 and Sp3. This response is androgen-specific because glucocorticoids in conjunction with the cotransfected glucocorticoid receptor were not able to activate SCGB 2A1 expression in LNCaP cells. Thus, SCGB 2A1 was unexpectedly identified as a target gene for a non-ARE mediated action of the androgen receptor. Future investigations will focus on the exact mechanism of this indirect hormone response.|