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Testis-specific regulation of cholesterogenic genes and the role of transcription factor CREM

Cholesterogenic lanosterol 14alpha-demethylase (CYP51), the most conserved member of the citochrome P450 superfamily, is expressed at unusually high level in spermatids due to the testis specific transcripts. The immediate product of the CYP51 reaction is FF-MAS, that is converted further to testis meiosis activating sterol T-MAS. MAS are intermediates in cholesterol biosynthesis, found to accumulate in gonads, with the ability to trigger resumption of oocyte meiosis in vitro. We have established that the expression of cholesterogenic genes during sexual development of the rat testis is discordant and proposed that discrepancy in expression of pre-MAS and post-MAS genes is a result of a testis-specific transcriptional regulation (Fon Tacer et al., J Lip Res, 2002, 43:82-89). It is well established for CYP51 that in addition to sterol/SREBP dependent regulation, cAMP/CREMtau-dependent regulation also exists. CREMtau is expressed in haploid male germ cells and is responsible for expression of male germ cell-specific genes. CREM knock out males are sterile. They lack the testis-specific transcripts of CYP51. In order to evaluate the role of CREM in regulation of other cholesterogenic genes, DNA microarray experiments on a limited cholesterol homeostasis chip and Northern analysis have been performed. According to the expression pattern, cholesterogenic genes cluster into three distinct groups. As was expected from Northern blot results, CYP51 is down-regulated in testis of CREM-knockout mice compared to expression in testis of wild type animals. The same pattern is shown also for two other genes involved in cholesterol homeostasis, the LDL-receptor and cholesterol 12alpha-hydroxylase. The second group is represented by early genes of the biosynthetic pathway: HMG CoA synthase, FPP synthase and squalene synthase, which show no changes in expression level in testis of knockout mice compared to wild type animals. It was very surprising to find also the third group of genes that are upregulated in testis of knockout mice. This group is represented by late cholesterogenic genes, the sterol C5-desaturase, C-4 sterol methyl oxydase, sterol delta7-hydroxylase and STAR. This is the first observation of transcriptional upregulation due to the lack of transcriptional factor CREM. However, it has not yet been established in which cell type of the testis (germ cells, Leydig cells or Sertoli cells) this upregulation does exist. Upregulation of genes in knockout animals may be either a direct consequence of the lack of CREM inhibitory forms (CREMalpha, beta, ICER) or an indirect impact of disrupted spermatogenesis in CREM knockout males. Defective spermatogenesis could be a signal for higher prooduction of testosterone via the gonadotrophin trigger, that results in a higher expression of cholesterol.synthesizing and steroid-producing genes in Leydig cells.

cholesterogenic genes; DNA microarray; expression patterns; spermatogenesis; rat testis

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