Metabolism of Polyunsaturated Fatty Acid in Rat Testis : Expression and regulation of delta5-desaturase, delta6-desaturase, stearoyl-CoA desaturase 1, stearoyl-CoA desaturase 2 and the nuclear receptors peroxisome proliferator-activated receptor alpha, delta and gamma
In mammalian cells essential polyunsaturated fatty acids (PUFAs) are converted to longer PUFAs by alternating steps of elongation and desaturation. In contrast to other PUFA-rich tissues, the testis is continuously drained of these fatty acids, as spermatozoa are transported to the epididymis. Alteration of the germ cell lipid profile from spermatogonia to condensing spermatids and mature spermatozoa had been described prior to this work, but the male gonadal gene expression of the desaturases, responsible for the PUFA-metabolism, was still not established.
The focus of this study was to characterise the expression and regulation of stearoyl-Coenzyme A (CoA) desaturase 1 (SCD1), stearoyl-CoA desaturase 2 (SCD2), delta5- and delta6-desaturase in rat testis. Desaturase gene expression was detected in testis, epididymis and fractionated cells from seminiferous tubulus using Northern blot analysis. For the first time SCD1 and SCD2 expression are demonstrated in rat testis and epididymis. Both SCDs are expressed in epididymis, while testis mainly contains SCD2. Examination of the testicular distribution of delta5- and delta6-desaturase and SCD1 and -2 shows that all four desaturases seem to be localised in the Sertoli cells, with far lower expression in germ cells. In light of earlier published results showing that germ cells are richer in PUFAs than Sertoli cells, this strengthens the hypothesis of a lipid transport from the Sertoli cells to the germ cells.
As opposed to what is shown in liver, delta5- and delta6-desaturase mRNA levels in Sertoli cells are upregulated by dexamethasone. Furthermore, dexamethasone induces SCD2 mRNA. Insulin also upregulates these three genes in the Sertoli cell, while SCD1 mRNA is downregulated by both insulin and dexamethasone.
Delta5- and delta6-desaturase, SCD1 and SCD2 are all upregulated by follicle-stimulating hormone (FSH). A similar upregulation of the desaturases is observed when treating Sertoli cells with (Bu)2cAMP, indicating that the desaturase upregulation observed with FSH treatment results from elevated levels of cAMP. Testosterone has no influence on the desaturase gene expression, at age 19 days. Thus, FSH seems to be a key regulator of the desaturase expression in the Sertoli cell.
Neither SCD1 or -2, nor delta5- and delta6-desaturase are upregulated in whole testis tissue in response to two weeks feeding of a fat free diet. This makes a great contrast to what is seen in liver and kidney. Both in liver and kidney SCD1 and delta5- and delta6-desaturase are upregulated, accompanied by increased levels of fatty acids in the n-9 family. In liver the Mead acid (20:3(n-9)) to arachidonic acid (20:4(n-6)) ratio reached 0,75. Also in testis increased n-9 levels are seen; 20:3(n-9)/20:4(n-6)=0,36. However, this is probably resulting from decreased supply of n-3 and n-6 substrates. At present we hypothesise that the desaturases in testis are insensitive to essential fatty acid deficiency (EFAD), and furthermore, that the vacant desaturase response is due to differences on the transcription factor level.
The low testicular responsiveness to changes in the PUFA pool is confirmed as Sertoli cells treated with arachidonic or tetradecylthioacetic (TTA) acid only show a weak to non-excising downregulation of the desaturases (less than 1,5-fold). The downregulation of the desaturases in response to PUFAs in hepatocytes has been reported to be more than 4-fold. However, the fact that the net effect of TTA, a strong PPAR-alpha agonist, is desaturase-repressive, indicates that some nuclear mediator of desaturase repression must be present in low doses, also in testis.
Peroxisome proliferator-activated receptor (PPAR) alpha, delta and gamma are expressed in rat testis, with PPAR-alphaƒnshowing a gene expression pattern quite similar to delta5- and delta6-desaturase, as the Sertoli cell is the cell type with the highest expression. Furthermore, PPAR-gamma expression also dominates in the Sertoli cells. PPAR-delta expression is for the first time demonstrated in rat germ cells, with high levels in round spermatids. PPAR-alpha and -gamma are known to control numerous genes related to lipid metabolism including PUFA desaturation. Hence, the co-localisation of the desaturases and e.g. PPAR-alpha can not be coincidental. Since PPAR-alpha expression in the Sertoli cells has been proven to be stage specific, and regulated by FSH, the induction of the desaturases in response to FSH reported here, could be resulting from a combined primary FSH-cAMP-CREB action and secondary FSH-PPAR-alpha-ligand action.
Finally, we show that PPAR-alpha is slightly downregulated in testis in response to the fat free diet, while upregulated in liver and kidney. Acknowledging that PPAR-alpha induces delta5- and delta6-desaturase, and that the same genes are upregulated in the EFAD liver and kidney, we speculate if the increased level of PPAR-alpha mRNA adds to the SREBP-1c-governed induction of delta5- and delta6-desaturase under EFAD conditions in these tissues. Furthermore, the small downregulation of PPAR-alpha in the EFAD testis could explain how an abolished desaturase repression seen in PUFA-treated Sertoli cells, would disappear in whole testis tissue from fat free diet-fed rats.