The importance of mitochondria as
The importance of mitochondria as site of aerobic oxidation of metabolic fuels, such as FA, has been recognized for a long time. It has been observed that the number of mitochondria may vary in response to cellular metabolic requirements and hormonal environment (Gao et al., 2014). In order to sustain the changing energetic demands of spermatogenesis, a careful regulation of mitochondrial biogenesis may be predicted in Sertoli cells. A complex regulatory network, which includes nuclear respiratory factors (NRF1 and NRF2) and mitochondrial transcription factor A (Tfam), coordinates mitochondrial biogenesis (Kelly and Scarpulla, 2004). NRF1 stimulates transcription of the Tfam gene by binding to an NRF1 response element in the promoter and subsequently, Tfam increases the transcription of mtDNA-encoded gene targets. Although the presence of NRF1, NRF2 and Tfam in the testis had been demonstrated (Escrivá et al., 1999), no previous studies in Sertoli Nicotine had analyzed whether hormones were involved in the transcriptional regulation of these genes and thus in mitochondrial biogenesis. Therefore, we decided to investigate whether FSH and bFGF, hormones that regulate carbohydrate and lipid metabolism in Sertoli cells, were able – concomitantly with the regulation of energetic metabolism – to regulate the expression of NRF1, NRF2 and Tfam and consequently mtDNA content in this cell type. The present study shows that FSH, but not bFGF, increases the expression of NRF1, NRF2 and Tfam. In accordance with this augment, FSH also stimulates mtDNA content. These results suggest that the trophic hormone participates in the control of mitochondrial biogenesis in Sertoli cells and reinforce the idea that it is essential to coordinate processes involved in energy homeostasis that may be necessary to maintain spermatogenesis. Previous studies on the hormonal regulation of the expression of genes involved in mitochondrial biogenesis had been performed in various cell types. To this respect, it has been observed that testosterone increases mRNA levels of genes involved in mitochondrial biogenesis in skeletal muscle, which suggests a role in testosterone-induced increase in energy expenditure (Usui et al., 2014). Moreover, it has been observed that estradiol upregulates mitochondrial markers in skeletal muscle and L6 miotubes (Capllonch-Amer et al., 2014) and also in mammary gland and in uterus (Ivanova et al., 2013). Furthermore, it has been shown that thyroid hormone has a profound effect on mitochondrial biogenesis that is related with its effects on oxygen consumption and metabolic rate (Weitzel et al., 2003). The results presented herein suggest that in Sertoli cells, FSH leads to an increase in mitochondrial content in order to provide the machinery to utilize energetic substrates that will fulfill the high-energy demands of spermatogenesis. As mentioned in the introduction, we have previously shown that only PPARβ/δ activation simultaneously regulates the expression of genes involved in FA metabolism and lactate production suggesting that this nuclear receptor may have a relevant physiological role in the seminiferous tubules. Hormones and growth factors, acting through their cognate receptors and regulating signal transduction, can play a role in determining the activity of PPARs and hence their effects on gene expression (Vanden Heuvel, 1999). Based on the latter observations, we hypothesized that PPARβ/δ activation might participate in the mechanism utilized by FSH and bFGF to regulate Sertoli cell energetic metabolism. Regarding FSH, no effects of GSK – a PPARβ/δ antagonist – were observed, indicating that this nuclear receptor is not involved in FSH action. As for the effects of bFGF on CPT1 mRNA levels, fatty acid oxidation and lactate production, all of them were inhibited in the presence of GSK. The latter results suggest a prominent role of PPARβ/δ activation in bFGF regulation of Sertoli cell metabolism.