Ernesto J. Podestá

Discrepancies between adenosine 3':5'-cyclic monophosphate (cAMP) and steroid production have been frequently observed in isolated target cells stimulated by low concentrations of trophic hormone. This dissociation is particularly marked in the interstitial cells of the testis, where testosterone production is elicited by gonadotropin concentrations in the picomolar range. Because of these observations, and a disparity between steroidogenesis and protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37) activation in Leydig cells, the role of cAMP as a mediator of the acute steroidogenic response has been questioned. This problem has been further analyzed by assay of free and occupied cAMP-binding sites of the regulatory subunit of protein kinase in basal and hormone-stimulated cells. Free sites were measured by a [(3)H]-cAMP-binding assay, and occupied sites were measured by radioimmunoassay of endogenous cAMP eluted from receptor protein. After stimulation of purified Leydig cells with 0.1-10 pM human chorionic gonadotropin, a dose-dependent decrease in available [(3)H]cAMP-binding sites was observed, with no change in binding affinity. The reduction in cAMP-binding sites was equivalent to the increase in occupancy of cAMP receptors by endogenous nucleotide formed during gonadotropin action. Fractional occupancy of cAMP receptors rose progressively from basal values of 0.2-0.40 to full saturation as intracellular cAMP rose 10- to 30-fold during hormone stimulation. The testosterone dose-response curve was coincident with the initial part of the cAMP-receptor occupancy curve. These changes in endogenous cAMP binding to the regulatory subunit were accompanied by a significant increase in protein kinase activity in gonadotropin-stimulated Leydig cells. These observations provide direct evidence for the role of cAMP and protein kinase during hormonal activation of steroidogenesis in the Leydig cell by low concentrations of gonadotropin.

Although the contribution of mitochondrial dynamics (a balance in fusion/fission events and changes in mitochondria subcellular distribution) to key biological process has been reported, the contribution of changes in mitochondrial fusion to achieve efficient steroid production has never been explored. The mitochondria are central during steroid synthesis and different enzymes are localized between the mitochondria and the endoplasmic reticulum to produce the final steroid hormone, thus suggesting that mitochondrial fusion might be relevant for this process. In the present study, we showed that the hormonal stimulation triggers mitochondrial fusion into tubular-shaped structures and we demonstrated that mitochondrial fusion does not only correlate-with but also is an essential step of steroid production, being both events depend on PKA activity. We also demonstrated that the hormone-stimulated relocalization of ERK1/2 in the mitochondrion, a critical step during steroidogenesis, depends on mitochondrial fusion. Additionally, we showed that the SHP2 phosphatase, which is required for full steroidogenesis, simultaneously modulates mitochondrial fusion and ERK1/2 localization in the mitochondrion. Strikingly, we found that mitofusin 2 (Mfn2) expression, a central protein for mitochondrial fusion, is upregulated immediately after hormone stimulation. Moreover, Mfn2 knockdown is sufficient to impair steroid biosynthesis. Together, our findings unveil an essential role for mitochondrial fusion during steroidogenesis. These discoveries highlight the importance of organelles' reorganization in specialized cells, prompting the exploration of the impact that organelle dynamics has on biological processes that include, but are not limited to, steroid synthesis.

ERK1/2 is known to be involved in hormone-stimulated steroid synthesis, but its exact roles and the underlying mechanisms remain elusive. Both ERK1/2 phosphorylation and steroidogenesis may be triggered by cAMP/cAMP-dependent protein kinase (PKA)-dependent and-independent mechanisms; however, ERK1/2 activation by cAMP results in a maximal steroidogenic rate, whereas canonical activation by epidermal growth factor (EGF) does not. We demonstrate herein by Western blot analysis and confocal studies that temporal mitochondrial ERK1/2 activation is obligatory for PKA-mediated steroidogenesis in the Leydig-transformed MA-10 cell line. PKA activity leads to the phosphorylation of a constitutive mitochondrial MEK1/2 pool with a lower effect in cytosolic MEKs, while EGF allows predominant cytosolic MEK activation and nuclear pERK1/2 localization. These results would explain why PKA favors a more durable ERK1/2 activation in mitochondria than does EGF. By means of ex vivo experiments, we showed that mitochondrial maximal steroidogenesis occurred as a result of the mutual action of steroidogenic acute regulatory (StAR) protein -a key regulatory component in steroid biosynthesis-, active ERK1/2 and PKA. Our results indicate that there is an interaction between mitochondrial StAR and ERK1/2, involving a D domain with sequential basic-hydrophobic motifs similar to ERK substrates. As a result of this binding and only in the presence of cholesterol, ERK1/2 phosphorylates StAR at Ser(232). Directed mutagenesis of Ser(232) to a non-phosphorylable amino acid such as Ala (StAR S232A) inhibited in vitro StAR phosphorylation by active ERK1/2. Transient transfection of MA-10 cells with StAR S232A markedly reduced the yield of progesterone production. In summary, here we show that StAR is a novel substrate of ERK1/2, and that mitochondrial ERK1/2 is part of a multimeric protein kinase complex that regulates cholesterol transport. The role of MAPKs in mitochondrial function is underlined.

The acyl-CoA synthetase 4 (ACSL4) is increased in breast cancer, colon and hepatocellular carcinoma. ACSL4 mainly esterifies arachidonic acid (AA) into arachidonoyl-CoA, reducing free AA intracellular levels, which is in contradiction with the need for AA metabolites in tumorigenesis. Therefore, the causal role of ACSL4 is still not established. This study was undertaken to determine the role of ACSL4 in AA metabolic pathway in breast cancer cells. The first novel finding is that ACSL4 regulates the expression of cyclooxygenase-2 (COX-2) and the production of prostaglandin in MDA-MB-231 cells. We also found that ACSL4 is significantly up-regulated in the highly aggressive MDA-MB-231 breast cancer cells. In terms of its overexpression and inhibition, ACSL4 plays a causal role in the control of the aggressive phenotype. These results were confirmed by the increase in the aggressive behaviour of MCF-7 cells stably transfected with a Tet-off ACSL4 vector. Concomitantly, another significant finding was that intramitochondrial AA levels are significantly higher in the aggressive cells. Thus, the esterification of AA by ACSL4 compartmentalizes the release of AA in mitochondria, a mechanism that serves to drive the specific lipooxygenase metabolization of the fatty acid. To our knowledge, this is the first report that ACSL4 expression controls both lipooxygenase and cyclooxygenase metabolism of AA. Thus, this functional interaction represents an integrated system that regulates the proliferating and metastatic potential of cancer cells. Therefore, the development of combinatory therapies that profit from the ACSL4, lipooxygenase and COX-2 synergistic action may allow for lower medication doses and avoidance of side effects.