Acyl-CoA synthetase 4 modulates mitochondrial function in breast cancer cells.

Mitochondria are dynamic organelles that respond to cellular stress through changes in global mass, interconnection, and subcellular location. As mitochondria play an important role in tumor development and progression, alterations in energy metabolism allow tumor cells to survive and spread even in challenging conditions. Alterations in mitochondrial bioenergetics have been recently proposed as a hallmark of cancer, and positive regulation of lipid metabolism constitutes one of the most common metabolic changes observed in tumor cells. Acyl-CoA synthetase 4 (ACSL4) is an enzyme catalyzing the activation of long chain polyunsaturated fatty acids with a strong substrate preference for arachidonic acid (AA). High ACSL4 expression has been related to aggressive cancer phenotypes, including breast cancer, and its overexpression has been shown to positively regulate the mammalian Target of Rapamycin (mTOR) pathway, involved in the regulation of mitochondrial metabolism genes. However, little is known about the role of ACSL4 in the regulation of mitochondrial function and metabolism in cancer cells. In this context, our objective was to study whether mitochondrial function and metabolism, processes usually altered in tumors, are modulated by ACSL4 in breast cancer cells. Using ACSL4 overexpression in MCF-7 cells, we demonstrate that this enzyme can increase the mRNA and protein levels of essential mitochondrial regulatory proteins such as nuclear respiratory factor 1 (NRF-1), voltage-dependent anion channel 1 (VDAC1) and respiratory chain Complex III. Furthermore, respiratory parameters analysis revealed an increase in oxygen consumption rate (OCR) and in spare respiratory capacity (SRC), among others. ACSL4 knockdown in MDA-MB-231 cells led to the decrease in OCR and in SCR, supporting the role of ACSL4 in the regulation of mitochondrial bioenergetics. Moreover, ACSL4 overexpression induced an increase in glycolytic function, in keeping with an increase in mitochondrial respiratory activity. Finally, there was a decrease in mitochondrial mass detected in cells that overexpressed ACSL4, while the knockdown of ACSL4 expression in MDA-MB-231 cells showed the opposite effect. Altogether, these results unveil the role of ACSL4 in mitochondrial function and metabolism and expand the knowledge of ACSL4 participation in pathological processes such as breast cancer.

Specific cellular microenvironments for spatiotemporal regulation of StAR and steroid synthesis.

For many years, research in the field of steroid synthesis has aimed to understand the regulation of the rate-limiting step of steroid synthesis, i.e. the transport of cholesterol from the outer to the inner mitochondrial membrane, and identify the protein involved in the conversion of cholesterol into pregnenolone. The extraordinary work by B Clark, J Wells, S R King, and D M Stocco eventually identified this protein and named it steroidogenic acute regulatory protein (StAR). The group's finding was also one of the milestones in understanding the mechanism of nonvesicular lipid transport between organelles. A notable feature of StAR is its high degree of phosphorylation. In fact, StAR phosphorylation in the acute phase is required for full steroid biosynthesis. As a contribution to this subject, our work has led to the characterization of StAR as a substrate of kinases and phosphatases and as an integral part of a mitochondrion-associated multiprotein complex, essential for StAR function and cholesterol binding and mitochondrial transport to yield maximum steroid production. Results allow us to postulate the existence of a specific cellular microenvironment where StAR protein synthesis and activation, along with steroid synthesis and secretion, are performed in a compartmentalized manner, at the site of hormone receptor stimulation, and involving the compartmentalized formation of the steroid molecule-synthesizing complex.

New insights into signal transduction pathways in adrenal steroidogenesis: role of mitochondrial fusion, lipid mediators, and MAPK phosphatases.

Hormone-receptor signal transduction has been extensively studied in adrenal gland. Zona glomerulosa and fasciculata cells are responsible for glucocorticoid and mineralocorticoid synthesis by adrenocorticotropin (ACTH) and angiotensin II (Ang II) stimulation, respectively. Since the rate-limiting step in steroidogenesis occurs in the mitochondria, these organelles are key players in the process. The maintenance of functional mitochondria depends on mitochondrial dynamics, which involves at least two opposite events, i.e., mitochondrial fusion and fission. This review presents state-of-the-art data on the role of mitochondrial fusion proteins, such as mitofusin 2 (Mfn2) and optic atrophy 1 (OPA1), in Ang II-stimulated steroidogenesis in adrenocortical cells. Both proteins are upregulated by Ang II, and Mfn2 is strictly necessary for adrenal steroid synthesis. The signaling cascades of steroidogenic hormones involve an increase in several lipidic metabolites such as arachidonic acid (AA). In turn, AA metabolization renders several eicosanoids released to the extracellular medium able to bind membrane receptors. This report discusses OXER1, an oxoeicosanoid receptor which has recently arisen as a novel participant in adrenocortical hormone-stimulated steroidogenesis through its activation by AA-derived 5-oxo-ETE. This work also intends to broaden knowledge of phospho/dephosphorylation relevance in adrenocortical cells, particularly MAP kinase phosphatases (MKPs) role in steroidogenesis. At least three MKPs participate in steroid production and processes such as the cellular cycle, either directly or by means of MAP kinase regulation. To sum up, this review discusses the emerging role of mitochondrial fusion proteins, OXER1 and MKPs in the regulation of steroid synthesis in adrenal cortex cells.

Modulation of albumin-induced endoplasmic reticulum stress in renal proximal tubule cells by upregulation of mapk phosphatase-1.

High amounts of albumin in urine cause tubulointerstitial damage that leads to a rapid deterioration of the renal function. Albumin exerts its injurious effects on renal cells through a process named endoplasmic reticulum (ER) stress due to the accumulation of unfolded proteins in the ER lumen. In addition, albumin promotes phosphorylation and consequent activation of MAPKs such as ERK1/2. Since ERK1/2 activation promoted by albumin is a transient event, the aims of the present work were to identify the phosphatase involved in their dephosphorylation in albumin-exposed cells and to analyze the putative regulation of this phosphatase by albumin. We also sought to determine the role played by the phospho/dephosphorylation of ERK1/2 in the cellular response to albumin-induced ER stress. MAP kinase phosphatase-1, MKP-1, is a nuclear enzyme involved in rapid MAPK dephosphorylation. Here we present evidence supporting the notion that this phosphatase is responsible for ERK1/2 dephosphorylation after albumin exposure in OK cells. Moreover, we demonstrate that exposure of OK cells to albumin transiently increases MKP-1 protein levels. The increase was evident after 15 min of exposure, peaked at 1 h (6-fold) and declined thereafter. In cells overexpressing flag-MKP-1, albumin caused the accumulation of this chimera, promoting MKP-1 stabilization by a posttranslational mechanism. Albumin also promoted a transient increase in MKP-1 mRNA levels (3-fold at 1 h) through the activation of gene transcription. In addition, we also show that albumin increased mRNA levels of GRP78, a key marker of ER stress, through an ERK-dependent pathway. In line with this finding, our studies demonstrate that flag-MKP-1 overexpression blunted albumin-induced GRP78 upregulation. Thus, our work demonstrates that albumin overload not only triggers MAPK activation but also tightly upregulates MKP-1 expression, which might modulate ER stress response to albumin overload.