Cell Biology
C.S. Sureka, C. Armpilia in Radiation Biology for Medical Physicists, 2017
The endoplasmic reticulum (ER) is an interconnected network of tubules, vesicles (cellular chemical reaction chambers for organizing cellular substances), and cisternae (flattened sacs) within cells. It extends from the nuclear membrane to the cell membrane. The functions of the endoplasmic reticulum are synthesis and export of proteins and membrane lipids. There are two types of ER. They are rough ET and smooth ET. Rough ER consists of a lot of ribosomes, which give a rough appearance. But, ribosomes are not present in smooth ER (a part of Figure 1.1). The main role of the rough ER is the synthesis of proteins (enzymes). But, the smooth ER synthesizes lipids and steroids, metabolizes carbohydrates, and regulates the calcium concentration and drug metabolism (drug inactivation too). The quantity of both rough and smooth ER in a cell depends upon the metabolic activities of the cell.
Environmental toxicants on Leydig cell function
C. Yan Cheng in Spermatogenesis, 2018
CYP17A1 is a P450 cytochrome enzyme. It is located in the smooth endoplasmic reticulum. CYP17A1 catalysis depends on cytochrome P450 oxidoreductase, cytochrome b5, and phosphorylation.31 Cytochrome b5 acts to facilitate CYP17A1 17, 20-lyase. Human CYP17A1 is phosphorylated on serine and threonine residues by a cAMP-dependent protein kinase and the phosphorylated CYP17A1 has increased 17,20-lyase activity.31 Human CYP17A1 requires cytochrome P450 oxidoreductase for the transfer of electrons. Both human and rat CYP17A1 enzymes catalyze two steps of reaction: 17α-hydroxylation and 20-lyation. Each reaction requires cofactor NADPH.14 Human CYP17A1 primarily catalyzes pregnenolone to 17-hydroxyprogenolone and then to dehydroepiandrosterone (Figure 20.1).32 However, rat CYP17A1 primarily catalyzes progesterone into 17-hydroxyprogesterone and then to androstenedione.33
The Stimulation of Steroid Biosynthesis by Luteinizing Hormone
Mario Ascoli in Luteinizing Hormone Action and Receptors, 2019
The conversion of Δ5-3β-hydroxysteroids to Δ4-3-ketosteroids is catalyzed by the enzymes Δ5-3β-hydroxsteroid dehydrogenase and 3-ketosteroid-Δ4-Δ5-isomerase. The enzymes are associated with the smooth endoplasmic reticulum (SER137). Although several studies have reported that Δ5-3β-hydroxysteroid dehydrogenase-isomerase activity can be detected in mitochondrial preparations from adrenal tissue138,139 and ovarian follicles,140 cross-contamination of the subcellular compartments indicated that the Δ5-3β-hydroxysteroid dehydrogenase-isomerase activity found in the mitochondrial preparations was probably a redistribution artifact.140,141 The oxidized form of nicotinamide adenine dinucleotide (NAD+) is required as a hydride acceptor for the Δ5-3β-hydroxysteroid dehydrogenase activity.142,143 The isomerization that follows the dehyrogenase reaction was originally thought to be catalyzed by the same enzyme, but dehydrogenase and isomerase isolated from adrenal microsomes have been physically separated.144
Protective role of PERK-eIF2α-ATF4 pathway in chronic renal failure induced injury of rat hippocampal neurons
Published in International Journal of Neuroscience, 2023
Qi Chen, Jingjing Min, Ming Zhu, Zhanqin Shi, Pingping Chen, Lingyan Ren, Xiaoyi Wang
The endoplasmic reticulum is one of the most important organelles in eukaryotic cells. It is not only the site for protein translation and synthesis as well as calcium ion storage, but also a participant in the transmission and processing of various cell signals. In addition, one of the major functions of the endoplasmic reticulum is to serve as a site for synthesizing secretory and integral membrane proteins.5,6 When cells are stimulated by hypoxia, an imbalance of calcium ions or a change in their concentration occurs in the internal environment, accompanied with the accumulation of some unfolded proteins in the endoplasmic reticulum, resulting in an imbalance between the structure and function of the endoplasmic reticulum. At this time, the corresponding signal pathway is activated to further trigger the endoplasmic reticulum stress (ERS) response.7 Unfolded protein response activation can be triggered in the following three ways: (1) inhibition of protein translation to prevent the production of more folded proteins; (2) induction of the folding of unfolded proteins by the endoplasmic reticulum chaperone; (3) activation of endoplasmic reticulum associated degradation pathways to remove unfolded proteins accumulated in the endoplasmic reticulum.8 However, under prolonged or severe stress, the unfolded protein response initiates programmed cell death.
Ultrastructural changes in resistance arterioles of normotensive and hypertensive premenopausal women with uterine fibroids
Published in Ultrastructural Pathology, 2023
LM Brewster, ID Perrotta, Z Jagernath, Z Taherzadeh, GA van Montfrans
Arterioles from untreated hypertensive women also exhibited endothelial cell alterations of mitochondria and endoplasmic reticulum at higher magnifications, but here most mitochondria appeared swollen with disarrangement of cristae combined with partial or total cristolysis. Also, endoplasmic reticulum tubules with bound ribosomes were markedly enlarged (Figure 4, Panel A). Endothelial cell junctions were generally maintained but, in some instances edematous changes appeared beneath the endothelium. Some endothelial cells exhibited alterations of chromatin organization that acquired a finely granular appearance (Figure 4, Panel B). Smooth muscle cells apparently reshaped from fusiform to a more rounded form and appeared to migrate through the fragmented IEL to the intima (Figure 4, Panels C, D). These migrating cells contained numerous dense bodies (reminiscent of the contractile phenotype) along the plasma membrane while in the cytoplasm the myofilaments network appeared barely organized. ER dilation and mitochondrial swelling were also observed (Figure 4, Panels C, D).
Lithium effects on vesicular trafficking in hepatocellular carcinoma cells
Published in Ultrastructural Pathology, 2019
Iuliia Taskaeva, Nataliya Bgatova, Izabella Gogaeva
Histologically, tumors arising from HCC-29 cells exhibited hepatocyte-like cellular morphologies (Figure 1a) while their nuclei had a typical morphology with irregular shapes and numerous clusters of heterochromatin. The membranes of the rough endoplasmic reticulum were not abundant and both multiple polysomes and ribosomes were distributed throughout the cytoplasm. Structural abnormalities in mitochondria, such as homogenous mitochondrial matrices with cristae fragmentation, were also found. Multivesicular bodies and lysosomes were not numerous and were only occasionally distributed throughout the cytoplasm (Figure 1b). These bodies had typical morphological features such as central, round vacuoles with a few intraluminal vesicles and no connections to the vacuolar membrane. Lysosomes, on the other hand, appeared as spherical organelles which contained relatively homogeneous, electron-dense content. We could therefore confirm that the cytoplasm of HCC-29 cells contained multiple tubular and vesicular structures (Figure 2). These vesicles are referred to as endosomes due to their location at the cellular periphery and included predominantly electron-transparent content with accidental intraluminal vesicles. Endosomes in HCC-29 cells were abundant, with various shapes and sizes, and were occasionally associated with short tubules.