What is the structure and function of rough and smooth endoplasmic reticulum?

Smooth endoplasmic reticulum

The smooth endoplasmic reticulum (Fig. 1.5A) is associated with carbohydrate metabolism and many other metabolic processes, including detoxification and synthesis of lipids, cholesterol and steroids. The membranes of the smooth endoplasmic reticulum serve as surfaces for the attachment of many enzyme systems, e.g. the enzyme cytochrome P450, which is involved in important detoxification mechanisms and is thus accessible to its substrates, which are generally lipophilic. The smooth endoplasmic reticulum in hepatocytes (which are able to store glycogen) contains the enzyme glucose-6-phosphatase, which converts glucose-6-phosphate to glucose, a step in gluconeogenesis. The membranes also cooperate with the rough endoplasmic reticulum and the Golgi apparatus to synthesize new membranes; the protein, carbohydrate and lipid components are added in different structural compartments.

Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum functions in many metabolic processes. It synthesizes lipids, phospholipids as in plasma membranes, and steroids. Cells that secrete these products, such as cells of the testes, ovaries, and skin oil glands, have an excess of smooth endoplasmic reticulum. The smooth endoplasmic reticulum also carries out the metabolism of carbohydrates and steroids. In muscle cells, the smooth endoplasmic reticulum regulates calcium ion storage. The smooth endoplasmic reticulum like the rough endoplasmic reticulum is connected to the nuclear envelope. The smooth endoplasmic reticulum comprises tube-like structure located near the cell periphery. These tubules or tubes sometimes branch forming a network that is reticular in appearance. The network of smooth endoplasmic reticulum allows for an increased surface area to be devoted to storage of key enzymes.

Triacylglycerols produced in the liver on the smooth endoplasmic reticulum can only be transiently stored

The liver has the unique capacity to offload stored TAG by producing lipoprotein complexes also containing cholesterol, phospholipids, and apolipoproteins (the latter also synthesized on the endoplasmic reticulum) for export in the form ofvery-low-density lipoprotein (VLDL). The VLDL is then assembled in the endoplasmic reticulum, transferred to the Golgi apparatus, and released into the bloodstream. To mobilize the transiently stored TAG, a lipolytic reaction occurs. It results in the formation of DAGs, which can then again enter the TAG synthetic pathway of VLDL assembly. The nature of this lipase is as yet not known but is of significant medical interest due to the complications of fatty liver disease. It is also possible that some TAGs enter the VLDL in the Golgi by the fusion of a primordial VLDL and an already-existing lipid droplet to produce the larger VLDL particles.

VLDL, once released into the bloodstream, is acted upon bylipoprotein lipase (LPL). This enzyme is found attached to the basement membrane glycoproteins of capillary endothelial cells and is active against both VLDL and chylomicrons (Chapter 33). The fatty acid of the TAG stored in adipose tissue will therefore reflect the mixture of fatty acids in the diet (delivered by chylomicrons) and the endogenous fatty acids delivered by VLDL. The latter will be composed of recirculated fatty acids (from adipose tissue) and DNL fatty acids from the liver. Beyond this, there might be a small component of DNL fatty acids generated within the tissue.

In the fed state, when adipose tissue is actively taking up fatty acids from the lipoproteins and storing them as TAG, the adipocytes synthesize LPL and secrete it into the capillaries of the adipose tissue. This increased synthesis and secretion of LPL is stimulated by insulin. Increased insulin levels also stimulate the uptake of glucose by adipose tissue and promote glycolysis. This has the net effect of producing increasing amounts of α-glycerophosphate, and it facilitates the synthesis of TAG within adipocytes. The skeletal muscle capillary bed also has LPL, but it is inhibited by insulin. Instead,LPL is activated in skeletal muscle by its contractions or by adrenergic stimulation.

Insulin is an important hormone in relation to fatty acid synthesis and storage. It promotes glucose uptake in both the liver and adipose tissue. In the liver, by increasing fructose-2,6-bisphosphate levels, it stimulates glycolysis, thus increasing pyruvate production. By stimulating dephosphorylation of pyruvate dehydrogenase complex and thus activating this enzyme, insulin promotes production of acetyl-CoA, stimulating the TCA cycle and increasing citrate levels, which, in turn, through stimulation of the acetyl-CoA carboxylase, increase the rate of fatty acid synthesis (see alsoChapter 31).

Clinical box

Lipid abnormalities in alcoholism

A 36-year-old woman attending a well-woman clinic was found to have serum concentrations of triglyceride 73.0 mmol/L (6388 mg/dL) and cholesterol 13 mmol/L (503 mg/dL). After some initial prevarication, she admitted to drinking three bottles of vodka and six bottles of wine per week. When she discontinued alcohol, her triglyceride concentrations decreased to 2 mmol/L (175 mg/dL), and her cholesterol concentration decreased to 5.0 mmol/L (193 mg/dL). Three years later, the woman presented again with an enlarged liver and return of the lipid abnormality. Liver biopsy indicated alcoholic liver disease with steatosis (infiltration of the liver cells with fat).

Smooth Endoplasmic Reticulum

sER is a membrane-bound network of tubules (see Figs. 1-1 and 1-3) without surface ribosomes. sER is not involved in protein synthesis. Its main function is the synthesis of lipids, steroids, and carbohydrates, as well as the metabolism of exogenous substances, such as drugs or toxins. Cells, such as hepatocytes, that are important for synthesis of lipids and metabolism of drugs or toxins have abundant sER, as do cells that produce steroid hormones, such as adrenocortical cells and certain testicular or ovarian cells. Cells with abundant sER have pale eosinophilic, finely vacuolated cytoplasm.

Lipid Synthesis by the Smooth Endoplasmic Reticulum

The endoplasmic reticulum also synthesizes lipids, especially phospholipids and cholesterol. These lipids are rapidly incorporated into the lipid bilayer of the endoplasmic reticulum, thus causing the endoplasmic reticulum to grow more extensive. This process occurs mainly in the smooth portion of the endoplasmic reticulum.

To keep the endoplasmic reticulum from growing beyond the needs of the cell, small vesicles calledER vesicles ortransport vesicles continually break away from the smooth reticulum; most of these vesicles then migrate rapidly to the Golgi apparatus.

Localization of Organelles

Smooth endoplasmic reticulum (SER) is an organelle that is likely to be involved in sequestering calcium. Depending on the particular brain regions, few, many, or most of the dendritic spines contain SER (Figure 1). For example, only about 14% of the hippocampal CA1 spines contain SER, and most of it occurs laminated with dense-staining material into a structure known as the spine apparatus (as in Figure 1) in the large, complex spines. In contrast, nearly 100% of the cerebellar Purkinje spines contain SER in a tubular network. Polyribosomes are also present in or near the base of some dendritic spines (Figure 2(d)), suggesting that local protein synthesis of dendritic mRNAs can occur in the spines. In immature neurons, synaptic activity can shift the distribution of polyribosomes from the shaft to spines. Similarly, endosomal compartments including coated pits and vesicles, large vesicles, tubules, and multivesicular bodies are restricted to a subpopulation of dendritic spines that differs from spines that contain SER. Mitochondria rarely occur in dendritic spines and are usually restricted to those that are very large, complex, and highly branched. However, during periods of active synapse formation and remodeling in cultured neurons, mitochondria can localize to smaller dendritic spines.

Smooth Endoplasmic Reticulum

The smooth ER is a continuous extension of the rough ER, located more distally from the nucleus. Whereas the rough ER is shaped like flattened hollow pancakes in many cell types, the smooth ER is usually more tubular in structure, forming a lacelike reticulum. It is an important site of lipid metabolism (e.g., cholesterol biosynthesis), and, for example, in liver cells, is the site where various membrane-associated detoxifying enzymes (e.g., cytochrome P450 enzymes) oxidize and otherwise act to modify toxic hydrophobic molecules (e.g., phenobarbital), making them less toxic and more water soluble.

The lumen of the smooth ER also serves as an important storage site for intracellular Ca2+. Smooth ER membranes contain ligand-regulated Ca2+ channels that open in response to the hormone-generated second messenger inositol 1,4,5-triphosphate (IP3). The cytosol of all cells is virtually Ca2+ free under resting conditions, and the transient appearance of Ca2+ in the cytosol after its release from the ER stores serves to initiate any of a number of cellular responses to extracellular signals, depending on the cell type. The ER membrane also possesses numerous Ca2+ pumps that bring the transiently released Ca2+ back into the ER lumen. Muscle contraction is initiated by transient release of Ca2+ from a specialized form of smooth ER in muscle fibers, known as the sarcoplasmic reticulum.

Smooth Endoplasmic Reticulum

The SER has a variety of functions that are often more prominent in certain cell types whose roles require an enhanced SER ability. Four common functions are the mobilization of glucose from glycogen, calcium storage, drug detoxification, and the synthesis of lipids. Glucose is stored as the polymer glycogen in close proximity to the SER, especially in liver, kidney, and intestinal cells that specialize in glucose homeostasis. To be released for use, individual glucose units are excised from glycogen and converted to glucose-1-phosphate, which is then converted to glucose-6-phosphate. However, to be exported from the cell for use by other cells, the glucose must traverse the plasma membrane; to do this, the phosphate must be removed. The enzyme that removes phosphate, glucose-6-phosphatase, is an SER-bound protein, prominent in liver, kidney, and intestine, which are organs that are glucose reservoirs. Type 1 glycogen storage disease (Von Gierke disease), one of about a dozen diseases that affect glycogen metabolism, is due to a genetic deficiency of glucose-6-phosphatase. Patients with this disease can store glycogen but cannot break it down, and with time glycogen accumulates, enlarging the liver. The disease causes chronic low blood sugar, abnormal growth, and is frequently fatal.

The SER is a storage site for calcium within cells. Calcium is pumped into the SER by active transport and released in response to hormonal signals. This is particularly important in muscle cells where the SER is so prominent it has a special name, the sarcoplasmic reticulum. Calcium is released in response to signaling pathways initiated on neurotransmitter binding to the cell-surface receptors.

Cytochrome P450s are a large family of enzymes resident in the membrane of the SER that use oxygen and nicotinamide adenine dinucleotide phosphate (NADPH) to hydroxylate a wide variety of substrates, including steroids and drugs. Hydroxylation often increases the solubility of hydrophobic drugs, facilitating clearance from the body, and selected cytochrome P450 enzymes are up-regulated in response to different drugs. This up-regulation can be large enough to cause dramatic expansion of the SER membrane. For example, chronic barbiturate use leads to expansion of the SER caused by induction of detoxifying cytochrome P450 enzymes. The increased inactivation of the drug requires larger barbiturate doses to achieve an effect, which is part of the addictive spiral in chronic users. Carcinogens, such as polycyclic aryl hydrocarbons, are also hydroxylated by SER-associated cytochrome P450 enzymes, which frequently enhances their carcinogenic activity.

Phospholipids, ceramide, and sterols are primarily synthesized in mammalian cells by enzymes in the ER, usually associated with the cytoplasmic leaflet of the SER. Exceptions to this include mitochondria that make selected phospholipids and peroxisomes that can biosynthesize cholesterol and some other lipids. The initial step in phospholipid synthesis is the condensation of two molecules of fatty acyl coenzyme A (CoA) with glycerol phosphate to make phosphatidic acid (Fig. 4-5). Each molecule of fatty acyl CoA is added separately, enabling the cell to control the type of fatty acid esterified to the 2 and 3 positions of the glycerol, with position 2 often containing an unsaturated fatty acid. Free fatty acids in the cytosol are usually bound to a fatty acid–binding protein and are converted to the fatty acyl-CoA derivatives that are substrates for the acyl-transferase enzymes in the cytosolic side of the membrane. A phosphatase removes the phosphate from phosphatidic acid to make diacyl glycerol, and in the polar head group, either cytidine-diphosphoethanolamine (CDP-ethanolamine) or cytidine-diphosphocholine (CDP-choline) is added (see Fig. 4-5).

Phospholipids are assembled in the cytoplasmic leaflet of the ER and must then be translocated to the other half of the bilayer to distribute a particular phospholipid between the two monolayers. The spontaneous flipping of phospholipids from one monolayer to the other is extremely slow and proteins termed flippases have evolved that catalyze the flipping of specific lipids. Phospholipids are often asymmetrically distributed in the two halves of a bilayer; for example, phosphatidylcholine and sphingomyelin are predominantly in the luminal face (or topologically equivalent extracellular face) of the membrane, whereas phosphatidylethanolamine and phosphatidylserine are mainly on the cytosolic face. Because the distribution of a phospholipid on the two sides of the membrane depends on the type of flippase present, it is believed that the asymmetric distribution of phospholipids in the two halves of a membrane is achieved by control of flipping, although it is unclear how the process is regulated to achieve the diversity of lipid asymmetry that is observed with different membranes.

Ceramide, the precursor of phosphosphingolipids and glycosphingolipids, is synthesized in the ER from serine and palmitoyl CoA. Phosphosphingolipids are also made in the ER. Glycosphingolipids, such as gangliosides, are made when ceramide reaches the Golgi complex and is glycosylated on the luminal face of the Golgi complex by glycosyl transferases. Glycosphingolipids are found only on the extracellular (luminal) side of membranes, suggesting that there are no flippases for this type of lipid.

The committed step in cholesterol synthesis, the production of mevalonate, is catalyzed by 3-hydroxy-3-methlyglutaryl-CoA reductase (HMG-CoA reductase), an integral membrane proteins of the SER. Other enzymes involved in the process of making cholesterol, as well as metabolically modifying cholesterol, are also ER residents. Although initially made on the cytosolic side of the SER, cholesterol is found on both sides of the membrane, and evidence exists that cholesterol flippases catalyze the flipping.

Not only are many lipids unevenly arranged in two halves of the bilayer, but most membranes maintain a unique lipid composition. For example, the ER of mammalian cells is typically 50% or more phosphatidylcholine, with less than 10% each for sphingomyelin and cholesterol, whereas the plasma membrane contains less than 25% phosphatidylcholine and more than 20% each for sphingomyelin and cholesterol. Thus, once a lipid is incorporated into the ER, it must not only be transported to other membranes, but the characteristic compositions of the destination membranes must be maintained. There are three basic thought processes on how this occurs. One is vesicle-mediated transport whereby vesicles bud from the donor membrane and fuse to a target membrane, thus moving lipids from one membrane to the other. Known pathways of vesicular transport whereby vesicles originating from the ER move sequentially through the Golgi apparatus and on to endosomes or the plasma membrane have been reported (see later). Ceramides that require glycosylation in the Golgi complex to make glycosphingolipids are believed to use this pathway. Cholesterol, however, can go from the ER to other membranes bypassing the Golgi complex, and evidence exists for a type of vesicle that buds from the ER carrying cholesterol and selected phospholipids that transports lipids to other membranes without passing through the Golgi complex. A second idea in lipid transport is that lipid transfer proteins directly extract a lipid from a membrane and shield the lipid in a hydrophobic pocket while the protein diffuses through the cytosol and deposits the lipid in an acceptor membrane. The third idea in lipid transport is that the ER makes transient contact with another membrane, passing lipids from the ER to the other membrane.

Hepatotoxicity secondary to enzyme induction

Proliferation of smooth endoplasmic reticulum (SER) is a well-documented change seen secondary to administration of xenobiotics that induce cytochrome P450 enzymes (Guengrich, 2007). Hepatocytes have a full complement of cytochrome P450s (CYP), since one of the liver’s main functions is metabolism of foreign substances. Induction most commonly occurs in centrilobular hepatocytes. Not all xenobiotics that induce cytochrome P450s cause proliferation of smooth endoplasmic reticulum. Proliferation is dependent on the subset of CYP that are induced, as no single agent induces all CYP. Cytochrome P450 enzymes are grouped into families CYP1, CYP2, CYP3, etc., with capital letters designating subfamilies CYP1A, CYP1B, CYP1C, etc., and numerals further identifying individual enzymes (e.g. CYP1A1). Not all CYP inducers cause SER proliferation and liver enlargement (Barka and Popper, 1967); however, SER proliferation is nearly pathognomonic for CYP induction.

Classic inducers of SER proliferation are phenobarbital and its analogs. Phenobarbital is metabolized by, and induces, cytochrome P4502B, the major membrane protein of SER (2d). Proliferation can be rapid and dramatic, with morphologic changes occurring within 4 days. Liver weight and size are increased. By light microscopy in HE sections, centrilobular hepatocytes are hypertrophied due to increased amounts of homogeneous eosinophilic cytoplasm. SER proliferation begins in centrilobular hepatocytes (Figure 54.5). With increasing magnitude, proliferation will extend into midzonal and rarely to periportal hepatocytes. Withdrawal of the inciting xenobiotic is accompanied by a rapid regression of changes (Cheville, 1994). Effete SER forms aggregates and is removed from the cell by autophagocytosis (Feldman et al., 1980).

Hepatotoxicity secondary to hepatocellular enzyme induction can occur through increased activation of xenobiotics to hepatotoxins (Zimmerman, 1999). Subsets of CYP inactivate xenobiotics while others activate to reactive electrophiles (Greaves, 2007). Aflatoxin B1, a mycotoxin, is activated to a number of metabolites, including exo-8,9-epoxide, an hepatocarcinogen. Acetaminophen, an analgesic, is activated to a reactive iminoquinone. Trichloroethylene (TCE), an industrial toxicant and previously used anesthetic, is activated to TCE oxide, which forms unstable protein adducts. Troglitazone, an antidiabetes drug, is metabolized to electrophilic reactive metabolites.