Characterization of a Novel Intestinal Glycerol-3-phosphate Acyltransferase Pathway and Its Role in Lipid Homeostasis

Abstract

Dietary triglycerides (TG) are absorbed by the enterocytes of the small intestine after luminal hydrolysis into monacylglycerol and fatty acids. Before secretion on chylomicrons, these lipids are reesterified into TG, primarily through the monoacylglycerol pathway. However, targeted deletion of the primary murine monoacylglycerol acyltransferase does not quantitatively affect lipid absorption, suggesting the existence of alternative pathways. Therefore, we investigated the role of the glycerol 3-phosphate pathway in dietary lipid absorption. The expression of glycerol-3-phosphate acyltransferase (GPAT3) was examined throughout the small intestine. To evaluate the role for GPAT3 in lipid absorption, mice harboring a disrupted GPAT3 gene (Gpat3 ؊/؊ ) were subjected to an oral lipid challenge and fed a Western-type diet to characterize the role in lipid and cholesterol homeostasis. Additional mechanistic studies were performed in primary enterocytes. GPAT3 was abundantly expressed in the apical surface of enterocytes in the small intestine. After an oral lipid bolus, Gpat3 ؊/؊ mice exhibited attenuated plasma TG excursion and accumulated lipid in the enterocytes. Electron microscopy studies revealed a lack of lipids in the lamina propria and intercellular space in Gpat3 ؊/؊ mice. Gpat3 ؊/؊ enterocytes displayed a compensatory increase in the synthesis of phospholipid and cholesteryl ester. When fed a Western-type diet, hepatic TG and cholesteryl ester accumulation was significantly higher in Gpat3 ؊/؊ mice compared with the wild-type mice accompanied by elevated levels of alanine aminotransferase, a marker of liver injury. Dysregulation of bile acid metabolism was also evident in Gpat3-null mice. These studies identify GPAT3 as a novel enzyme involved in intestinal lipid metabolism.The ability of an organism to efficiently extract energy-rich nutrients from its diet is critical for survival. Triglycerides (TG) 2 are energy-dense lipid molecules; however, their biophysical properties preclude direct absorption in the gut. The primary site for the absorption of TG and other dietary lipids is the small intestine. Lipid absorption is a multistep process that begins with the intraluminal emulsification of fat by bile acids, which in turn facilitates hydrolysis by multiple lipases. TG hydrolysis leads to the liberation of monoacylglycerol (MAG) and free fatty acid (FFA). Absorption of these hydrolyzed products occurs in the epithelial monolayer lining the villi and involves both passive diffusion and facilitated transport. After absorption, FFAs are primarily reesterified into TG and either temporarily stored as cytosolic lipid droplets (CLDs) or packaged into chylomicrons and secreted into the lymphatic vessels of the lamina propria through the basolateral surface of enterocytes (1). There are two primary routes for TG synthesis, namely, the MAG and glycerol 3-phosphate (G3P) pathways. Both pathways produce diacylglycerol (DAG) as a common intermediate. In the MAG pathway DAG is directly synthesized from sn-2 MAG throug...

Diet-dependent retinoid effects on liver gene expression include stellate and inflammation markers and parallel effects of the nuclear repressor Shp

“…This reversal is shown in Figure 7B for phosphogluconate dehydrogenase (Pgd) and a regulator of glycogen synthesis, Ppp1r3b [36]. Additional genes include steroyl CoA desaturase ( Scd1 ), malic enzyme ( Me1 ), fatty acid synthase ( Fasn ), another regulator of glycogen synthesis, Ppp1r3g , and Agpat9/Gpat3 , a limiting enzyme in triglyceride synthesis [37]. …”

Recent discoveries on absorption of dietary fat: Presence, synthesis, and metabolism of cytoplasmic lipid droplets within enterocytes

“…The MGAT pathway accounts for approximately 70 - 80% of newly synthesized TAG within enterocytes in the postprandial state [42, 43]. The G3P pathway is also present in the small intestine and although it has long been thought to be only a minor contributor to TAG synthesis in the postprandial state [44], emerging evidence suggests it may play a more significant role [45]. …”

“…In the small intestine, two isoforms of GPAT are expressed: GPAT3 (high expression, jejunum) and GPAT4 (moderate expression) [45, 52, 53]. Increased Gpat activity was observed in high fat fed obese compared to lean Zucker rats [54], suggesting it may play a role in mediating intestinal TAG metabolism in response to chronic high fat feeding.…”

“…Increased Gpat activity was observed in high fat fed obese compared to lean Zucker rats [54], suggesting it may play a role in mediating intestinal TAG metabolism in response to chronic high fat feeding. In addition, Gpat3 -deficient mice have decreased TAG secretion and increased TAG storage in enterocytes following a dietary fat challenge [45]. The lysophophatidic acid formed during this first reaction is then acylated by acylglycerolphosphate acyltransferase (AGPAT) enzymes, also known as lysophosphatidic acid acyltransferases (LPAATs), to form phosphatidic acid.…”

“…In addition, specific isoforms of enzymes that activate FAs to form acyl-CoAs for incorporation into TAG (ACSL1, ACSL3, and ACSL4) have been identified on CLDs in several cell types (reviewed in [118]). In enterocytes, TAG synthesis enzymes that have been identified on CLDs are ACSL3, ACSL4, ACSL5, GPAT3 and MGAT2 in mice and humans (reviewed in [45, 119]). Of these, ACSL3 [120], ACSL5 [97], and GPAT3 [45] have been validated by confocal or electron microscopy to be present on CLDs in enterocytes.…”