Adipose saturation reduces lipotoxic systemic inflammation and explains the obesity paradox

Abstract

Obesity sometimes seems protective in disease. This obesity paradox is predominantly described in reports from the Western Hemisphere during acute illnesses. Since adipose triglyceride composition corresponds to long-term dietary patterns, we performed a meta-analysis modeling the effect of obesity on severity of acute pancreatitis, in the context of dietary patterns of the countries from which the studies originated. Increased severity was noted in leaner populations with a higher proportion of unsaturated fat intake. In mice, greater hydrolysis of unsaturated visceral triglyceride caused worse organ failure during pancreatitis, even when the mice were leaner than those having saturated triglyceride. Saturation interfered with triglyceride's interaction and lipolysis by pancreatic triglyceride lipase, which mediates organ failure. Unsaturation increased fatty acid monomers in vivo and aqueous media, resulting in greater lipotoxic cellular responses and organ failure. Therefore, visceral triglyceride saturation reduces the ensuing lipotoxicity despite higher adiposity, thus explaining the obesity paradox.

Fig. 1. Relationship of human SAP to cutoff BMI and composition of dietary fat intake.

(A) Studies using a BMI cutoff of ≤25 are shown in pink, those associating SAP with a BMI of ≥30 are shown in gold, and studies showing a BMI of >30 was not associated with severe AP (SAP) are show shown in blue. The respective countries from which the study originated are shown in pink, green, and blue, with a checkered pattern for countries with studies showing different outcomes. Box plot comparing the per capita per year saturated fat (from dairy, cattle, and palm oil) consumption in countries with BMI cutoffs of >30 and those with ≤25 BMI (B) and %UFA in dietary fat intake (C). (D) Meta-analysis showing OR for SAP for studies using a BMI cutoff of ≤25 (pink background) and those using a BMI cutoff of >30 (blue-yellow background). The overall OR is shown at the bottom (orange background).

Fig. 2. Dietary and adipose fat composition and parameters of local, systemic severity of CER pancreatitis in UFA- and SFA-fed ob/ob mice.

(A1) Table comparing the fatty acid composition of fat pad triglycerides (TG) and diets of mice given the SFA- and UFA-enriched diets. Body weights (A2), body fat (A3), and body fat as a percentage of body weight (A4) in the SFA- and UFA-fed mice. Serum amylase (B1) and lipase (B2) in control (CON) mice and after 24 hours of AP (CER). Local pancreatic injury seen histologically (C1) and quantified as acinar necrosis (C2) as a percentage of total parenchymal area and percentage of acinar necrosis adjacent to the FN (shown in yellow rectangles), termed as “% peri-fat acinar necrosis” (C3). Systemic injury measured as renal injury [TUNEL staining showing brown nuclei in (D1) and serum BUN in (D2)], lung TUNEL positivity highlighted with arrows in (D3), and shown as number per high-power field (HPF) in (D4), along with shock (as measured by a drop in carotid pulse distention) in (E1), serum calcium (E2), and survival curve (E3) of mice with CER AP. NS, not significant.

Fig. 3. Effect of visceral fat composition on its lipolysis and associated inflammatory response.

(A) Gross appearance of the FN in SFA-fed (top) and UFA-fed mice with pancreatitis. (B1) PNLIP, IKB-α, and α-tubulin (α-Tub) amounts on Western blotting (B2) in the fat pads of SFA- or UFA-fed control (Con) mice or those with CER pancreatitis (CER). Cytokine mRNA levels in fat pads (C1) shown as an increase over control levels or serum levels of IL-6 (C2), MCP-1 (C3), or TNF-α (C4) in control mice or those with CER pancreatitis after they had been fed with UFA- or SFA-enriched diets. Serum NEFA showing OA (D1), LA (D2), total serum UFA (C3), total serum NEFA (D4), PA (D5), palmitoleic acid (D6), total serum SFA (D7), and serum SFA as a percentage of total NEFA (D8) in control mice or those with CER pancreatitis after they had been fed with UFA- or SFA-enriched diets. Photo credit: Krutika Patel, Mayo Clinic, AZ.

Fig. 4. Effect of intraperitoneal GTL or GTP on IL12,18-induced pancreatitis in lean CD-1 mice.

Serum amylase (A1), lipase (A2), and trends and survival (A3) during IL12/18 pancreatitis alone (IL12,18), with GTL (IL12,18 + GTL) or GTP (IL12,18 + GTP). Note similar amylase and lipase at day 2 in all groups and progressive worsening in the GTL group, which has reduced survival. (B1 to B3) Representative H&E-stained images of the pancreas of mice belonging to the groups mentioned below the image. Note the increased necrosis at the periphery of the lobules in IL12,18 + GTL group (black arrows). Box plots showing serum glycerol (C1) and NEFA (C2) concentrations in control (CON) mice and in the different pancreatitis groups. (D1 to D3) Representative TUNEL-stained images of the kidneys of mice belonging to the groups mentioned below the image. Note the increased positivity in the tubules of the IL12,18 + GTL group (black arrows), which also has a significantly higher BUN than other groups on ANOVA (*) (D4). Box plots of the serum resistin (E1), IL-6(E2), TNF-α (E3), and MCP-1(E4) control (CON) mice and in the different pancreatitis groups. * indicates significantly higher than control on ANOVA, and # indicates a significant reduction compared to the IL12,18 + GTL group.

Fig. 5. Influence of triglyceride composition on their lipolytic interaction, lipolysis and resulting biological effects.

Lipase activity (A1), NEFA generation (A2), cytosolic calcium (A3), and mitochondrial depolarization (A4) of acini alone (control) or with 300 μM GTL, GTL with 50 μM orlistat (GTLO), GTP, or GTO. (B1 to B3) Effect of substituting LA in GTL (red) with palmitate at Sn-3 (LLP; magenta) or at Sn-1 and Sn-3 (PLP; cyan) or oleate and palmitate at Sn-2 and Sn-3, respectively (LOP; light brown) on NEFA generation over 15 min by acini (B1), acinar mitochondrial depolarization (B2), and cytosolic calcium increase (B3). (C1) Time course of NEFA generation from GTL, LLP, and LOP (100 μM) by PNLIP (12 nM). (C2 and C3) hPNLIP-triglyceride interaction, showing raw heat per second (ΔQ/Δt) as a function of PNLIP/triglyceride ratio (C3) and measured peaks (C2). (D1) GTL docked into 1LPA via induced fit protocol. PNLIP molecule (top) and zoomed in LBD (bottom) are shown with catalytic triad (dark blue), distances, and GlideScores (kilocalories per mole). LA, OA, and PA are in red, cyan, and green, respectively, with receptor interaction surface in yellow and oxygen atoms in orange. (D2 to D4) LLP, LOP, and GTP docked into 1LPA.

Fig. 6. Effects of NEFA unsaturation on their in vivo effects and unbound monomeric behavior.

Serum MCP-1 (A), BUN (B), NEFA (C), uNEFA (D), and dsDNA (E) after intraperitoneally administering C16:0 (green), C18:1 (blue), and C18:2 (red) as mentioned below the x axis. Values are at necropsy if moribund or euthanasia at 72 hours. (F1) ITC thermograms of injecting NEFA or solvent (0.34% DMSO for C16:0 and PBS for C18:1 or C18:2) at concentrations mentioned below the NEFA into PBS (pH 7.4) at 37°C, and corresponding enthalpograms (F2) and CMCs (F3) shown as shaded rectangles in the areas where they intersect the x axis. Effect of exogenous albumin (final concentrations of 0.5%) on the cytosolic calcium (Cai) (G) and mitochondrial depolarization (ψm) (H and I) induced by 100 μM of the indicated NEFA added at 100 s. (J and K) Comparison of the dose responses of increase in mitochondrial depolarization (Δψm) after 60 s of addition of different concentration of C18:1 (blue) or C18:2 (red) in acini (J) and HEK293 cells (K). * indicates significant difference (P < 0.05) from the previous lower concentration by paired t test. # indicates a significant difference between C18:2 and C18:1 for that specific concentration on t test.

Fig. 7. Schematic summarizing how dietary fat composition affects visceral fat necrosis and causes the obesity paradox.

The impact of consuming a Western diet enriched in saturated fat from dairy and red meat (left) or one enriched in unsaturated fat from vegetable oil and fish (right) are shown. In the event of AP, the relatively more saturated adipocyte triglyceride shown in blue (, left) or unsaturated triglyceride shown in red (, right) is exposed to pancreatic lipase, principal among which is PNLIP (zoomed circles). Despite being lesser in amount, the more unsaturated triglyceride is hydrolyzed more into the lipotoxic NEFA (), which are stable as monomers at higher concentrations than saturated NEFA, that form micelles () at lower concentrations. This results in worse systemic inflammation, injury, and organ failure in those with a higher unsaturated fat consumption, despite having less adiposity. This pathophysiology can explain the obesity paradox.