Abstract 321: Lymphatic derived HDL Is an Effective Donor for the Trans-Intestinal Cholesterol Efflux Pathway That Is Impaired During Insulin Resistance

Introduction: Emerging evidence shows that the proximal small intestine secretes cholesterol into the intestinal lumen via the trans-intestinal cholesterol excretion (TICE) pathway. It is thought TICE can contribute up to 30-40% of fecal neutral sterol excretion. Plasma apoB containing lipoproteins are known to donate cholesterol to the TICE pathway whilst conflicting evidence exists on the role for plasma HDL. Due to the anatomical proximity of lymphatic vessels to the basolateral membrane of enterocytes, it may serve as a candidate donor to TICE, but to date this has not been tested. Objective: To determine if HDL derived from mesenteric lymph can act as cholesterol donor for TICE in JCR:LA cp rats as a model of insulin resistance (IR). Methods: Mesenteric lymph was collected following intralipid infusion via lymphatic cannulation from control rats. Lymph HDL was isolated using ultra-density centrifugation and labeled with H3 cholesterol. Jejunal explants were obtained from control and IR rats (as a model of reduced TICE) fed chow or a high fat/cholesterol diet. TICE was measured with Ussing Chambers as appearance of H3-cholesterol labeled lymph HDL using micelles as acceptors. Results: Relative to free cholesterol [FC; used as marker of non-specific lipid permeability], lymph derived HDL TICE was 77% higher in control tissue (n=5, p<0.05) under chow fed conditions, suggestive of an effective donor for TICE. Lymph HDL TICE was also reduced (89%, p<0.05) in IR rats compared to control. Furthermore SR-B1 mRNA was reduced (-65%) in enterocytes from IR rats compared to control, and may explain reduced TICE by lymph HDL in IR. Under conditions of high fat/cholesterol fed diet, TICE from FC [and mannitol as a marker of paracellular transport] was increased in both control and IR rats, suggesting an elevated non-specific permeability of lipids by the basolateral membrane. Conclusions: Consistent with evidence that the lymphatics have a role in reverse cholesterol transport, this data supports that lymph HDL is an effective donor for TICE possibly by the SR-B1 pathway. While we have shown that the lymph HDL TICE pathway may be impaired during insulin resistance, a high fat/cholesterol diet may exacerbate lipid permeability via non-specific efflux pathways.