To test this hypothesis, HDL labeled with 3H-cholesteryl ester was injected into mice in which VLDL catabolism was blocked with P-407. P-407 has been previously Vandetanib hypothyroidism demonstrated to not affect the integrity of the HDL particle in contrast to tyloxapol (10). Using this approach, we were able to recover HDL-derived cholesteryl ester within VLDL of wild-type mice, and hepatic overexpression of SR-BI resulted in significantly increased recovery (954 �� 67 vs. 1441 �� 174 cpm/200 ��l, P < 0.05, Fig. 7A). In contrast, HDL-derived cholesteryl ester was almost absent within VLDL of SR-BI knockout mice compared with controls (1253 �� 49 vs. 148 �� 13 cpm/200 ��l, P < 0.001, Fig. 7B). Fig. 7. HDL-derived cholesterol is resecreted by the liver within VLDL particles.

A: Wild-type mice investigated on day 7 following injection with either AdSR-BI or the control adenovirus AdNull. B: SR-BI knockout mice and wild-type controls. Mice were injected … To further substantiate these observations, pulse-chase experiments were carried out in primary hepatocytes isolated from wild-type and SR-BI knockout mice each injected with the control adenovirus AdNull as well as from wild-type mice following injection with AdSR-BI. Because the SR-BI expression status impacts on the cellular uptake of HDL cholesteryl ester, counts within VLDL were normalized for the total counts recovered from the respective well. Under basal conditions without oleate added, hepatocytes from SR-BI knockout mice secreted significantly less labeled cholesterol within VLDL compared with wild-type control hepatocytes (1.

8 �� 0.1 vs. 3.4 �� 0.5%, P < 0.05, Fig. 8A). On the other hand, significantly higher counts were recovered within VLDL from hepatocytes isolated from wild-type mice injected with AdSR-BI compared with wild-type control cells (5.1 �� 0.1%, P < 0.05, Fig. 8A). When the experiment was performed with oleate present, HDL-derived labeled cholesterol within VLDL did not increase significantly in SR-BI knockout hepatocytes (2.5 �� 0.4%, Fig. 8B), whereas recovered counts within VLDL were increased by 72% in wild-type hepatocytes (5.9 �� 0.3%, P < 0.05 compared with SR-BI knockout hepatocytes as well as to conditions without oleate, Fig. 8B). SR-BI overexpression resulted, under these conditions, in an even further increase in HDL-derived counts within VLDL by 94% (9.9 �� 1.3%, P < 0.05 compared with wild-type hepatocytes as well Batimastat as to conditions without oleate, Fig. 8B). Taken together, these data indicate that in liver, a metabolic shunt might exist between the HDL catabolic and the VLDL secretion pathways and suggest that SR-BI might represent a central link within this process. Fig. 8. HDL-derived cholesterol is resecreted within VLDL particles by isolated primary hepatocytes.