The molecular mechanism of how hepatocytes maintain cholesterol homeostasis has become much more transparent with the discovery of sterol regulatory element binding proteins (SREBPs) in recent years. These membrane pr...The molecular mechanism of how hepatocytes maintain cholesterol homeostasis has become much more transparent with the discovery of sterol regulatory element binding proteins (SREBPs) in recent years. These membrane proteins aremembers of the basic helix-loop-helix-leucine zipper (bHLHZip) family of transcription factors. They activate the expression of at least 30 genes involved in the synthesis of cholesterol and lipids. SREBPs are synthesized as precursor proteins in the endoplasmic reticulum (ER), where they form a complex with another protein, SREBP cleavage activating protein (SCAP). The SCAP molecule contains a sterol sensory domain. In the presence of high cellular sterol concentrations SCAP confines SREBP to the ER. With low cellular concentrations, SCAP escorts SREBP to activation in the Golgi. There, SREBP undergoes two proteolytic cleavage steps to release the mature, biologically active transcription factor, nuclear SREBP (nSREBP). nSREBP translocates to the nucleus and binds to sterol response elements (SRE) in the promoter/enhancer regions of target genes. Additional transcription factors are required to activate transcription of these genes. Three different SREBPs are known, SREBPs-1a, -1c and -2. SREBP-1a and -1c are isoforms produced from a single gene by alternate splicing. SREBP-2 is encoded by a different gene and does not display any isoforms. It appears that SREBPs alone, in the sequence described above, can exert complete control over cholesterol synthesis, whereas many additional factors (hormones, cytokines, etc.) are required for complete control of lipid metabolism. Medicinal manipulation of the SREBP/SCAP system is expected to prove highly beneficial in the management of cholesterol-related disease.展开更多
Advanced closed chamber system was used to study the fate of phenanthrene (3 rings PAHs) in the presence of linear alkylbenzene sulphonates(LAS). The results showed mineralization and metabolism of phenanthrene are f...Advanced closed chamber system was used to study the fate of phenanthrene (3 rings PAHs) in the presence of linear alkylbenzene sulphonates(LAS). The results showed mineralization and metabolism of phenanthrene are fast in the “culture solution lava plant air' model ecological system. The distribution proportions of applied 14 C activity in this simulative ecological system were 41%—45%, 14% to 10% and 1% in plant, lava and culture solution respectively, and 18% to 29%, 11% to 8% recovered in the forms of VOCs and CO 2. Main parts of the applied 14 C activity exist in two forms, one is polar metabolites(25%) which mainly distribute in the root(23%), the other is unextractable part(23%) which have been constructed into plant root(8 98%), shoot(0 53%) or bonded to lava(13 2%). The main metabolites of phenanthrene were polar compounds(25% of applied 14 C activity), and small portion of 14 C activity was identified as non polar metabolites(6% of applied 14 C activity) and apparent phenanthrene(1 91% of applied 14 C activity). Phenanthrene and its metabolites can be taken up through plant roots and translocated to plant shoots. The presence of LAS significantly increased the the concentration of 14 C activity in the plant and production of VOCs, at the same time it decreased the phenanthrene level in the plant and the production of CO 2 at the concentration of 200 mg/L.展开更多
文摘The molecular mechanism of how hepatocytes maintain cholesterol homeostasis has become much more transparent with the discovery of sterol regulatory element binding proteins (SREBPs) in recent years. These membrane proteins aremembers of the basic helix-loop-helix-leucine zipper (bHLHZip) family of transcription factors. They activate the expression of at least 30 genes involved in the synthesis of cholesterol and lipids. SREBPs are synthesized as precursor proteins in the endoplasmic reticulum (ER), where they form a complex with another protein, SREBP cleavage activating protein (SCAP). The SCAP molecule contains a sterol sensory domain. In the presence of high cellular sterol concentrations SCAP confines SREBP to the ER. With low cellular concentrations, SCAP escorts SREBP to activation in the Golgi. There, SREBP undergoes two proteolytic cleavage steps to release the mature, biologically active transcription factor, nuclear SREBP (nSREBP). nSREBP translocates to the nucleus and binds to sterol response elements (SRE) in the promoter/enhancer regions of target genes. Additional transcription factors are required to activate transcription of these genes. Three different SREBPs are known, SREBPs-1a, -1c and -2. SREBP-1a and -1c are isoforms produced from a single gene by alternate splicing. SREBP-2 is encoded by a different gene and does not display any isoforms. It appears that SREBPs alone, in the sequence described above, can exert complete control over cholesterol synthesis, whereas many additional factors (hormones, cytokines, etc.) are required for complete control of lipid metabolism. Medicinal manipulation of the SREBP/SCAP system is expected to prove highly beneficial in the management of cholesterol-related disease.
文摘Advanced closed chamber system was used to study the fate of phenanthrene (3 rings PAHs) in the presence of linear alkylbenzene sulphonates(LAS). The results showed mineralization and metabolism of phenanthrene are fast in the “culture solution lava plant air' model ecological system. The distribution proportions of applied 14 C activity in this simulative ecological system were 41%—45%, 14% to 10% and 1% in plant, lava and culture solution respectively, and 18% to 29%, 11% to 8% recovered in the forms of VOCs and CO 2. Main parts of the applied 14 C activity exist in two forms, one is polar metabolites(25%) which mainly distribute in the root(23%), the other is unextractable part(23%) which have been constructed into plant root(8 98%), shoot(0 53%) or bonded to lava(13 2%). The main metabolites of phenanthrene were polar compounds(25% of applied 14 C activity), and small portion of 14 C activity was identified as non polar metabolites(6% of applied 14 C activity) and apparent phenanthrene(1 91% of applied 14 C activity). Phenanthrene and its metabolites can be taken up through plant roots and translocated to plant shoots. The presence of LAS significantly increased the the concentration of 14 C activity in the plant and production of VOCs, at the same time it decreased the phenanthrene level in the plant and the production of CO 2 at the concentration of 200 mg/L.
