Image Map

huge guy curling bar
SREBP-1 Proteins – Part II
by: Tom Rayhawk

In Part 1 of this article series, we delved into the structure, manufacturing, and ultimate functions of the SREBP-1c proteins. You should now appreciate that this is a protein you want to repress, especially if you are interested in maximizing your lean body mass to fat ratio.

Here in Part 2, we will consider different nutrients and supplementation/nutrition strategies and their impact on SREBP-1c levels.

The good news is that there are many methods that can be employed to lower srebp-1c activity and its deleterious effects on adipose tissue accumulation.

A rather unconventional and unexpected SREBP-1c antagonist Indinavir, a protease inhibitor, often used in AIDS patients. A surprising effect/benefit of indinavir is the reduction of lipogenic gene/enzyme expression, including SREBP-1c. This drug and other protease inhibitors are interesting as they reduce not only srebp-1c but prevent the elevation of PPAR? and C/EBP?, which also affect the transcription of adipocytes; however, complications arise from its use due to its inhibition of lipoprotein lipase and GLUT4. This specifically the case with fat cells, where these will most certainly reduce the uptake of glucose and fatty acids thus eliminating the initiation of triglyceride synthesis.

Unfortunately this can lead to insulin resistance and eventual diabetes. Furthermore, overweight individuals already typically express a greater than normal plasma concentration of fatty acids; so Indinavir would exacerbate the problem.

Another interesting finding observed by researchers at the Washington University School of Medicine was that reductions in fat mass were apparent but isolated to the lower extremities (thighs, hips, etc.) (1) While this is great from an aesthetic point of view, it is abdominal tissue that correlates strongly with metabolic disorders such as hyperlipidemia, elevated blood pressure and increased risk of diabetes/insulin resistance. By failing to impact these fat tissue regions, protease inhibitors do not prevent or reduce the repercussions of both subcutaneous and deep visceral adipose tissue.

Fatty acids, or more specifically, unsaturated fatty acids play a major role in minimizing the manufacturing of the SREBP-1c protein. Many fitness enthusiasts are aware of the benefits of EPA and DHA intake, such as a reduction in the pro-lipogenic protein PPAR gamma and a subsequent increase in PPAR alpha, a lipolytic or beta-oxidation-inducing protein. However, several other unsaturated fatty acids are capable of inhibiting lipogenic genes and proteins, such as the SREBP-1c variant.

CLA or conjugated linoleic acid has received considerable attention in both the medicinal and supplementation industry. Initially, there did seem to be support for its use as a ‘repartitioning’ agent, as CLA improved feed efficiency in mammals, by reducing fat storage pathways and enhancing beta-oxidation through an increase in lipolytic proteins (2).

 

Their effects, however, seem to be isomer specific. There are two isomers of CLA – cis-9, trans-11 (abbreviated c9,t-11) and trans-10,cis-12 (abbreviated t10,c-12). In essence the ‘good’ characteristics of CLA seem to be attributed to the c9, t11 isomer. In fact, the two seem to have opposing actions. The chart below compares the impact of the two isomers on blood triglycerides, non-esterified fatty acids (NEFA), glucose and insulin. Note the great disparity in the characteristics exhibited by the two isomers in Table 1.

Table 1.
Fasting plasma triglycerides, non-esterified fatty acids (NEFA), glucose and insulin concentrations, HOMA and revised QUICKI in apolipoprotein E knockout mice fed a high fat high cholesterol diet supplemented with 1% (w/w) linoleic acid (control), 1% (w/w) cis9, trans11-CLA or 1% (w/w) trans10, cis12-CLA for 12 weeks

Control (n = 10)

cis9, trans11-CLA (n = 9)

trans10, cis12-CLA (n = 9)


Triglycerides (mmol/L)

2.2 ± 0.5

1.5 ± 0.6*

6.5 ± 4.0*

NEFA (mg/dL)

3.2 ± 1.0

2.1 ± 1.0

8.6 ± 4.1*

Glucose (mmol/L)

17.3 ± 2.8

14.4 ± 2.3*

22.6 ± 6.4*

Insulin (pmol/L)

295.7 ± 102.7

217.8 ± 23.2*

304.5 ± 105.3

HOMA

32.7 ± 12.8

20.3 ± 4.8*

44.6 ± 20.3

revised QUICKI

0.21 ± 0.01

0.22 ± 0.01*

0.18 ± 0.01**

Values represent the mean ± SD. HOMA: homeostasis model for insulin resistance; (revised) QUICKI: quantitative insulin sensitivity check index. Significantly different from the control group:
* P < 0.05;
** P < 0.01.

As the table above shows, the implications of CLA use depend on the type of isomer used. As it relates to this article, CLA has a negative effect on SREBP-1c action, thereby reducing hyperlipidemia and lipogenic gene expression. Tumor Necrosis Factor Alpha causes apoptosis in adipocytes and upregulation of uncoupling protein 2 in brown adipose tissue. These characteristics are also both negative regulators of adipocyte accumulation and CLA intake is associated with and increase in TNF-alpha(3).

Another study specifically observed the effects of feeding one group of rats a c9,t11, dominant-diet and the other group a t10,c12 based diet. Amazingly, the t10-c12 have 4 times the lipid content in their livers compared to the t10,c12 group (4). However, the effects of these two isomers display remarkably different properties when one considers the majority of literature performed on both. So I had to throw a spanner in the works – sorry – but you should be aware that in other studies the t10,c12 variant is the most potent isomer. In fact, a 2003 study found it to actually reduce triglyceride content of adipocytes (4). When applied to SREBP-1c, the t10,c12 isomer may actually prevent the active form of the protein from being released from the endoplasmic reticulum much like EPA (4). This same study found no such effect from c9,t11. This is in contrast to other research which provides supportive evidence for c9,t11’s – but not t10,c12 – SREBP-1c reducing effects (1,2,3,5,6).

What should we make of all of this? I’m going to have to utilize a cliché here and recommend waiting until further and more in-depth research of CLA’s isomers uncovers which variant does what and if that ‘what’ is significantly beneficial in humans. This is pretty discouraging when one considers that CLA is not exactly a new compound and has been the target of a decent amount of research already.

A more obvious inhibitor of SREBP-1c expression is leptin, the hormone released from adipocytes, in response to energy surplus or deficit. Several studies demonstrate that in leptin unresponsive subjects, an elevation of mRNA of SREBP-1c is observed (7). Up regulation of leptin gene expression has been correlated with a reduction in SREBP-1c production (8). The implications here are obvious and repetitive if you read Mind and Muscle Magazine with any regularity. Simply put, the utilization of carb-ups or ‘refeeds’ or supplementation with other leptin manipulating nutrients during hypo caloric conditions is advantageous for the dieter.

A compound that has recently been attracting more respect for its body composition enhancing potential is berberine, an isoquinoline alkaloid. Research strongly supports its ability to inhibit expression of lipogenic proteins and enzymes, including SREBP-1c (9). Its effects are broader, though, as berberine reduces the levels of other proteins that induce differentiation of pre-adipocytes into mature fat cells (10). Apparently, berberine’s benefits are numerous relative to the lipogenic/lipolytic tug-of-war. Specifically, treatment with this alkaloid reduces the expression of PPAR gamma, Fatty Acid Synthase (FAS), C/EBP , and 11ß-HSD1, the enzyme responsible for the conversion of inactive cortisone to cortisol.

 

The latter effect indicates berberine’s potential therapeutic use in complications caused by increased visceral adipose tissue – whose expression is enhanced in the presence of excessive cortisol – and its relation to Metabolic Syndrome X. On the flip side berberine use has demonstrated high correlations with elevated MAPk, PPARalpha, and Uncoupling proteins, all of which enhance fatty acid oxidation (9,10).

Activating Transcription Factor 6 (ATF6) is an endoplasmic reticulum-bound protein that upon proteolytic cleavage translocates to the nucleus to affect target genes. Its ability to inhibit SREBP-1c is unique indeed. ATF6 actually binds to the SREBP-1c/SRE complex, effectively crippling the ability of SREBP1c to affect gene transcription. ATF6 is activated upon glucose deprivation, which initiates a cascade of effects leading to the eventual inactivation of SREBP1c. (11)

The biggest player, though, in reducing the impact of SREBP-1c are unsaturated fatty acids, specifically EPA and DHA. As mentioned in Part 1, the liver x receptor (LXR) enhances the actions of SREBP-1c by binding to liver x receptor response elements (LXRE’s) contained within the SREBP promoter region of DNA (12,). Upon the activation of this domain, up regulation of lipogenic genes ensue, including SREBP-1c. EPA has been shown to inhibit the ability of the LXR’s to bind to their response elements (,6, 13). It has been proposed that this occurs via an increase in PPAR alpha expression, which cripples LXR’s capacity to bind to the LXRE’s, ultimately reducing transcription of SREBP-1c.

EPA has a greater ability to reduce SREBP-1c levels compared to DHA (): this is one of the reasons the ratios of EPA:DPA found in various fish and fish body oil supplements are important (14,16). It should be noted, though, that DHA is still a very potent inhibitor and has other cognitive-enhancing effects via its ability to increase synaptic membrane fluidity in neurons, which can enhance learning and memory (15).

Arachodonic Acid (AA) an unsaturated fatty acid involved in the inflammatory response to exercise has also demonstrated the ability to inhibit insulin’s ability to activate the LXR-LRE complex and its activation of SREBP-1c transcription (6, 7, 8). In fact it exceeds EPA in this regard (7,13). However, it does not possess EPA’s ability to act during post-transcription to prevent the mature and thus active form of SREBP-1c from being cleaved from the endoplasmic reticulum and made free to interact with the nuclear domain. One must consider that AA has its double bond on the 6th position as opposed to EPA/DHA where the double bond is on the third carbon.

What does this mean? The ratio of n-6/n-3 polyunsaturated fatty acids in a diet has often been discussed, as the typical American diet has far too many n-6 fatty acids – which has been associated with elevated blood cholesterol and triglyceride levels.

HMG CoA reductase inhibitors reduce cholesterol synthesis by inhibiting this HMG CoA reductase from producing mevalonate, the initial product of cholesterol. Discussed in the previous article, the srebp-1a variant plays a greater role in cholesterol homeostasis. However, one of the means for activation of srebp-1c is via the interaction of the LXR receptor with cholesterol intermediates, which are elevated when srebp-1a levels are increased (17). Compactin, an inhibitor of HMG CoA reductase has been shown to reduce SREBP-1c expression via this indirect mechanism of diminishing cholesterol production.

Nutritional approaches have recently compared the impact of a high versus a low-carbohydrate intake. Here is the lowdown – high carbohydrate diets increase lipogenic gene expression. This is supported in literally hundreds of studies (here are two18,19). As alluded to in the discussion of ATF6, glucose deprivation as would be experienced on a ketogenic or reduced carbohydrate diet, upregulates ATF6 leading to its unique inactivation of the SREBP-1c protein. Also, the use of various fatty acids discussed earlier in this article would certainly be a prudent choice (fish body oils, arachidonic acid).

Of the simple 6-carbon sugars, fructose is the most potent activator of SREBP-1c (20).This actually occurs through a newly discovered process by inducing the production of RNA-binding motif on the X chromosome(RBMX) which amplifies the activity of the SREBP-1c promoter. This allows for activators (lxr-rxr, insulin, etc.) of this promoter to exhibit greater efficacy, ultimately amplifying SREBP-1c transcription Glucose (blood sugar) which is ultimately what all non-fibrous carbohydrates metabolize into.

As alluded to in Part 1, glucose can act independently of insulin to elevate lipogenic gene expression such as SREBP-1c (21). This is somewhat oversimplified since considerations must be made as to a dieter’s intensity and absolute amount of activity and lean muscle mass to body fat ratio; as this will greatly enhance the ability of muscle to be more glucose-responsive relative to fat tissue in comparison to sedentary and/or obese individuals. As mentioned earlier, the judicious use of glucose and carbohydrate loading or ‘refeeds’ can act as a negative regulator of SREBP-1c via the temporary elevation of leptin levels.

As one can deduce from the information offered in this article, there is an abundant number of means to inhibit the actions of SREBP-1c. In fact, this is truly a brief overview as there are many nutrients/dietary strategies that have proven effective against this lipogenic protein not referenced in this article.

Hopefully both Parts 1 and 2 of this series have given you a greater understanding and appreciation for both what SREBP-1c is, what it’s capable of when activated and the power you have to impair its negative downstream effects on body composition.

 

References

1. Alterations in thigh subcutaneous adipose tissue gene expression in protease inhibitor-based highly active antiretroviral therapy. Juan Chaparro,1 Dominic N. Reeds,1 Weidong Wen, E. Xueping, Samuel Klein, Clay F. Semenkovich, Kyongtae T. Bae, Erin K. Quirk, William G. Powderly, Kevin E. Yarasheski, and Ellen Li*. Metabolism. 2005 May; 54(5): 561-567

2. Conjugated linoleic acid improves feed efficiency, decreases subcutaneous fat, and improves certain aspects of meat quality in stress-genotype pigs. B. R. Wiegand, F. C. Parrish Jr, J. E. Swan, S. T. Larsen and T. J. Baas. Journal of Animal Science, Vol 79, Issue 8 2187-2195

3. Isomer-Dependent Metabolic Effects of Conjugated Linoleic Acid. Insights From Molecular Markers Sterol Regulatory Element-Binding Protein-1c and LXR. Helen M. Roche1, Enda Noone1, Ciaran Sewter2, Siobhan Mc Bennett3, David Savage2, Michael J. Gibney1, Stephen O’Rahilly2, and Antonio J. Vidal-Puig2 Diabetes 51:2037-2044, 2002

4. The Inhibitory Effect of trans-10, cis-12 CLA on Lipid Synthesis in Bovine Mammary Epithelial Cells Involves Reduced Proteolytic Activation of the Transcription Factor SREBP-11,2 Daniel G. Peterson3, Elvina A. Matitashvili and Dale E. Bauman. J. Nutr. 134:2523-2527, October 2003

5. Contrasting effects of t10,c12- and c9,t11-conjugated linoleic acid isomers on the fatty acid profiles of mouse liver lipids. D.S. Kelley, G.L. Bartolini, J.M. Warren, V.A. Simon, B.E. Mackey, K,L. Erickson. LIPIDS 2004 Feb;39(2):135-41.

6 Isomer-specific regulation of metabolism and PPAR signaling by CLA in human preadipocytes J. Mark Brown*, Maria Sandberg Boysen , Søren Skov Jensen , Ron F. Morrison*, Jayne Storkson , Renee Lea-Currie**, Michael Pariza , Susanne Mandrup and Michael K. McIntosh1,*. Journal of Lipid Research, Vol. 44, 1287-1300, July 2003

7.Leptin, troglitazone, and the expression of sterol regulatory element binding proteins in liver and pancreatic islets. Kakuma T, Lee Y, Higa M, Wang Z, Pan W, Shimomura I, Unger RH. Proc Natl Acad Sci 2000 Jul 18;97(15):8536-41

8. Leptin decreases lipogenic enzyme gene expression through modification of SREBP-1c gene expression in white adipose tissue of aging rats. Hashimoto M, Hossain S, Shimada T, Shido O. Metabolism 2005 Aug;54(8):1041-7

9. Berberine reduces the expression of adipogenic enzymes and inflammatory molecules of 3T3-L1 adipocyte. Choi BH, Ahn IS, Kim YH, Park JW Lee SY, Hyun CK, Do MS. Exp Mol Med 2006 Dec 31;38(6):599-605

10. Berberine, a Natural Plant Product, Activates AMP-Activated Protein Kinase With Beneficial Metabolic Effects in Diabetic and Insulin-Resistant States. Yun S. Lee1,2, Woo S. Kim1,2, Kang H. Kim1,2, Myung J. Yoon1, Hye J. Cho1, Yun Shen3,4, Ji-Ming Ye3, Chul H. Lee5, Won K. Oh5, Chul T. Kim5, Cordula Hohnen-Behrens3, Alison Gosby3, Edward W. Kraegen3, David E. James3, and Jae B. Kim1,2 Diabetes 55:2256-2264, 2006

11. ATF6 modulates SREBP2-mediated lipogenesis. Lingfang Zeng,1 Min Lu,1 Kazutoshi Mori,2 Shengzhan Luo,3 Amy S Lee,3 Yi Zhu,1 and John Y-J Shyy1a EMBO J. 2004 February 25; 23(4): 950–958.

12. POLYUNSATURATED FATTY ACID REGULATION OF GENES OF LIPID METABOLISM. Harini Sampath¬ James M. Ntambi. Annual Review of Nutrition. Vol. 25: 317-340 (Volume publication date August 2005).

13. Polyunsaturated Fatty Acids Suppress Sterol Regulatory Element-binding Protein 1c Promoter Activity by Inhibition of Liver X Receptor (LXR) Binding to LXR Response Elements. Tomohiro Yoshikawa, Hitoshi Shimano §, Naoya Yahagi, Tomohiro Ide , Michiyo Amemiya-Kudo, Takashi Matsuzaka , Masanori Nakakuki , Sachiko Tomita, Hiroaki Okazaki, Yoshiaki Tamura, Yoko Iizuka, Ken Ohashi, Akimitsu Takahashi , Hirohito Sone , Jun-ichi Osuga, Takanari Gotoda, Shun Ishibashi, and Nobuhiro Yamada J. Biol. Chem., Vol. 277, Issue 3, 1705-1711, January 18, 2002.

14. A low fish oil inhibits SREBP-1 proteolytic cascade, while a high-fish-oil feeding decreases SREBP-1 mRNA in mice liver: relationship to anti-obesity. Teruyo Nakatani, Hyoun-Ju Kim, Yasushi Kaburagi, Kazaki Yasuda, and Osamu Ezaki. Journal of Lipid Research, Vol. 44, 369-379, February 2003

15 Docosahexaenoic acid-induced protective effect against impaired learning in amyloid beta-infused rats is associated with increased synaptosomal membrane fluidity. Hashimoto M, Hossain S, Shimada T, Shido O. Clin Exp Pharmacol Physiol 2006 Oct;33(10):934-9.

16. The Interrelated Effects of n-6/n-3 and Polyunsaturated/Saturated Ratios of Dietary Fats on the Regulation of Lipid Metabolism in Rats. Joon Ho Lee, Michiyo Fukumoto, Harumi Nishida, Ikuo Ikeda and Michihiro Sugano. Journal of Nutrition Vol. 119 No. 12 December 1989, pp. 1893-1899

17. Expression of sterol regulatory element-binding protein 1c (SREBP-1c) mRNA in rat hepatoma cells requires endogenous LXR ligands.. Russell A. DeBose-Boyd,* Jiafu Ou,* Joseph L. Goldstein,† and Michael S. Brown.. Proc Natl. Acad Sci 2001 Feb 13;98(4):1477-82.

18. The Role of SREBP-1c in Nutritional Regulation of Lipogenic Enzyme Gene Expression. Angela K. Stoeckman and Howard C. Towle. J. Biol. Chem., Vol. 277, Issue 30, 27029-27035, July 26, 2002

19. Carbohydrate responsive element binding protein (ChREBP) and sterol regulatory element binding protein-1c (SREBP-1c): Two key regulators of glucose metabolism and lipid synthesis in liver. Dentin R. Girard, J. Postic, C. Biochimie. Volume 87, Issue 1 SPEC. ISS., January 2005, Pages 81-86

20. Stearoyl-CoA Desaturase 1 Gene Expression Is Necessary for Fructose-mediated Induction of Lipogenic Gene Expression by Sterol Regulatory Element-binding Protein-1c-dependent and -independent Mechanisms. Makoto Miyazaki , Agnieszka Dobrzyn , Weng Chi Man , Kiki Chu , Harini Sampath , Hyoun-Ju Kim ¶, and James M. Ntambi J. Biol. Chem., Vol. 279, Issue 24, 25164-25171, June 11, 2004

21. Central role for liver X receptor in insulin-mediated activation of Srebp-1c transcription and stimulation of fatty acid synthesis in liver. Tadashi Takemoto, Yoshihiko Nishio, Osamu Sekine, Chikako Ikeuchi, Yoshio Nagai, Yasuhiro Maeno, Hiroshi maegawa, Hiroshi Kimura, and Atsunori Kashiwagi. Proc Natl Acad Sci USA 2004 Aug 3;101(31):11245-50. Epub 2004 Jul 20.

×