Papel de la lipasa sensible a hormonas en el testículo de ratón. Implicación de los lípidos y los receptores "scavenger" clase B en fertilidad

Resumen

Spermatogenesis occurs in a series of proliferation and differentiation stages. There is a tight relationship between fertility and changes in cholesterol and lipids metabolism during spermatogenesis. Cholesterol is required for steroidogenesis and spermatogenesis, and is essential for germ cell development and fertility. Leydig cells require a continuous supply of cholesterol as a precursor for the synthesis of steroid hormones, while in the seminiferous tubules, cholesterol is involved in the proliferation and differentiation of germ cells to spermatozoa. In testis, scavenger receptor class B (SR-B) mediates selective uptake of cholesterol esters from HDL, which are hydrolyzed to unesterified cholesterol by hormone sensitive lipase (HSL). HSL is an intracellular neutral lipase that hydrolyzes triacylglycerols, diacylglycerols, monoacylglycerols, cholesteryl esters and retinyl esters. In mice, HSL deficiency results in male sterility caused by a major defect in spermatogenesis. In the present study we found that HSL knockout (HSL -/-) mice testis presented altered spermatogenesis associated with decreased sperm counts, sperm motility and infertility. Defects included multinucleation of spermatides, abnormal shapes and reduced elongating spermatids. Many epithelial cells in the seminiferous tubules were vacuolated and the amount of Leydig cells was increased. In Wildtype (HSL +/+) testis, HSL is expressed in elongating spermatides; SR-BI in Leydig cells and spermatides; SR-BII is expressed in spermatocytes and spermatides but not in Leydig; and LIMPII is present in both Sertoli and Leydig cells. The lack of HSL induced augmented expression of SR-BI, SR-BII and LIMPII in mice testis, this allows an increased uptake of cholesterol esters from HDL mediated by these receptors, and prevents their hydrolysis to free cholesterol, which facilitates the accumulation of cholesterol esters in the epithelial cells of the testis and limits the availability of free cholesterol required for steroidogenesis and spermatogenesis in the mice testis. The lack of HSL also induces an activation of several intracellular signaling pathways mediated by class B scavenger receptor such as ERK, AKT and SRC, important in the processes of differentiation and cell proliferation during spermatogenesis. HSL is a key enzyme in the mobilization of cholesterol esters and fatty acids from intracellular stores. In our study we found that the lack of HSL results in a change in the composition of sterols in the testis, an increase of total cholesterol and FF-MAS, a LXR (liver X receptor) ligands. The testes contain high concentrations of polyunsaturated fatty acids (PUFAs) and specific PUFA are essential for spermatogenesis. We determined the fatty acid composition and the mRNA levels of key enzymes involved in fatty acid metabolism in testis of HSL -/- mice. HSL deficiency altered fatty acid composition in the testis but not in plasma; a decrease in the essential n-6 PUFA linoleic acid and the n-3 PUFA linolenic acid, and an increase in the corresponding synthesis intermediates C22:4n-6 and C22:5n-3 without changes in docosapentaenoic or docosahexaenoic acids. Mead acid, which has been associated with an essential fatty acid deficit leading to male infertility, was increased in the testis from HSL -/- mice. Moreover, the expression of SCD-1, FADS1, and FADS2 was increased while expression of ELOVL2, an essential enzyme for the formation of very-long PUFAs in testis, was decreased. It is suggested that the changes in fatty acid metabolism observed in testes from HSL -/- male mice contribute the infertility of these animals. Lipid raft are cholesterol-rich membrane domains and constitute major platforms for initiation, propagation and maintenance of signal transduction events, and are involved in cholesterol traffic. Ours results show that the absence of HSL in the testes of mice alters the structure of plasma membrane microdomains, lipid raft. Caveolin-1, a lipid raft associated protein, was shifted to non-raft fractions in membranes from HSL -/- testis. This redistribution of caveolin-1 from lipid rafts to denser membrane domains (non-raft) indicated a disruption of lipid rafts. Moreover, SR-BI and SR-BII are anchored to plasma membrane lipid rafts. The lack of HSL results in an increase of expression SR-BI in plasma membrane domains, lipid raft and non-raft in testis. The sterols membrane domains composition also is altered: desmosterol content decreases in the lipid raft domain, while in non-raft, cholesterol and T-MAS increase in testis of HSL -/-. These changes confirm the alteration of plasma membrane domains, lipid raft and non-raft.