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Regular Article |
a Department of Biological Sciences, Northern Arizona University, Flagstaff, Arizona 86011
b Division of Vascular Biology, La Jolla Institute for Molecular Medicine, San Diego, California 92121
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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corpus luteum, ovary, ovulation, progesterone
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The lipolytic enzyme hepatic lipase (HL) participates in providing cholesterol substrate for steroidogenesis. HL facilitates selective uptake and mobilization of cholesterol by hydrolyzing phospholipids and triglycerides of high-density lipoprotein (HDL) [1], which is the primary source of plasma cholesterol for rodent steroidogenic tissues [2]. Both human and rodent steroidogenic tissues have two different forms of HL expressed from a single gene. Extracellular HL is a 59-kDa glycosylated enzyme expressed and secreted from parenchymal cells of the liver [3, 4]. The extracellular form is bound to heparan sulfate proteoglycans on the vascular endothelium of the liver and steroidogenic tissues, especially within corpora lutea of the ovary [5]. Recently, an intracellular form of HL was found to be expressed primarily in steroidogenic tissues [6]. The intracellular form is a 47-kDa enzyme that is expressed from the same gene as the full-length form but lacks the first two exons and signal sequence [7, 8]. In steroidogenic tissues, >90% of mRNA for HL codes for the truncated form; therefore, little to no full-length extracellular HL is made within these tissues [7]. The truncated HL retains the catalytic site that is present in the extracellular HL. The expression of extracellular and intracellular HL forms by steroidogenic tissues and their role in HDL metabolism suggest that HL participates in providing cholesterol to support steroidogenesis.
Many studies have indicated that both the extracellular and intracellular forms of HL are involved directly in ovarian steroidogenesis, although the relative contribution of each form remains unclear. HL enzymatic activity of ovary homogenate (containing both the extracellular and intracellular HL forms) is correlated with progesterone production during the rat estrous cycle, with highest activity localized within the corpora lutea during lactation and pseudopregnancy [9]. Release of the extracellular HL by heparin perfusion decreased HL activity by 25%–70%, suggesting that total HL activity in the ovary relies on the contribution of both the extracellular and intracellular forms of HL [10]. Total HL activity was increased several fold in superovulated rats, but antibody inhibition of the extracellular HL did not affect progesterone production [11], suggesting that the intracellular HL may play a significant role in steroidogenesis. Vieira-van Bruggen et al. [12] showed that expression of truncated HL mRNA and intracellular HL protein was increased several fold in ovaries of eCG/hCG-superovulated rats, again suggesting the potential importance of the intracellular HL to ovarian steroidogenesis.
These experimental observations indicate that both the extracellular and intracellular forms of HL are involved in ovarian steroidogenesis, but what impact, if any, HL has on reproductive success has been untested. In the study presented here, we used a genetically modified mouse strain lacking the HL gene (HL-/-) and thus lacking the expression of both the intracellular and extracellular forms of HL, allowing us to address whether HL is necessary for optimum female reproductive performance.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Breeding pairs of wild-type (WT) and HL-/- C57BL/6J mice were purchased from the Jackson Laboratory (Bar Harbor, ME). All mice were housed in the Northern Arizona University Animal Care Facility and treated according to an animal protocol approved by the Northern Arizona University Institutional Animal Care and Use Committee in compliance with the National Research Council Guide for Care and Use of Laboratory Animals. Mice were kept on a 12L:12D cycle at 25°C with free access to food and water. Males and females were housed individually except during breeding, which began at 7 wk of age. Breeding pairs were housed together for 2 wk and then separated. Females were observed daily following separation for the presence of pups, and the date of birth and number of pups born were recorded for each female. Pups were weaned at 29 days of age, and the number of surviving pups and the litter sex ratio was recorded. One week after pups were weaned, the mothers were bred to a different male. This breeding regimen resulted in females being bred approximately once every 2 mo. Females were bred until they did not become pregnant (no pups for 4 wk following separation from the male) for two consecutive breedings. Pregnancy success was calculated for each female through the first four breedings as the number of litters produced/number of attempted breedings. Pup mortality was calculated for each litter as the difference between the number of pups born and the number of pups weaned.
Response to Superovulatory Treatment
Immature WT and HL-/- mice were superovulated at 29 days of age. Each female received a 100-µl i.p. injection containing 12 IU eCG (Calbiochem, La Jolla, CA) or sterile saline. The time of eCG injection was considered 0 h. Forty-eight hours post-eCG, the mice received a second 100-µl i.p. injection of 20 IU hCG (National Institutes of Health, Washington, DC) or sterile saline. There were three treatment groups: 1) saline-injected WT mice, 2) eCG/hCG-injected WT mice, and 3) eCG/hCG-injected HL-/- mice. Animals were killed by carbon dioxide inhalation followed by cervical dislocation at 3, 6, 12, or 24 h post-eCG and at 3, 6, 12, 24, 48, or 72 h post-hCG. At least two saline-injected WT mice, five superovulated WT mice, and five superovulated HL-/- mice were killed at each time point.
Plasma collection Trunk blood was collected from each mouse into a heparinized 1.5-ml microcentrifuge tube and stored at 4°C until centrifugation. Within 1 h of collection, the blood samples were centrifuged at 10 000 rpm for 10 min at 4°C. The plasma fraction was removed and stored at -80°C until further processing.
Tissue collection One randomly selected ovary from each animal was rapidly removed, trimmed of fat, snap frozen in liquid nitrogen, and stored at -80°C for ovary weight and hemorrhage site determination. Some of the remaining ovaries were fixed in Bouin solution (Sigma Chemical Co., St. Louis, MO) and embedded in paraffin, and others were snap frozen and stored at -80°C for RNA extractions. A small piece of liver was also removed from some animals and snap frozen for RNA extraction. The uteri and adrenal glands from each mouse were removed, trimmed, and placed into preweighed microcentrifuge tubes for weight determination.
Ovary weight and hemorrhagic site determination The ovaries frozen for weight and hemorrhagic site determination were thawed at room temperature. The ovaries were placed into preweighed microcentrifuge tubes and reweighed. After weighing, each ovary was examined under a dissecting microscrope, and the number of hemorrhagic sites was recorded. The number of hemorrhagic sites per microgram of ovary was also calculated.
Ovarian sections Ovaries removed at 24 h post-eCG and at 72 h post-hCG were embedded in paraffin, serially sectioned at 7 µm, and stained with hemotoxylin and eosin. Every 15th ovarian section (to avoid double-counting ovarian structures) was examined by observers who were blinded to time point, hormone treatment, and mouse genotype. The number of large antral follicles, corpora lutea, and oocytes trapped within corpora lutea was recorded for each section.
Progesterone RIA Steroids were ether extracted from the plasma samples. Recovery from the extraction was determined by calculating the percentage recovery of a 3H-progesterone internal standard added to the plasma prior to extraction. Progesterone content of the extracted plasma samples was measured by specific RIA as described previously [13]. RIA results were calculated by a four-parameter logistic analysis using the software AssayZap (Biosoft, Ferguson, MO) and adjusted for extraction recovery.
Reverse transcriptase-polymerase chain reaction for HL Total ovarian and liver RNA was prepared from frozen tissues using an RNeasy Mini RNA Extraction Kit (Qiagen, Valencia, CA). One microgram of total RNA, with a 260/280 absorbance ratio of >1.5 as determined by spectrophotometric reading, was used for a reverse transcriptase-polymerase chain reaction (RT-PCR) assay as described previously [14]. Specific oligonucleotide primers were designed from the reported cDNA sequence for mouse hepatic lipase (GenBank accession X58426). The forward primer sequence included nucleotides 673–690: 5'-GGACGCCATTCATACCTT-3', and the reverse primer sequence included nucleotides 1246–1263: 5'-ATCAACTCGCCGATGTCT-3' (Operon Technologies, Alameda, CA). These primers were used to amplify a 591-base pair (bp) product from a region of the cDNA common to both the extracellular and intracellular forms of HL [6]. The amplified product was visualized by gel electrophoresis using a 1.2% agarose (Sigma) gel. The size of the product was estimated by comparison to a DNA ladder (Hi-Low DNA Ladder; Minnesota Molecular, Minneapolis, MN) using the Electrophoresis Documentation and Analysis System 120 and 1D Image Analysis Software (Kodak Digital Science, Rochester, NY). To confirm the identity of the PCR product, it was purified using a QIAquick PCR Purification Kit (Qiagen) and digested with the restriction enzyme Pst1 (New England Biolabs, Beverly, MA). The resulting fragments were visualized by gel electrophoresis, and their sizes were estimated as described above.
Statistical Analysis
Means for litter size, percentage of pups surviving, pup weight, number of hemorrhagic sites/µg ovary, and number of large antral follicles, corpora lutea, and trapped oocytes were compared between WT and HL-/- mice using a Student t-test. The number of successful matings was compared between WT and HL-/- mice using a chi-square test. Means for number of hemorrhagic sites, ovary weight, adrenal weight, uterine weight, and serum progesterone concentration were compared across time points between saline-injected WT, superovulated WT, and superovulated HL-/- mice using a two-way ANOVA. Differences were assumed to be significant at P < 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In all previous studies of HL, rat tissues have been used, so we first confirmed that HL mRNA is expressed in the mouse liver and ovary. We used RT-PCR with specific primers that recognized both the extracellular and intracellular forms to detect HL expression in both liver and ovary tissue from superovulated WT mice. We amplified a product of the expected length of 591 bp from both liver and ovary tissue (Fig. 1) and used restriction digest analysis to verify that the product represented the HL cDNA sequence (data not shown). These results confirmed that HL mRNA is expressed in mouse in both the liver and ovary.
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Hepatic Lipase Deficiency Resulted in Significantly Reduced Litter Size
To determine whether HL plays a significant role in reproduction, we compared reproductive performance of WT and HL-/- females through their first four breedings. Pregnancy success did not differ significantly between WT and HL-/- females (
2 test, P > 0.05; Table 1). However, HL -/- females had significantly smaller litters than did WT females (Student t-test, P < 0.05; Table 1), producing on average 1.7 fewer pups per litter. Although fewer pups were produced, HL-/- pups did not have greater mortality compared with WT pups (Student t-test, P > 0.05; Table 1), suggesting that HL deficiency does not affect postpartum pup survival. The results showed that HL deficiency reduced optimal reproductive performance.
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Hepatic Lipase Deficiency Resulted in Smaller Ovaries after hCG Treatment
We used eCG/hCG hormone stimulation to synchronize ovarian responses and compared the weights of the ovaries from the three treatment groups to estimate follicular development. Throughout the superovulation regimen, the saline-treated WT ovaries remained small and unchanged in weight; therefore, all time points were combined (Fig. 2, open bar). In contrast, the eCG/hCG-treated WT and HL-/- mice showed significant increases in ovary weight (20%–80%) throughout the superovulation regimen (Fig. 2, saline control vs. eCG/hCG-treated animals, two-way ANOVA, P < 0.05). During the post-eCG/pre-hCG time frame, ovarian weight did not differ significantly between WT and HL-/- mice (Fig. 2, post-eCG/pre-hCG time points, two-way ANOVA, P > 0.05), suggesting that follicular development and maturation were unaffected by HL deficiency during early and middle stages of the induced follicular phase. However, eCG/hCG-treated HL-/- ovaries were significantly smaller than WT ovaries during the post-hCG phase of the regimen (Fig. 2, post-hCG time points, two-way ANOVA, P < 0.05). The most dramatic differences occurred during the first 12 h and 72 h post-hCG injection when eCG/hCG-treated HL-/- ovaries were 20%–40% smaller than WT ovaries.
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Effects of Hepatic Lipase Deficiency on Weight Were Specific to the Ovary
Two hypotheses were proposed to explain decreased ovary weight in HL-/- mice: 1) reduced total body weight resulting in smaller ovaries or 2) smaller steroidogenic tissues due to a developmental impact of HL deficiency. To examine the first hypothesis, we compared body weights of 19- and 29-day-old WT and HL-/- mice. At 19 days, body weight did not differ significantly between WT and HL-/- pups (Fig. 3, Student t-test, P > 0.05), but at 29 days (the first day of the eCG injection), HL-/- mice weighed significantly more (
8%) than WT mice (Fig. 3, Student t-test, P < 0.05). If the larger overall body size of the HL-/- mice at the time of superovulation were factored in, the differences in ovarian weights between WT and HL-/- mice (Fig. 2) would be even more pronounced.
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To determine whether HL deficiency had a developmental impact on another steroidogenic tissue, we compared adrenal glands of saline-treated WT and eCG/hCG-treated WT and HL-/- mice throughout the superovulation regimen. Adrenal gland weights did not differ across time points of the superovulation regimen nor between WT and HL-/- mice with or without the eCG/hCG treatment (two-way ANOVA, P > 0.05; Table 2). Together, these results suggested that the effects of HL deficiency are specific to the ovary and not due to developmental differences between WT and HL-/- mice.
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Hepatic Lipase Deficiency Did Not Influence Uterine Growth
To examine the role of HL in gonadotropin-induced steroid production, we compared estrogen-sensitive uterine weights of saline-treated WT and eCG/hCG-treated WT and HL-/- mice. As expected eCG/hCG treatment stimulated uterine growth throughout the superovulation regimen (two-way ANOVA, treatment effect, P < 0.05, data not shown), but there was no significant difference between WT and HL-/- mice (two-way ANOVA, genotype effect, P > 0.05; Table 2). These data were consistent with the lack of difference in ovarian weights between WT and HL-/- mice at 24 h post-eCG (Fig. 2) and suggest that estrogen production by developing follicles was not influenced by the absence of HL.
Hepatic Lipase Deficiency Attenuated Post-hCG Progesterone Production
Plasma progesterone was measured by specific RIA and compared among saline-treated WT and eCG/hCG-treated WT and HL-/- mice. Plasma progesterone remained low and unchanged in saline-treated WT animals throughout the stimulation regimen. Therefore, all time points were combined (Fig. 4, open bar). Progesterone levels in the eCG/hCG-treated WT and HL-/- mice were not significantly different from those of saline-treated WT mice during the post-eCG/pre-hCG phase (Fig. 4, two-way ANOVA, post-eCG time points, P > 0.05), which was expected because progesterone is not the major steroid product during the early and middle follicular phases of growth. In contrast, by 3 h post-hCG, progesterone levels in both the WT and HL-/- mice were significantly increased (2- to 8-fold) compared with saline-injected WT mice (Fig. 4, saline vs. eCG/hCG-treated animals, two-way ANOVA, P < 0.05). By 72 h post-hCG, progesterone production in both WT and HL-/- mice had returned to the levels in the saline-injected WT mice. After hCG, the HL-/- mice produced significantly less progesterone than did the WT mice (Fig. 4, two-way ANOVA, post-hCG time points, P < 0.05). These differences were most dramatic within the first 6 h following hCG and at 48 h post-hCG, when HL-/- mice produced 25%–40% less progesterone than did WT mice.
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HL-/- Ovaries Ovulated Fewer Oocytes than Did WT Ovaries
To determine whether HL plays a role in ovulation, we compared the number of ovarian hemorrhagic sites in saline-treated WT and eCG/hCG-treated WT and HL-/- mice. As expected, immature saline-treated WT ovaries did not have hemorrhagic sites. Both eCG/hCG-treated WT and HL-/- ovaries had hemorrhagic sites after hCG treatment. However, eCG/hCG-treated HL-/- ovaries had significantly fewer hemorrhagic sites than did eCG/hCG-treated WT ovaries, ranging from 1 to 7 fewer hemorrhagic sites after hCG treatment (Fig. 5, two-way ANOVA, P < 0.05). When the number of hemorrhagic sites was normalized to ovarian weight and averaged over the entire post-hCG time course, the HL-/- ovaries had about half of the hemorrhagic sites per microgram of ovary than did WT ovaries (Fig. 6, Student t-test, P < 0.05).
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HL-/- Ovaries Had More Trapped Oocytes in Corpora Lutea
Given reduced progesterone production and fewer ovulated oocytes in response to eCG/hCG in HL-/- ovaries, we anticipated that there would be more trapped oocytes in corpora lutea that represented unruptured follicles. When we examined the ovarian sections, the number of large antral follicles did not differ significantly between WT and HL-/- ovaries examined at 24 h post-eCG (Table 3). However, the number of corpora lutea was significantly lower in HL-/- ovaries than in WT ovaries at 72 h post-hCG. There were 3 times as many oocytes trapped within corpora lutea in eCG/hCG-treated HL-/- ovaries than in WT ovaries at 72 h post-hCG (Table 3, Student t-test, P < 0.05).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Because all steroids come from cholesterol, why was it that in the absence of HL progesterone production was reduced without altering estrogen levels? Estrogen production is regulated by the amount of androgen substrate synthesized by the theca cells. Theca cells are well vascularized and have ready access to HDL to meet their cholesterol demand. Therefore, what limits androgen production in theca cells is the level of expression of P450 17
-hydroxylase, C 17-20 lyase, the enzyme that converts pregnenolone and progesterone into testosterone and androstenedione, respectively. Consequently, estrogen production is indirectly related to the amount of progesterone available for conversion into androgen. In contrast, progesterone production by cells luteinizing in response to the LH surge is limited by the amount of available cholesterol substrate. Without HL, the reduced progesterone production was likely due to less cholesterol substrate being made available for utilization. Also, estrogen peaks at levels in the picograms per milliliter range in plasma. However, during the extreme steroidogenic demand of the LH surge, progesterone levels reach high levels, in the nanograms per milliliter range, in plasma so that up to 1000-fold increase in cholesterol substrate is needed to support the dramatic increase in progesterone in the plasma.
After hCG treatment, HL-/- ovaries weighed significantly less than WT ovaries. The ovaries in HL-/- mice were smaller apparently because of their attenuated response to gonadotropin stimulation. There were fewer corpora lutea in the HL-deficient ovaries, so when these corpora lutea regressed there was a more dramatic drop in ovarian weight in the HL-/- ovaries than in the WT ovaries at 72 h after the hCG dose. Decreased ovarian weight in the HL-/- mice was not attributed to developmental deficiencies or general deficits in steroidogenic tissues. Age-matched HL-/- mice weighed more than their WT counterparts. If an allometric adjustment were factored in for the greater HL-/- body size, the post-hCG HL-/- ovaries would be even smaller, relatively, than those of matched WT ovaries. The adrenal gland weights in each of the 2 genotypes were equivalent. Yet another potential explanation for the smaller HL-/- ovaries was the development of fewer antral follicles in response to eCG, so when hCG was injected fewer follicles were ready to respond with their final growth phase. However, this explanation was not supported; the numbers of large antral follicles in the HL-/- ovaries was the same as that in the WT ovaries following eCG priming. These data suggest that the HL-/- ovaries were smaller after the hCG injection because of the attenuation of late-stage progesterone-dependent follicular maturation.
Progesterone is essential for the final maturation of preovulatory follicles that follows the LH surge [15]. If fewer ovulations in HL-/- ovaries were due only to reduction of final follicular growth, then normalizing the number of hemorrhagic sites to ovary weight should eliminate the difference between the strains. Despite normalizing the number of hemorrhagic sites to ovary weight, eCG/hCG-treated HL-/- mice still had significantly fewer hemorrhagic sites. Thus, these results suggest that in addition to decreasing hCG-dependent follicular growth, HL deficiency may have also led to impaired release of the oocytes. This conclusion was supported by the observation that eCG/hCG-treated HL-/- ovaries had significantly more oocytes trapped in corpora lutea and fewer corpora lutea than did WT ovaries. Trapped oocytes have been reported in eCG/hCG-stimulated progesterone receptor-deficient mice, which cannot successfully ovulate in spite of having preovulatory follicles [16].
Successful ovulation requires the physical release of the mature oocyte. Expulsion of the oocyte is dependent on progesterone induction of proteolytic enzymes that degrade the follicle wall [17]. The dependence on progesterone for ovulation has been demonstrated by using anti-progesterone antiserum [18], blocking progesterone production [19], and blocking the progesterone receptor with antagonists [20, 21] and in studies with anovulatory progesterone receptor-deficient mice [22]. Two proteases, A disintegrin and metalloproteinase with thrombospondin-like motifs (ADAMTS-1), and cathepsin L, a lysosomal cysteine protease, are transcriptional targets of the progesterone receptor, and both are induced by LH-stimulated progesterone produced prior to ovulation [17]. We suggest that the reduced production of progesterone in the HL-/- ovaries that occurred just after hCG stimulation resulted in reduced expression of proteases needed for the degradation of the walls of all of the preovulatory follicles ready to release their oocytes.
HL expression is greatest in the luteinized rat ovary and is dramatically higher in superovulated rat ovaries [11]. Additionally, expression of mRNA for the truncated HL parallels progesterone production in superovulated rats [12]. Our observation that HL deficiency results in decreased progesterone production just after hCG stimulation is consistent with these previous results, but exactly how and which form of HL supports ovarian steroidogenesis remains unclear. Gonadotropin-stimulated steroidogenesis is dependent on the availability of cholesterol substrate. HL probably participates in steroidogenesis by facilitating the provision of cholesterol substrate.
In vitro and in vivo extracellular HL facilitates selective uptake of cholesteryl esters from HDL via scavenger receptor class B, type I (SR-BI) [23–26]. Extracellular HL enhances selective uptake by acting both as a ligand for HDL and as a lipolytic enzyme, remodeling the HDL particle to promote cholesteryl ester transfer. SR-BI and HL are coparticipants in steroidogenesis, as indicated by the upregulation of SR-BI mRNA expression in HL-/- mouse adrenal gland [27]. Also, inhibition of extracellular HL by blocking antibodies causes significant increases in adrenal SR-BI expression similar to the compensatory response in HL-/- mouse adrenal gland [28]. Thus, in the absence of HL ovarian selective uptake of cholesteryl esters by SR-BI may have been significantly compromised, limiting progesterone production and diminishing ovarian responses distal to progesterone production. The dependence on extracellular HL to support steroidogenesis may differ in the mouse because in mice two thirds of the HL activity is in plasma, whereas in rats and humans HL activity is undetectable in plasma [29].
It is unknown whether the expression of ovarian HL is altered by the absence of SR-BI. SR-BI-deficient mice have been generated, but study of HL expression and its participation in facilitating selective uptake by steroidogenic tissues in these mice has not been reported. SR-BI-deficient female mice are infertile [30]. Pseudopregnant SR-BI-deficient mice have levels of serum progesterone 6 days after superovulation similar to those of control WT mice [30]. The SR-BI-deficient mice have no obvious defects in their estrous cycles or number of oocytes ovulated [30]. However, a detailed study of ovarian progesterone production within 12 h of hCG injection, the time frame in our studies when the absence of HL resulted in a significant decrease in progesterone production, has not been reported for SR-BI-deficient mice.
The role of the truncated intracellular HL in ovarian steroidogenesis is undefined. The truncated HL may also be involved with SR-BI in facilitating cholesteryl ester transfer from HDL or in the management of the transfer of phospholipids. Intracellular HL may facilitate the mobilization of cholesterol from cholesteryl ester-rich lipid droplets that are so prominent in steroidogenically active ovarian cells, a role that was previously suggested by Vieira-van Bruggen et al. [12]. The pattern of immunolocalized HL in the WT mouse adrenal gland is similar to that seen when lipid vesicles in the cells are stained with oil red O [31]. Thus, without intracellular HL there may be less efficient use of cholesteryl ester stores needed during periods of extreme demand, such as in response to the LH surge or the supraphysiologic hCG-mediated stimulation of progesterone production. Another scenario could involve HL in the processing of HDL as it is routed through the newly described retroendocytic pathway, which has been shown to occur with HDL selective uptake by the liver via SR-BI [32, 33].
HL deficiency in the mouse results in a mild dyslipidemia with an increased subfraction of larger, less dense HDL particles [34]. These large cholesterol-rich HDL particles are less efficient at supporting ovarian steroidogenesis [35]. Thus, the in vivo decrease in ovarian progesterone production could have been due to HDL subfraction differences in the HL-/- mice. An in vitro investigation of HL-/- ovarian cell steroidogenesis could differentiate between the contribution of altered HDL and that of HL deficiency to abnormalities in ovarian function.
HL deficiency occurs in humans and is associated with a more profound dyslipidemia than that observed in the HL-/- mouse [36]. It is not known whether HL-deficient women have compromised reproduction because the critical information, i.e., documented difficulty conceiving or maintaining pregnancy, has not been reported. Human ovaries produce very large quantities of progesterone shortly after the endogenous LH surge, on the order of a 5-fold increase in plasma concentration within the first 24 h [37]. Given the results presented here, we would expect that HL-deficient women would experience reproductive problems with fewer successful ovulations. Our findings indicate that HL, the extracellular or intracellular form or both forms, is required for optimal reproductive performance in the mouse.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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1 This work was supported by American Heart Association Predoctoral Awards 9804188P and 0010214Z (to R.L.W.) and American Heart Association Grant-in-Aid AGA6597 (to C.A.D.). C.A.D. is an Established Investigator of the American Heart Association. 
2 Correspondence: Cheryl A. Dyer, Department of Biological Sciences, Box 5640, Building 21, Northern Arizona University, South Beaver Street, Flagstaff, AZ 86011-5640. FAX: 520 523 7500; cheryl.dyer{at}nau.edu
Accepted: November 5, 2001.
Received: June 5, 2001.
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REFERENCES |
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-hydroxylase, C17-20 lyase. J Lipid Res 1998; 39:2406-2414This article has been cited by other articles:
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  C. Stocco, C. Telleria, and G. Gibori The Molecular Control of Corpus Luteum Formation, Function, and Regression Endocr. Rev., February 1, 2007; 28(1): 117 - 149. [Abstract] [Full Text] [PDF]
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 B. Perret, L. Mabile, L. Martinez, F. Terce, R. Barbaras, and X. Collet Hepatic lipase: structure/function relationship, synthesis, and regulation J. Lipid Res., August 1, 2002; 43(8): 1163 - 1169. [Abstract] [Full Text] [PDF]
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5 ovaries) ± SEM. Differences in ovary weight between the three treatment groups were analyzed across the post-eCG time points and across the post-hCG time points with separate two-way ANOVAs. Significant differences (P < 0.05) and the direction of the differences between the treatment groups are denoted in the figure by inequality (< or >) symbols. Nonsignificant differences are denoted by an equality symbol (=)
