The Complex Interplay Between Cholesterol and Prostate Malignancy

Abstract

Research into the topic of the role of cholesterol and prostate disease has been ongoing for many years, however our mechanistic and translational understanding is still poor. Recent evidence indicates that cholesterol lowering drugs reduce the risk of aggressive prostate cancer, however the studies in this area, performed over many years, reflect much controversy and uncertainty. Here we explore the entire literature on the relationship between circulating cholesterol and prostate cancer, with consideration and criticism of the older as well as the newer studies. We consider why low cholesterol is associated with both increased and decreased risk of advanced prostate cancer, and explain why both observations are probably correct. We discuss the conflicting results of randomized placebo-controlled trials of statin drugs vs. observational studies and demonstrate that a predominance of pravastatin in the randomized trials paints a distorted view of statin effects. Lastly, we discuss new data suggesting that a critical aspect of the role of cholesterol in prostate cancer progression is through its role in intratumoral steroidogenesis. With these points addressed, the data strongly point to hypercholesterolemia as a risk factor for prostate cancer progression and suggest clinical opportunities for the use of cholesterol lowering therapies to alter disease course.

“…it seems that the balance of evidence is…in favor of cholesterol’s playing at least some part in the growth of malignant tumors, and… in benign enlargement of the prostate.”.

—G. I. M. Swyer 1942

Cholesterol and prostate cancer: deconstructing a complex relationship

After over a hundred years of research into the topic of cholesterol and abnormal cell growth⁽¹⁾, much debate and substantial doubt remain concerning the effect of hypercholesterolemia on prostate disorders. These unresolved issues include whether drugs used to treat hypercholesterolemia alter the risk of prostate cancer. Much of the controversy stems from limitations in the existing literature and interpretations thereof that go beyond the data. In this review, we will highlight some of these points of debate and draw contrasts between established findings, inconsistencies between randomized trials and observational studies, and what we believe to be misinterpretations of published data sets. What emerges from a careful analysis of a large body of literature, going back many decades, is an integration of older findings and data in newer reports that support a conclusion that hypercholesterolemia is a risk factor for aggressive prostate cancer.

The prostate synthesizes cholesterol at rates equivalent to the liver and an age-dependent shift in cholesterol homeostasis allows cholesterol to accumulate in the prostate at high levels in older individuals⁽²⁾. Cholesterol makes up about 30% of the lipid content of plasma membranes and is an absolute requirement for new membrane synthesis. This neutral lipid also contributes to the physiological properties of cell membranes by regulating membrane fluidity, promoting negative membrane curvature, and by interdigitating with acyl chains of phospholipids to create “liquid ordered” membrane microdomains. Cholesterol-rich microdomains are believed to play an important role in signal transduction and in other physiologic mechanisms such as solute transport. Because of the multiple roles of cholesterol in the cell, perturbations in cholesterol metabolism might conceivably alter epithelial, stromal and inflammatory cell infiltrates in the prostate. In a scenario involving rapid cell proliferation, the requirement for cholesterol assembly into new membranes may be a rate-limiting step in the process of tissue growth. Tumor cell proliferation, in turn, may affect circulating cholesterol levels. Here we review various aspects of this complex relationship.

Cancer and its effects on circulating cholesterol: the U-shaped curve

The concept of the “U shaped curve” refers to the higher levels of mortality found on either end of the cholesterol level spectrum that many investigators discovered when studies were performed to assess the potential relationship between circulating cholesterol and mortality. Although it is well established that the increase in mortality at the high end of the cholesterol spectrum is due to cardiovascular disease, the higher level of mortality at the low end of the spectrum is somewhat more mysterious and appears to emanate from a variety of sources. These include an unexplained higher frequency of accidental deaths in individuals with low cholesterol and an association between endemic hepatitis and low cholesterol. However, another important reason for higher mortality at the low end of the U arises from the cholesterol-lowering effect of cancer itself; therefore, the low end of the curve likely includes an excess of subjects with pre-existing cancer.

Although earlier papers speculated on this point, evidence implicating a direct effect of cholesterol lowering on increased cancer risk was first reported in 1971 by Pearce & Dayton⁽³⁾. These investigators examined cancer incidence and cancer-specific deaths in two patient cohorts, one that received a control diet and the other an experimental cholesterol-lowering diet. The experimental diet was essentially the same as control, but with less cholesterol and with additional polyunsaturated fats. These authors found that within 10 years of the study’s initiation there was more total cancer (81 vs. 66), more cancer deaths (31 vs. 17) and, for our purposes, more prostate cancers (12 vs. 10) in the experimental diet cohort, although the prostate cancer differences are small. A review of 5 diet trials published in 1971⁽⁴⁾ did not support a cholesterol-lowering diet and cancer association (OR for cancer incidence was 1.15 95% CI (0.81–1.63); for cancer death it was 1.08 95% CI (0.71–1.69)) and other, subsequent, cholesterol-lowering diet trials did not confirm the observations of Pearce and Dayton⁽⁵⁾. A review of pre-statin cholesterol lowering trials in 1988⁽⁶⁾, in which 22 randomized trials including ≈40,000 individuals were discussed (only 2 of which are cited), found no evidence suggesting an association between low cholesterol and cancer risk. This contrasts with a 1990 meta-analysis⁽⁷⁾ of 6 cholesterol lowering trials, which used stringent criteria for selection of included studies. This review demonstrated a significant increase in overall cancer mortality (OR 1.43 95% CI (1.08–1.90)) that remained even when studies not including drugs (2 studies) were used in the analysis (OR 1.62 95% (CI 1.03–2.57)).

Randomized, placebo- or diet-controlled cholesterol lowering trials were largely equivocal in their conclusions concerning a cholesterol-cancer association, and population studies were equally ambiguous. Rose et al.⁽⁸⁾ (1974) pooled data from 6 prospective population studies (more on these studies below) to demonstrate a low cholesterol-colon cancer link, with colon cancer patients having on average a 10 mg/dL reduction in total cholesterol vs. the population mean. The conclusions of this early study were buttressed by a number of additional population studies that showed excess cancers in the cohorts with the lowest cholesterol.

We have reviewed 52^(9–60) population studies that reported on cholesterol and total cancer incidence and/or mortality that were published from 1972–2009. In total, these reports cover 79.5 million men and women ages 15–99 (average 38–63) from Finland, Yugoslavia, USA, New Zealand, Italy, France, Japan, China, Scotland, England, Norway, Israel, Australia and Sweden. 32 studies report an inverse association between cancer risk and cholesterol level, 16 show no association, 2 provided no statistics, 1 was a follow-up report, and 1 study was largely equivocal. Studies that demonstrate excess risk usually find this association more prominently in men and, most frequently, the associated cancers are those of the liver, colon, and lung (probably due to the high rates of mortality seen with these cancers). Many of these studies were long in duration (as long as 40 years); consequently, a number of the authors transformed the data by removing cancer cases that appeared in the studies’ early years (defined by authors as those occurring anywhere from 2–20 years after study initiation). In 12 of a total of 30 of these reports, removal of cancers that appeared early in the study either diminished or eliminated the significance of the low cholesterol-increased cancer risk association, suggesting that lower cholesterol was not the cause but the result of cancer.

One intriguing aspect of the above population studies that seems to strongly support the hypothesis that the inverse correlation between cancer and cholesterol level is due to an effect of cancer on cholesterol level is that there is no absolute ‘low level’ of cholesterol associated with cancer. For example, an association is found when the low cholesterol level for a population is ≤230⁽²⁶⁾ or ≤134⁽²⁵⁾, i.e., the 20% of any cohort with the lowest cholesterol in any population seems to have a greater prevalence of cancer than groups within that population with higher cholesterol. Logically, one would predict that if low cholesterol triggered cancer it would do so at a relatively uniform cholesterol level, and that if such an association existed, populations with low cholesterol would show an excess of certain cancers, an outcome that population studies do not support. Instead, if cancer reduces cholesterol level we would expect the 20% of the population with the lowest cholesterol to have excess cancer regardless of the endemic cholesterol level of the population, a prediction that is supported by the population studies when considered in aggregate.

A second interesting point that appears to demonstrate that low cholesterol is the result of the effect of cancer on the host, and not the cause of cancer, arises from studies that measured cholesterol proximal to time of death. Keys et al. show that men who died of cancer within 2 years of study onset had cholesterol values 9.48% lower than the average of all men at entry⁽²⁵⁾. Sherwin et al. demonstrated that men who developed cancer exhibited a 22.7 mg/dL drop in cholesterol level vs. matched survivors⁽⁴⁴⁾. The International Collaborative Group showed that individuals dying from cancer 1 year after cholesterol measurement had cholesterol levels that were 24–35 mg/dL lower than controls (i.e. those not dying); those dying 2–5 years after cholesterol measure demonstrated values that were 4–5 mg/dL lower than controls; and those dying of cancer 6–10 years after cholesterol measure had cholesterol values 2 mg/dL lower than controls⁽¹⁰⁾. These findings likely reflect the tendency of cancer to depress circulating cholesterol levels.

Particularly revealing are studies of cholesterol level variation over time in relation to cancer incidence or death. There are only a few of these reports because they require multiple cholesterol measures over time and few studies were designed to capture these data. The first such report we analyzed is by Sorlei and Feinleib (1982)⁽⁴⁶⁾, which used Framingham Heart Study data and showed that in some individuals who develop cancer, cholesterol levels are lower than the mean value up to 18 years prior to cancer diagnosis, while in other individuals cholesterol levels decline near the time of diagnosis. The robustness of the analysis is hampered by the fact that the authors split the group by decades of age and sex, thus making each cohort quite small. The authors write “Although the Framingham data are not conclusive, they do suggest that in some cancer cases where the serum cholesterol level was lower than that expected at as much as 16--18 years before cancer diagnosis, the depressed level was likely to be a precursor to the tumor growth. However, consistent with the metabolic consequences of tumor growth, the data show that in some cancer cases, serum cholesterol had decreased at measurements made close to the time of cancer diagnosis.” Pekkanen et al. 1992⁽³³⁾, studying Finnish men (aged 55–74), analyzed the change in cholesterol levels from 1959–1974 (every 5 years) in individuals that did not have cardiovascular disease in 1974. The authors found that older men (aged 65–74 in 1974) with the steepest decline in cholesterol had a higher risk of dying from cancer. In 1992 Pocock and Seed⁽³⁷⁾ reported cholesterol time trend data from British men and women with hypertension (aged 65–74) demonstrating that cholesterol levels fell an average of 11.2 mg/dL in men within a year prior to a cancer death. Sharp and Pocock (1997)⁽⁴²⁾, again using Framingham Heart Study data, demonstrate a number of highly relevant findings: 1) 61% of individuals demonstrate a 12.6 [95% CI (8.46–16.70)] mg/dL decline in cholesterol within 2 years prior to a cancer death; 2) The mean level of cholesterol 2–4 years prior to death from cancer is 10 mg/dL lower than the population norm; 3) The odds of dying of cancer within 4 years were increased (2.11 95% CI (1.41.–3.14)) in individuals whose decline in cholesterol was greater than 38 mg/dL.

Whether low cholesterol causes cancer or low cholesterol is the result of cancer has important implications for public health. Not surprisingly, the National Institutes of Health was concerned, especially in light of the large body of evidence that high cholesterol is associated with death from cardiovascular disease (nearly all the population studies of cholesterol and mortality verified this), and the international reaction to reduce cholesterol levels broadly in the human population. At least 3 NIH conferences^(61,62) were held to explore the relationship between cholesterol and mortality, and cancer mortality, specifically: the first was held in February of 1980, and the second soon after in May of the same year⁽⁶¹⁾; a third was held in October of 1990⁽⁶²⁾. Since the report of Jacobs⁽⁶²⁾ is the most thorough, with its inclusion of 19 population studies in its analysis, we will briefly explore this report with regards to cancer. After elimination of cancer deaths occurring within 5 years of the onset of each individual study, the cancer rate ratio for men with cholesterol <160 mg/dL was between 1.18 (p<0.05) and 1.23 (p<0.001), depending on how the studies were analyzed. For women the cancer rate ratio was 1.05 (not significant). Significant associations between low cholesterol and cancer were found for lung cancer in both men and women, and other cancers (not specified) in women. No association was found between low cholesterol and colon cancer. The summarized conclusion from the 3 NIH sponsored conferences was that there was insufficient data regarding the association between cholesterol and cancer. They cite multiple concerns, including: 1) the modest increase in cancer among the low cholesterol population, 2) the lack of a clear association in women, 3) the presence of the cancer-cholesterol association in some populations but not others, 4) the presence of a presumptive effect in some but not all studies, and 5) the absence of a plausible biological mechanism that would explain why “lower,” but still physiological levels of cholesterol might trigger cancer. These considerations, as well as an absence of information about the effect of disease on cholesterol levels over time, prevented the conferees at any of the 3 meetings mentioned above from adopting specific recommendations on cholesterol reduction therapy. The recommendation of all 3 NIH conferences on the subject of low cholesterol and mortality was not to alter the prescription of lowering cholesterol levels to improve public health, but to continue to study the issue.

The concern that low cholesterol might increase cancer risk persisted until the early 1990’s, however it has almost entirely disappeared in the post-statin era. Multiple meta-analyses demonstrate that statins, which potently reduce cholesterol levels, do not cause an increase in cancer^(63–65). This leaves the conclusion that cancer itself reduces circulating cholesterol as the only plausible explanation for the presumptive associations reported in the population studies described above.

What do the large population studies of overall disease incidence, mortality and cholesterol level, where prostate cancer was not an exclusive focus, indicate about prostate cancer, specifically? A small minority of the studies (10 out of 52 publications) we examined for this review include prostate cancer in the analysis, and the total number of prostate cancer cases combined in the studies is only 1,652. The following summarizes all of the major population studies that include prostate cancer as an endpoint. Knect et al.⁽²⁶⁾ reported a positive association between low cholesterol and increased prostate cancer risk (n=45), but did not follow their cohort for greater than 5 years. Kark et al. ⁽²⁴⁾ found that individuals with prostate cancer (n=12) had significantly reduced cholesterol levels. Hiatt et al.⁽¹⁹⁾ reported on 601 prostate cancers and found that removing cases occurring within the first two years after study inception eliminated the positive correlation between low cholesterol and cancer incidence. Williams et al.⁽⁵³⁾ reported on 44 prostate cancer cases, but do not comment specifically on a cholesterol-prostate cancer association. Wingard et al.⁽⁵⁴⁾ reported on 49 prostate cancer cases, but found no cholesterol-cancer association. Morris et al.⁽³¹⁾ found a significant low cholesterol-cancer association (n=26). These investigators show that the 5-year prostate cancer incidence rates decreased step-wise with lower to higher cholesterol levels (6.9 deaths/1000 cases when cholesterol was ≤204 mg/dL; 6.5/1000 in the 205–230 mg/dL quartile; 2.2/1000 in the 231–260 mg/dL quartile and 1.7 deaths/1000 cases in the ≥261 mg/dL quartile). Schatzkin et al.⁽⁴⁰⁾ reported on 95 prostate cancer deaths and found no cholesterol-cancer association. Tulinius et al.⁽⁴⁹⁾ describe no statistically significant association between cholesterol level and 524 incident prostate cancer cases. Davey-Smith et al.⁽⁴⁵⁾ reported on 92 prostate cancer deaths and in quintile analysis demonstrate a non-significant trend, with lower levels of cholesterol associated with reduced cancer risk; the HR corresponding to a 1 SD decline in cholesterol was 0.85 95% CI (0.69–1.04). Iso et al. found no low cholesterol-prostate cancer association in their 164 cases; instead they report a significant trend of higher levels of cholesterol associated with increased cancer risk (p for trend 0.0023), an association that disappeared when advanced cancers were removed from the analysis (p for trend 0.12). Collectively, these studies point to a modest association between low cholesterol and increased prostate cancer risk.

In contrast to the above reports that include limited numbers of prostate cancer cases, for the most part were of relatively short duration, and almost never report or comment on late stage disease (except as it pertains to death), studies that specifically address the potential association between serum cholesterol level and prostate cancer have large enough cohort sizes to allow a more thorough analysis. Thompson et al. 1989⁽⁶⁶⁾ found no cholesterol-prostate cancer association (n=100); the Asia Pacific Cohort Studies Collaboration 2007⁽⁶⁷⁾ (Huxley R.) reported on 308 prostate cancer deaths and found a greater number of deaths in the population with the highest cholesterol based on tertile analysis, but the difference was not significant (possibly due to small numbers). Platz et al. 2008⁽⁶⁸⁾ in a case-control analysis of men in the Health Professionals Follow-up Study demonstrated that men (n=698) with low cholesterol (Q1 in quartile analysis; actual cholesterol level is undefined because of assay complications) had a lower risk of high-grade prostate cancer (OR, 0.61 95% CI (0.39–0.98)). Platz et al. 2009⁽⁶⁹⁾ examined 1,251 incident prostate cancers and found that men with low cholesterol (<200 mg/dL) had a lower risk of high-grade disease (Gleason 8–10; OR, 0.41 95% CI (0.22–0.77)). Mondul et al. 2010⁽⁷⁰⁾ examined 438 incident prostate cancers and determined that men with cholesterol <240 mg/dL were less likely to develop high grade prostate cancer then men with cholesterol >240 mg/dL, results that were unchanged after eliminating cholesterol-lowering drug users. Batty et al. 2011⁽⁷¹⁾, in a study that included 578 prostate cancer deaths, reported a greater number of cancer-specific deaths in the highest cholesterol tertile (<175 mg/dL HR 1.0 (reference population); 175–214 mg/dL HR 1.07 95% CI (0.87–1.32); >214 mg/dL HR 1.35 95% CI (1.11–1.65) p-value for trend = 0.003). Clearly, these reports do not support a low cholesterol-increased prostate cancer risk association, but instead suggest that men with high cholesterol are either at increased risk for prostate cancer or castrate-resistant disease.

A question of PSA

The answer to the question of why some studies support an association between low cholesterol and prostate cancer, while others support an association with high cholesterol, most likely reflects when the study was conducted. The FDA approved serum prostate specific antigen (PSA) as a prostate cancer biomarker in 1994, forever changing the diagnostic landscape in the field. With PSA testing, men now generally present clinically with early stage disease, years before any clinical symptoms would otherwise appear. Thus, cancer populations considered in studies published prior to 1994 include many more advanced cancers than studies published in the last 10 years. This important milestone also suggests that cholesterol readings in the pre-PSA era have a greater chance of being a product of tumor metabolism, leading to a low cholesterol-cancer association, whereas cholesterol measures in post-PSA studies are more likely to reflect the cholesterol environment prior to the development of cancer. This would lead to a positive correlation between high cholesterol and prostate cancer risk.

We propose a unifying model that reconciles the data from the pre- and post-PSA studies (Fig. 1). The most recent evidence indicates that high circulating cholesterol is a risk factor for prostate cancer⁽⁷²⁾. In the pre-PSA era a low cholesterol reading is more likely to be associated with a higher risk of a prostate cancer death; in the post-PSA era this association is reversed. This contradiction can be explained by the vastly different patient cohorts (with regard to extent of disease progression) analyzed in the older vs. the new studies, by preclinical findings that high circulating cholesterol promotes prostate cancer growth^(73,74) and, by epidemiological data showing higher cholesterol levels increase prostate cancer risk. Additional evidence includes the apparent protective effects of long-term statin drug therapy on prostate cancer risk (considered below).

Figure 1.

Theoretical representation of the relationship between low cholesterol and the risk of prostate cancer death in the pre- and post-PSA eras.

However, there are important caveats that alter this simple equation, and these are best understood by considering the period of time between a “low” cholesterol measure and a prostate cancer diagnosis vs. the relative risk of prostate cancer death (Fig. 1). In both the pre- and post PSA eras a low cholesterol measure within 1 year of a prostate cancer diagnosis raises the risk of a prostate cancer death, while in both the pre- and post-PSA eras a low cholesterol measure >6 years prior to a prostate cancer diagnosis reduces the risk of a prostate cancer death (Fig. 1, left end of the curves vs. the right end of the curves). Between 1 and approximately 6 years prior to a prostate cancer diagnosis the tendency for low cholesterol to correlate with increased risk of prostate cancer death is different in the pre- and post-PSA eras. In the pre-PSA era a low cholesterol measure about 1–6 years prior to prostate cancer diagnosis raises the risk of a prostate cancer death, while in the post-PSA era a low cholesterol measure ~1–6 years prior to a prostate cancer diagnosis decreases the risk of prostate cancer death.

The statin controversy

Recent epidemiologic studies from a number of groups have shown that cholesterol-lowering drugs (primarily 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, known as “statins”) may lower prostate cancer risk, and in particular, the risk of advanced disease^(75–87). Platz et al.⁽⁸⁴⁾ assessed potential statin drug effects specifically on prostate cancer in an analysis powered to measure differences in cancer incidence and progression in a total of 2,579 cancer cases, with 316 cases of advanced disease. The adjusted relative risk of castration resistant cancer among statin users in this study was 0.51 95% CI (0.30–0.86)) and of metastatic or fatal disease was 0.39 95% CI (0.19–0.77)) for statin users vs. nonusers. These authors also demonstrated that risk of advanced disease was lower with longer statin use (the relative risk for statin users of <5 years of use was 0.60 95% CI (0.35 –1.03)) and for ≥5 years of use was 0.26 95% CI (0.08–0.83)). In contrast to advanced disease, this study found no association between statin use and prostate cancer risk, suggesting that incidence alone is inadequate for evaluating potential chemopreventives in this disease.

Several more recent prospective studies from independent groups have largely confirmed the conclusions of Platz et al.^(79,82–84) that statins reduce the risk of aggressive prostate cancer. Flick et al. performed a case-control analysis using a population of 69,047 participants in the California Men’s Health Study that included 888 total cases of prostate cancer, with 131 advanced cases. They concluded that use of statins for ≥5 yrs was associated with a 28% lower disease risk (adjusted rate ratio of 0.72 95% CI (0.53–0.99)). Jacobs et al. reported on a case-control study using 55,454 men from the Cancer Prevention Study II Nutrition Cohort, which included 3,413 cases of prostate cancer, with 317 cases of advanced disease. This group did not show a change in overall prostate cancer risk in the statin group; however, they found a marginally significant effect on advanced disease among the statin users (adjusted rate ratio of 0.60 95% CI (0.36–1.00)). Murtola et al. presented a case-control study of a large study population using data from the Finnish Cancer Registry, the Population Register Center, and the Social Insurance Institution of Finland (24,723 cases with an equal number of matched controls). This group found a significant reduction of risk of advanced prostate cancer in users of atorvastatin, lovastatin, and simvastatin (approximately 77% of the statin drug usage in the cohort), with an adjusted odds ratio of 0.61 95% CI (0.37–0.98); 0.61 95% CI (0.43–0.85); and 0.78 95% CI (0.61–1.01), respectively (the overall odds ratio for all statins was 0.75 95% CI (0.62–0.91)). Murtola et al. also noted that non-statin cholesterol-reducing drugs (fibrates specifically) modestly reduced the risk of advanced prostate cancer, but these data did not reach significance, possibly a result of a small sample size. This latter point is interesting. We have proposed that the chemopreventive effects of statin drugs arise predominantly or exclusively from cholesterol lowering, not from other statin effects, such as inhibition of isoprenoid synthesis⁽⁷²⁾; consequently, if this hypothesis is true, we would expect that other methods of cholesterol reduction would also modify risk. In summary, recent observational studies of statin effects on prostate cancer risk, which contain large numbers of subjects, are largely supportive of the hypothesis that statins reduce the risk of advanced prostate cancer.

Although the recent literature indicates that long-term statin therapy is chemopreventive against aggressive prostate cancer, large randomized trials of statin drugs that report on cancer (including prostate cancer) do not support this claim^(63–65). There are a number of reasons that these two types of studies tend to present very different pictures regarding prostate cancer risk and statin use.

Large randomized placebo controlled trials of statins include small numbers of prostate cancers. We reviewed 49^(88–136) trial reports that included 134,516 individuals and identified only 5 prostate cancer deaths and 1,142 incident prostate cancer cases, and these trials were of relatively short duration (4.2 years on average). Observational studies of prostate cancer risk and statin use include many more prostate cancer cases (77,325 in a total study population of 4,168,049), and they include follow-up to 14 years. This explains some of the discrepancy between randomized, placebo-controlled studies and observational studies, but there are some additional important differences that deserve attention.

Too much pravastatin?

Of the 34 separate randomized placebo-controlled trials that we analyzed that reported on the LDL reduction, including all the major statin trials (some of the 49 reports we evaluated were various analyses of the same initial trial, and others did not report on LDL reduction), 14 were trials of pravastatin^{(90–93,101,102,105,110–112,114,116,119,121–123,127,128,135,136)}, 6 were simvastatin trials^{(88,89,94,96,98,100,118,129,130,133,134)}, 6 were atorvastatin trials^{(99,113,115,117,120,126,131)}, 4 were trials of fluvastatin^{(107–109,124,125)}, 3 were lovastatin trials^{(97,103,106,132)} and 1 was a rosuvaststin trial⁽¹⁰⁴⁾ (Table 1). Of the 56,095 individuals randomized to any statin, 46.5% were randomized to pravastatin. This contrasts greatly with statins used as reported in the observational studies, in which only 1.6% of the patients took pravastatin. This lower percentage of prevastatin is more similar to overall statin sales in which pravastatin only represents about 6% of the total⁽⁷²⁾. It is important to note this large difference in the use of a specific statin between these two types of reports. Statins are usually considered as an undifferentiated group in population studies, as if one statin drug was chemically and physiologically equivalent to another. However, statins are chemically diverse and have significantly different potencies with respect to cholesterol lowering. This large difference in pravastatin use in the various studies may have important consequences with regards to outcomes.

Table 1. Number of randomized, placebo-controlled statin trials that recorded LDL reduction, and percentage of LDL reduction achieved.

LDL reduction was calculated by subtracting the levels post-treatment with pre-treatment values in the statin cohort^{(88–93,95–97,99–102,104,106,107,109,110,113–117,119–122,124–128,130–132,135,136)}. In some studies, the placebo group also demonstrated LDL reduction. In these cases this value was subtracted from the reduction achieved in the treatment group.

Statin Trials	Number of studies	LDL reduction (%)
Pravastatin	14	24.3 ± 6.5
Simvastatin	6	32.6 ± 8.8
Atorvastatin	6	33.4 ± 8.9
Fluvastatin	4	25.5 ± 9.3
Lovastatin	3	30.6 ± 8.6
Rosuvastatin	1	41

Pravastatin is the weakest of the statin drugs in terms of its ability to inhibit HMG-CoA reductase (IC₅₀ 6.93 nM; Ki 23 × 10⁻¹⁰)⁽⁷²⁾ and consequently does not reduce total or LDL cholesterol to the same extent as other statins. This difference in potency seems to be reflected in the results of randomized studies of statin effects on cardiovascular disease. Although all statins seem to be similarly effective in reducing risk of cardiovascular disease, pravastatin was less effective than any other statin used in reducing LDL cholesterol (Table 1). In our analysis of these published data, pravastatin reduced LDL by an average of 24.3%. In contrast, simvastatin reduced LDL cholesterol by 32.5%, a difference that is statistically significant (p=0.01). Moreover, we calculated that the total percentage decline in LDL from all the randomized studies was 28.5%. However, if the patients randomized to take a statin were given statins in the percentage used in observational studies, we estimate that the decrease in LDL cholesterol would be about 32%, a potential difference of 3.5 mg/dL in LDL levels. Because so many patients were randomized to pravastatin, pharmacological cholesterol lowering in the study population is not as great as would be expected if the patients were taking statins outside of the study setting (i.e., in the proportions used by the general public). The consequences of this skewed mix of statins for detecting a statin effect on cancer incidence or progression is unknown. However, coupled with the short duration of most randomized studies, patient crossover from placebo to statin (as required by usual care), and patient selection bias (as we discussed in prior works)^(72,137), this could entirely explain the differences in statin association with prostate cancer risk in randomized, placebo-controlled studies and observational studies.

How does cholesterol increase the risk of lethal prostate cancer?

As mentioned above, cholesterol plays a vital role in establishing the properties of functional animal cell membranes. It also affects various signaling pathways and proteins, either by direct conjugation to proteins, i.e. sonic hedgehog, or by modifying the activities of membrane proximal signaling pathways and proteins such as the cell survival kinase, AKT^(138,139). Certain signal transduction pathways appear to be highly sensitive to manipulations in circulating cholesterol levels^(73,74). In vivo, hypercholesterolemia causes increased tumor angiogenesis, reduces tumor apoptosis and increases tumor cell proliferation^(73,74). The relative contribution(s) of these mechanisms to prostate cancer progression is unknown. We have extensively reviewed the above mechanisms in previous commentaries^{(72,140–143)}.

Recent studies have provided another mechanism for tumor-promoting effects of cholesterol that is highly relevant to prostate cancer. Prostate tumors respond to circulating androgen through the action of the androgen receptor (AR), a nuclear receptor that drives prostate cancer cell proliferation and survival even under conditions of hormone suppression during late-stage disease^(144–146). Androgen Deprivation Therapy (ADT) remains the primary treatment strategy for advanced prostate cancer^(144,147). Despite widespread early responses to therapy, prostate cancer almost invariably becomes castration resistant and the tumor cells continue to grow despite circulating androgen at castrate levels^(144,145). In this phase of disease (termed CRPC), tumors become more aggressive. There are several theories about how this castration-resistant phenotype comes about: 1) gene amplification and/or mutation of the AR, allowing the receptor to be sensitive to low levels of androgen^(148–154); 2) residual androgen production from the adrenal glands⁽¹⁵⁵⁾; and 3) promiscuous receptor-ligand interactions^(150,156).

Although it was 30 years ago that Geller et al.⁽¹⁵⁷⁾ first reported that sufficient androgen to drive the AR remained in the prostate after ADT^(157,158), the significance of this observation was partially obscured because other work appeared to demonstrate that co-administration of the anti-androgen flutamide, along with castration, totally ablated DHT activity, suggesting that any residual disease is not driven by androgen, per se. Notably, this latter finding was based on an n=4 and employed an assay that did not detect DHT levels <300pg/g tissue⁽¹⁵⁹⁾. The combination of castration with an anti-androgen is commonly used to treat advanced prostate cancer. Indeed, results of a phase III trial using the CYP17 inhibitor, abiraterone, in men with CRPC showing a significant overall survival benefit, strongly suggesting that for many men, CRPC is still driven, at least in part, by androgen^(160–167). Several groups have presented data showing that prostate tumors in men receiving ADT contain androgen levels high enough to activate the AR^{(157,159–166,168–175)}.

Cholesterol and androgen synthesis

One major biological role of cholesterol is as the precursor for the synthesis of steroid hormones, including androgens. Enzymes and other proteins are essential in converting cholesterol into testosterone and dihydrotestosterone including STAR, obligatory for cholesterol transport into the mitochondria, CYP11A1 to convert cholesterol to pregnenolone, CYP17A & HSD3B to form androstenedione, and a 17β-hydroxysteroid dehydrogenase (e.g. AKR1C3 or HSD17B3) to convert androstenedione to testosterone.

Several lines of evidence indicate that prostate cancer cells in vivo synthesize their own androgens (i.e. de novo steroidogenesis), including in the castrate environment^{(173,176,177)}, in sufficient quantities to be biologically active. Locke et al.⁽¹⁷⁶⁾ demonstrated that all of the enzymes necessary for de novo androgen synthesis are expressed in LNCaP tumor xenografts, and that androgen-starved prostate cancer cells are capable of synthesizing DHT from acetic acid, suggesting that the entire pathway from acetate→cholesterol→DHT is intact in this model system⁽¹⁷⁶⁾. Montgomery et al. demonstrated that the full complement of enzymes comprising the steroidogenic pathways is present in the majority of human primary and metastatic prostate cancers examined⁽¹⁷³⁾, implying that de novo androgen synthesis is not merely an experimental phenomenon, but rather a potential underlying cause of disease progression in the CRPC phase of the disease.

The ability of prostate cancer cells to synthesize androgen from cholesterol may actually increase following hormone suppression because castration raises serum cholesterol levels^{(172,178–181)}, and because cholesterol synthesis increases in experimental prostatic tumors in the castrate condition⁽¹⁸²⁾. We have noted this increase in serum cholesterol in castrated mice. We find that hypercholesterolemia accelerates the growth of human prostate cancer xenografts following castration (Fig. 2). New data from our group (Mostaghel et al manuscript in preparation) suggest that hypercholesterolemia contributes to androgen synthesis in prostate tumors, even in the castrate environment. Thus, the combination of castration and a rise in circulating cholesterol may actually increase the risk of CRPC progression.

Figure 2. Tumor growth in castrated mice.

Castrated mice were fed isocaloric no cholesterol or high cholesterol diets for 80 days. LNCaP cells were subsequently injected subcutaneously and the animals continued on their respective diets for 22 days. (A) Tumor take. The numbers of tumor in each diet group were counted and the data are plotted as tumor take (% of all implantation sites) vs. Time (days). Significance was determined by logistic regression analysis. At all time points where tumors were present, the 2 cohorts differed significantly. P<0.05. n=40/group. (B) Longitudinal volume measurements. Tumors were measured at various time points by calipers starting at first appearance (day 1) and continued until tumor burden required the animals to be sacrificed (day 22). Data are plotted as tumor volume (mm³) per site vs. Time (days) ± SE. n=40/group.

In humans, there are two potential sources of intratumoral androgens in the castrate environment; they are either products of the adrenal glands (potentially as both precursors and as actual androgens), or they are created de novo through intratumoral steroidogenesis. After day 14.5 of embryogenesis⁽¹⁸³⁾, adrenal glands in the mouse begin to lose expression of 17 α-hydroxylase (CYP17) and express little to none of the enzyme into adulthood. CYP17 is an essential enzyme required to convert pregnenolone to androstenedione, and consequently the murine adrenals produce little to no androgen or androgen precursors (e.g. androstenediol)^(176,184). The lack of CYP17 in the adrenals of mice also explains why the major glucocorticoid present in the mouse is corticosterone instead of cortisol⁽¹⁸⁵⁾. In total, these data suggest, provocatively, that in the murine castrate environment prostatic tumors acquire the ability to synthesize T and DHT from intratumoral cholesterol, and do not require adrenal contributions. Whether this is also true in human patients remains to be established.

Cholesterol plays an essential role in steroidogenesis through its place as the precursor for all steroid hormones. However, we propose that cholesterol also plays an additional role as a steroidogenesis pathway agonist. Free cholesterol is cytotoxic and cells have multiple methods of reducing excess free cholesterol including: 1) upregulating the cholesterol efflux transporters, ABCA1, ABCG1, and ABCA7, 2) upregulating enzymes catalyzing the non-hepatic ‘acid’ pathway of bile acid synthesis, CYP27A1 and CYP7B1⁽¹⁸⁶⁾, 3) reducing expression of cholesterol receptors LDLR & SR-B1, 4) increasing cholesterol esterification through acyl-CoA cholesterol acyl transferase (ACAT), and 5) reducing cholesterol synthesis through regulation of HMG-CoA reductase expression. In addition, we propose that an additional mechanism for reducing the pool of free cholesterol is through the upregulation of the proteins/enzymes responsible for synthesizing androgens from cholesterol, i.e., de novo steroidogenesis within the tumor microenvironment. The relevance of these diverse mechanisms to prostate cancer remains to be tested in experimental models and in humans.

In summary, circulating cholesterol is a likely risk factor for aggressive prostate cancer. The multifaceted nature of the complex metabolic pathways in which cholesterol participates allows this lipid to play multiple roles in cancer progression. Further studies on the detailed mechanisms of cholesterol effects on prostate and other cancers are warranted, and could lead to new avenues for therapeutic intervention, particularly in controlling progression to late-stage disease.