**3. The balance of cholesterol and cancer**

Cholesterol accumulation in cancer cells and tumor tissues was discovered in cancer cells and tumor tissues started in earlier 1900s [12, 69, 70]. Since then, researchers have studied the relationship between cellular cholesterol and cancer in depth. Recent epidemiological studies suggest the correlation between serum cholesterol level and the risk of certain types of cancer [15, 71–74]. It is difficult to draw conclusions from epidemiological studies on whether cholesterol is a key factor of cancer incidence because of of their intrinsic limitations. On the other hand, experimental evidence from cell and animal models indicates that cholesterol plays a promotional role in cancer cell growth and cancer development and progression [57–60]. These findings support the notion that lowering cholesterol level may be a useful and effective strategy for cancer prevention and a therapeutic potential for cancer treatment.

#### **3.1. Lowering cholesterol level**

As described above, cholesterol homeostasis is controlled by its biosynthesis, catabolism, dietary absorption, transportation, and depletion [28–32]. Among these, cholesterol biosynthesis and absorption with low-density lipoprotein (LDL) receptor (LDLR) which mediates the endocytosis of cholesterol-rich LDL are key to elevate cellular cholesterol. By contrast, there are also two common avenues to achieve cholesterol lowering: (1) pharmacological treatment which inhibits cholesterol biosynthesis [41–45] and (2) dietary control that reduces cholesterol absorption [36, 75]. Meanwhile, cholesterol metabolite, 27-hydroxycholesterol, and other oxysterols can activate the liver X receptors (LXR), resulting in a reduction of intracellular cholesterol [76–78]. Modulation of LXR and their downstream targets has appeared to be involved in cholesterol and lipid metabolism in response to changes in cellular cholesterol status [76–78]. This also draws attention to the therapeutic interest of developing LXR agonists as a bona fide therapeutic approach in cancer treatment. The cross talk of LDLR-SREBP (sterol regulatory element-binding protein) signaling and LXR signaling in the regulation of cholesterol metabolism is potential as a new strategy to develop cancer therapeutic drugs and treatment regimen.

#### **3.2. Cholesterol-lowering drugs**

There are many different agents that can inhibit cholesterol biosynthesis at different enzymatic steps or reduce cholesterol level by different regulation pathways. **Table 1** summaries the targets and effects of different cholesterol-lowering agents. Statins, first marketed in 1987, are the most common drugs to lower cholesterol level. As structural analogues of HMG-CoA, statins inhibit HMG-CoA reductase to block the conversion of HMG-CoA to mevalonic acid in a rate-limiting step of cholesterol biosynthesis. Up to date, a number of different compounds in this class drugs have been developed: atorvastatin (Lipitor), cerivastatin (Baycol; withdrawn from the market in 2001), fluvastatin (Lescol), lovastatin (Mevacor), mevastatin (Compactin), pitavastatin (Livalo), pravastatin (Pravachol or Selektine), rosuvastatin (Crestor), and simvastatin (Zocor). They are effective for treating cardiovascular disease, atherosclerosis, dyslipoproteinemia, and liver disease [79–81] and are also recommended for those who do not meet their lipid-lowering goals through diet and lifestyle changes. Statins are also considered as an anticancer agent to prevent and treat cancer patients [42–44]. Because of multiple side effects of statins, such as muscle pain, increased risk of diabetes mellitus, and abnormalities in liver enzyme tests, many other enzymes that are involved in cholesterol biosynthetic pathway beyond HMG-CoA reductase are also being considered as targets for developing cholesterollowering drugs. These drugs include bisphosphonates which inhibit farnesyl-diphosphate synthase [82] and lonafarnib (SCH66366) and tipifarnib (R115777) which inhibit farnesyltransferase [83]. YM-53601, RPR-107393, and TAK-475 (Lapaquistat) can inhibit squalene synthase [84–86], and Ro 48-8071, BIBB515, and terbinafine (Lamisil) are potent inhibitors of 2,3 oxidosqualene cyclase or squalene epoxidase [87–89]. These agents are used in clinic and in clinic trials.

In addition, several another classes of compounds which can lower cholesterol level via different molecular mechanisms have recently been developed. Ezetimibe (Zetia), a cholesterol uptake-blocking drug, prevents cholesterol absorption from dietary intake [90]. Fibrate drugs (Gemfibrozil, Tricor, Atromid-S), an activator of peroxisome proliferator-activated receptor α (PPARα), can reduce very-low-density lipoprotein (VLDL) - and LDL-containing apoprotein B and increase HDL-containing apoprotein AI and AII [91, 92]. Cholestyramine, colestipol, and colesevelam, bile acid sequestrants, can remove bile acids from the body and further convert more plasma cholesterol to bile acids to reduce cholesterol level [93, 94]. Some other cholesterol-lowering agents are also on the market or available for research. Acyl-CoA:cholesteryl acyltransferase inhibitor (avasimibe or CI-1011) induces cholesterol 7-α-hydroxylase and increases bile acid synthesis [95]. Green tea or catechins can inhibit the intestinal absorption of dietary lipids [96]. Lomitapide (Juxtapid) inhibits the microsomal triglyceride transfer protein required for VLDL assembly and secretion [97]. Mipomersen is a second-generation antisense oligonucleotide targeted to human apolipoprotein B-100 which is the structural core of LDL cholesterol [98]. Anacetrapib is a novel inhibitor of cholesteryl ester transfer protein [99]. Evolocumab (AMG145) and alirocumab are monoclonal antibodies which inactivate the proprotein convertase subtilisin/kexin type 9 (PCSK9) and lower LDL level [100, 101]. Dynasore reduces labile cholesterol in the plasma membrane [102]. Some of these cholesterollowering drugs have demonstrated their anticancer property and have the potential of cancer pharmacological prevention [41–45].

absorption [36, 75]. Meanwhile, cholesterol metabolite, 27-hydroxycholesterol, and other oxysterols can activate the liver X receptors (LXR), resulting in a reduction of intracellular cholesterol [76–78]. Modulation of LXR and their downstream targets has appeared to be involved in cholesterol and lipid metabolism in response to changes in cellular cholesterol status [76–78]. This also draws attention to the therapeutic interest of developing LXR agonists as a bona fide therapeutic approach in cancer treatment. The cross talk of LDLR-SREBP (sterol regulatory element-binding protein) signaling and LXR signaling in the regulation of cholesterol metabolism is potential as a new strategy to develop cancer therapeutic drugs and

There are many different agents that can inhibit cholesterol biosynthesis at different enzymatic steps or reduce cholesterol level by different regulation pathways. **Table 1** summaries the targets and effects of different cholesterol-lowering agents. Statins, first marketed in 1987, are the most common drugs to lower cholesterol level. As structural analogues of HMG-CoA, statins inhibit HMG-CoA reductase to block the conversion of HMG-CoA to mevalonic acid in a rate-limiting step of cholesterol biosynthesis. Up to date, a number of different compounds in this class drugs have been developed: atorvastatin (Lipitor), cerivastatin (Baycol; withdrawn from the market in 2001), fluvastatin (Lescol), lovastatin (Mevacor), mevastatin (Compactin), pitavastatin (Livalo), pravastatin (Pravachol or Selektine), rosuvastatin (Crestor), and simvastatin (Zocor). They are effective for treating cardiovascular disease, atherosclerosis, dyslipoproteinemia, and liver disease [79–81] and are also recommended for those who do not meet their lipid-lowering goals through diet and lifestyle changes. Statins are also considered as an anticancer agent to prevent and treat cancer patients [42–44]. Because of multiple side effects of statins, such as muscle pain, increased risk of diabetes mellitus, and abnormalities in liver enzyme tests, many other enzymes that are involved in cholesterol biosynthetic pathway beyond HMG-CoA reductase are also being considered as targets for developing cholesterollowering drugs. These drugs include bisphosphonates which inhibit farnesyl-diphosphate synthase [82] and lonafarnib (SCH66366) and tipifarnib (R115777) which inhibit farnesyltransferase [83]. YM-53601, RPR-107393, and TAK-475 (Lapaquistat) can inhibit squalene synthase [84–86], and Ro 48-8071, BIBB515, and terbinafine (Lamisil) are potent inhibitors of 2,3 oxidosqualene cyclase or squalene epoxidase [87–89]. These agents are used in clinic and in

In addition, several another classes of compounds which can lower cholesterol level via different molecular mechanisms have recently been developed. Ezetimibe (Zetia), a cholesterol uptake-blocking drug, prevents cholesterol absorption from dietary intake [90]. Fibrate drugs (Gemfibrozil, Tricor, Atromid-S), an activator of peroxisome proliferator-activated receptor α (PPARα), can reduce very-low-density lipoprotein (VLDL) - and LDL-containing apoprotein B and increase HDL-containing apoprotein AI and AII [91, 92]. Cholestyramine, colestipol, and colesevelam, bile acid sequestrants, can remove bile acids from the body and further convert more plasma cholesterol to bile acids to reduce cholesterol level [93, 94]. Some other cholesterol-lowering agents are also on the market or available for research. Acyl-CoA:cholesteryl

treatment regimen.

clinic trials.

**3.2. Cholesterol-lowering drugs**

112 Cholesterol Lowering Therapies and Drugs


**Table 1.** Targets and effects of different cholesterol-lowering agents.

#### **3.3. Anticancer property of cholesterol-lowering drugs**

Accumulating evidence supports that deregulation of any steps in cell growth, proliferation, and migration may result in cell malignant transformation. More than a century ago, cholesterol was observed to accumulate in malignant tissues [69]. Now, more and more evidence shows that cholesterol plays a critical role in the regulation of cancer cell growth and proliferation and tumor progression [8, 10–18, 70]. The key regulators in cholesterol metabolism attract many researchers around the world to search for novel anticancer agents. Based on cholesterol biofunctions and experimental data, the role of cholesterol-lowering drugs may not limit on the property of LDL-cholesterol lowering but may also be involved in the prevention or treatment of cancer. Statins are the most common cholesterol-lowering drugs and are also the most studied drugs. Whether statins exhibit anticancer properties is based on experimental studies, epidemiological studies, and clinical studies. In experimental studies, statins reduce a variety of cancer cell viability (**Figure 3**) [75, 103–105]. The epidemiologic data also support that statins reduce the incidence of gastric cancer, breast cancer, advanced prostate cancer, colorectal cancer, and cholangiocarcinoma [105–109]. However, there are also some studies that do not support the association of statin use with cancer risk [110, 111]. In clinical studies, statins can significantly reduce prostate cancer-specific mortality and reduce the risk of biochemical recurrence among the patients treated with radiation therapy [112] and are also associated with improved survival in patients with metastatic renal cell carcinoma [113]. So far, statins show some promising results in certain types of cancer. The potential of statins in modern cancer prevention and treatment is very promising. Meanwhile, it is also important to search other cholesterol-lowering agents that are more effective and reduce adverse side effects. Some of these agents have already been studied at the different stages [89, 114].

**Figure 3.** Treatment of lovastatin reduces cell viability in different cancer cell lines. Different cancer cells were cultured in 96-well plates and treated with 10 μM lovastatin for 3 days; the samples analyzed cell viability by MTT assay (n = 16). The values of lovastatin treatment were statistically different from the controls. *P* < 0.05. M231, MDA-MB-231.

#### **3.4. Molecular mechanism of anticancer properties of cholesterol-lowering drugs**

**3.3. Anticancer property of cholesterol-lowering drugs**

114 Cholesterol Lowering Therapies and Drugs

Accumulating evidence supports that deregulation of any steps in cell growth, proliferation, and migration may result in cell malignant transformation. More than a century ago, cholesterol was observed to accumulate in malignant tissues [69]. Now, more and more evidence shows that cholesterol plays a critical role in the regulation of cancer cell growth and proliferation and tumor progression [8, 10–18, 70]. The key regulators in cholesterol metabolism attract many researchers around the world to search for novel anticancer agents. Based on cholesterol biofunctions and experimental data, the role of cholesterol-lowering drugs may not limit on the property of LDL-cholesterol lowering but may also be involved in the prevention or treatment of cancer. Statins are the most common cholesterol-lowering drugs and are also the most studied drugs. Whether statins exhibit anticancer properties is based on experimental studies, epidemiological studies, and clinical studies. In experimental studies, statins reduce a variety of cancer cell viability (**Figure 3**) [75, 103–105]. The epidemiologic data also support that statins reduce the incidence of gastric cancer, breast cancer, advanced prostate cancer, colorectal cancer, and cholangiocarcinoma [105–109]. However, there are also some studies that do not support the association of statin use with cancer risk [110, 111]. In clinical studies, statins can significantly reduce prostate cancer-specific mortality and reduce the risk of biochemical recurrence among the patients treated with radiation therapy [112] and are also associated with improved survival in patients with metastatic renal cell carcinoma [113]. So far, statins show some promising results in certain types of cancer. The potential of statins in modern cancer prevention and treatment is very promising. Meanwhile, it is also important to search other cholesterol-lowering agents that are more effective and reduce adverse side effects. Some of these agents have already been studied at the different stages [89, 114].

**Figure 3.** Treatment of lovastatin reduces cell viability in different cancer cell lines. Different cancer cells were cultured in 96-well plates and treated with 10 μM lovastatin for 3 days; the samples analyzed cell viability by MTT assay (n = 16). The values of lovastatin treatment were statistically different from the controls. *P* < 0.05. M231, MDA-MB-231.

Expression of HMG-CoA reductase gene can be regulated by genetic or dietary interaction [115], in which it is transcriptionally regulated by endoplasmic reticulum-based transcription factor, SREBP-2 [116], or high-fat diet feeding [117]. Statins inhibit HMG-CoA reductase to block cholesterol biosynthesis which attenuate cell proliferation and arrest cell cycle progression by interrupting growth-promoting signals and involving in RAS/RAF/MEK/ERK, PI3K/AKT/mTOR and Wnt/β-catenin signaling cascades [118, 119]. Statins also selectively induce proapoptotic potential in tumor cells and synergistically enhance proapoptotic potential of several cytotoxic agents. The mechanism for this effect has been demonstrated by disrupted binding of RhoA inhibitor GDIα which leads to increased levels of GTP-bound forms of RhoA, Rac1, and cdc42 proteins.These proteins induce apoptosis 1) by suppression of antiapoptotic proteins such as Bcl2 or activation of the superoxide-activated JNK pathway [120] or 2) by inhibiting Akt/mTOR pathway and inducing programmed cell death 4 expression in renal cell cancer cells [121]. Statins alter the angiogenic potential of cells by modulating apoptosis inhibitory effects of VEGF and decrease secretion of metalloproteases and suppress the rate of activation of multiple coagulation factors and thus prevent coagulation-mediated angiogenesis [122]. Statins suppress the Rho/Rho-associated coiled-coil-containing protein kinase pathways, thereby inhibiting cell migration, invasion, adhesion, and metastasis [123]. Other cholesterol-lowering agents have not been widely studied as statins. However, all cholesterol-lowering agents could affect membrane composition, in particular cholesterol-rich domain, termed lipid rafts. Membrane lipid rafts are highly ordered membrane domains that are enriched in cholesterol, sphingolipids, and gangliosides and selectively recruit certain classes of proteins (a large number of cancer-related signaling and adhesion molecules) and act as major modulators of membrane geometry, lateral movement of molecules, and traffic and signal transduction [52, 54]. Cholesterol-lowering drugs lead to membrane cholesterol depletion which could disrupt membrane lipid rafts, block the adhesion and migration processes of cancer cells, and induce cancer cell apoptosis [124, 125].

#### **4. Cholesterol-lowering drugs in cancer prevention and therapy**

A growing body of evidence from cell biology and animal models has strongly demonstrated the anticancer activity of cholesterol-lowering drugs such as statins [7, 83–89, 104–108]. Epidemiological studies also suggest an anticancer effect of statins evidenced by the reductions of cancer incidence and cancer-related mortality, although the association between statin use and cancer incidence based on different cancer remains controversial from different laboratories around the world. Statins as part of pharmacological cancer prevention and chemotherapy have generated interest in the oncology community and have been investigated in a variety of cancers at early and late stages and in the combination with chemotherapy and radiation therapy. Here, we summarize the current data that statin use affects cancer incidence and therapy.


**Table 2.** Effect of statins on cancer incidence.

#### **4.1. Cholesterol-lowering drugs in cancer prevention**

Cholesterol is accumulated in different solid tumors and cancer cells [12, 69–71, 126, 127], raising questions concerning the role of cholesterol in cancer cell growth, proliferation, and migration as well as tumor progression [57–61]. Although cholesterol-lowering drugs have also been shown to possess an important antitumor activity that reduces cell growth, proliferation, and migration through ERK-mediated and Akt-mediated signaling pathways and is capable of inducing apoptosis through extrinsic and intrinsic pathways using different cancer cells as models [43–45, 75, 78, 104, 118–123], it is still unclear whether statins are suitable to prevent the incidence of cancer. More than a hundred of epidemiological studies around the world have been performed to evaluate the effect of statin on the risk of cancer incidence [105, 108, 109, 126–142]. These studies have been focused on statin type, potency, lipophilic or hydrophobicity status, and duration of use. Due to the limitation of epidemiological studies with the patients different in age, sex, living regions, and life style, the results are controversial. **Table 2** summarizes the association of cancer risk and statin use in pancreatic cancer, gastric cancer, liver cancer, lung cancer, bladder cancer, breast cancer, prostate cancer, colorectal cancer, blood cancer, and other malignancies. The clinical studies have provided conflicting data regarding whether statins may reduce or may be no effect on the risk of cancer. It is clear that current data cannot rule out the association of statin use with the risk of some cancers. Analyses of larger numbers of cases, subgroup design (participant ethnicity or confounder adjustment), randomized controlled trials, and high-quality cohort studies with longer duration of follow-up are needed to further confirm this association. Meanwhile, we also need to study cancer patient genetic mutations and determine whether the effect of statins on cancer prevention and therapy is associated with genetic mutation. It is clear that defining the underlying mechanisms of how cholesterol lowering contributes to cancer prevention and the search for other cholesterol-lowering agents with better outcome has emerged as future objectives. Whether cholesterol-lowering agents are used in cancer prevention will be based on the analysis of responses to these agents with cancer patient genetic information.

#### **4.2. Cholesterol-lowering drugs in cancer treatment**

**Study No. of subjects/ studies**

116 Cholesterol Lowering Therapies and Drugs

**Results References**

[139]

[141]

[142]

Bonovas, 2008 12 studies No significant relationship between statins and pancreatic cancer risk [129] Khurana, 2007 483,733 Protective against the development of pancreatic cancer [130] Lin, 2016 19,727 Prevent *H. pylori*-associated gastric cancer [105] Singh, 2013 11 studies Prevent gastric cancer risk in both Asian and Western population [131] Tsan, 2012 33,413 Reduce the risk for hepatocellular carcinoma in HBV-infected patients [132] Chen, 2015 2,053 Decrease hepatocellular carcinoma in diabetic patients [133] Zhang, 2013 13 studies No association between statin use and risk of bladder cancer [134] Peng, 2015 3,174 Reduce the risk of cholangiocarcinoma [108] Yi, 2014 20 studies Preventive effects against hematological malignancies [135] Pradelli, 2015 14 studies Negatively associated with all hematological malignancies [136] Wang, 2013 20 studies Nonsignificant association between statin users and lung cancer risk [137] Bansal, 2012 27 studies Reduce the risk of total and advanced prostate cancer [138] Jacobs, 2007 55,454 Reduce the risk of advanced prostate cancer [109]

Undela, 2012 24 studies Do not support that statins have a protective effect against breast

Setoguchi, 2007 24,439 No effect in the risk of colorectal, lung, or breast cancer in older

Kuoppala, 2008 42 studies No effect on the incidence of lung, breast, or prostate cancer

Lytras, 2014 40 studies Do not support that statin users reduce the risk of colorectal cancer [140]

Cholesterol is accumulated in different solid tumors and cancer cells [12, 69–71, 126, 127], raising questions concerning the role of cholesterol in cancer cell growth, proliferation, and migration as well as tumor progression [57–61]. Although cholesterol-lowering drugs have also been shown to possess an important antitumor activity that reduces cell growth, proliferation, and migration through ERK-mediated and Akt-mediated signaling pathways and is capable of inducing apoptosis through extrinsic and intrinsic pathways using different cancer cells as models [43–45, 75, 78, 104, 118–123], it is still unclear whether statins are suitable to prevent the incidence of cancer. More than a hundred of epidemiological studies around the world have been performed to evaluate the effect of statin on the risk of cancer incidence [105, 108, 109, 126–142]. These studies have been focused on statin type, potency, lipophilic or hydrophobicity status, and duration of use. Due to the limitation of epidemiological studies with the patients different in age, sex, living regions, and life style, the results are controversial. **Table 2** summarizes the association of cancer risk and statin use in pancreatic cancer, gastric cancer, liver cancer, lung cancer, bladder cancer, breast cancer, prostate cancer, colorectal cancer, blood cancer, and other malignancies. The clinical studies have provided conflicting

Protect from stomach and liver cancer and from lymphoma Increase the incidence of both melanoma and nonmelanoma skin

cancer

patients

cancer

**4.1. Cholesterol-lowering drugs in cancer prevention**

**Table 2.** Effect of statins on cancer incidence.

Cholesterol is implicated in various cellular processes including the involvement of cell proliferation/apoptosis balance regulation in various types of cancers. Statins and other cholesterol-lowering agents are very common and effective medication used in preventing heart disease in those with high cholesterol, but no history of heart disease. The anticancer activity of these drugs has also attracted oncologists to consider whether cholesterol-lowering drugs can be a tool for cancer treatment. A variety of studies have focused on the effect of statins alone or in combination with other chemo- or/and immune-therapeutic drugs or radiation therapy on the treatment of different cancer patients. McKay et al. [113] showed some promising data that statin use improved survival in patients with metastatic renal cell carcinoma. Raval et al. found that statin significantly reduced the prostate cancer-specific mortality and improved the biochemical recurrence in certain subgroup of men with prostate cancer [112]. Song et al. found that statin use also reduces biochemical recurrence in men with prostate cancer after radical prostatectomy [143]. Statin use is related to reductions in overall and cancer-specific mortality [144] and associated with longer rates of survival [145] in colorectal cancer survivors. Two recent studies indicate that statin use is associated with improved overall survival in patients with resectable pancreatic ductal adenocarcinoma [146, 147]. Statin use also improves overall survival among patients undergoing resection for pancreatic cancer [148]. Lipophilic statins are associated with a reduced risk of breast cancer recurrence and inflammatory breast cancer [149]. Because statins negatively interfere with CD-20 and rituximab-mediated activity, statins have a negatively effect on clinical outcome in patients with rituximab-treated leukemia [150]. No association of statin use with patient survivals was also reported from colorectal cancer study [151]. Future studies are needed to further evaluate which cancer patients may benefit from statin treatment, what the best treatment is, and which cholesterol-lowering drugs are better to use in cancer treatment.

#### **5. Concluding remarks and future perspectives**

Cholesterol is tightly regulated by a physiological balance of cholesterol metabolism (biosynthesis and degradation), dietary absorption, transportation (efflux and influx), and depletion. Importantly, cholesterol is accumulated in cancer cells and tumor tissues and is implicated in various cellular processes including cell growth, proliferation, and migration. The increase and decrease incellular andcirculatingcholesterollevelshavedemonstratedthe involvement of cell proliferation/apoptosis balance regulation. This chapter reviewed our current understanding of how cholesterol metabolism contributes to cancer development and progression and cholesterol-lowering drugs may be associated with the therapeutic potential of cancer prevention and treatment. Current evidence cannot exclude the relevance of cancerrisk with statin use as seen in a variety of studies. Whether the genetic mutations of cancer patients are associated with the response of statins is also unknown. It is clear that more studies are needed to better characterize potential statin-mediated mechanisms that prevent cancerincidence. On the other hand, statins alone or used in combination with certain anticancer drugs or radiation therapy can improve survival in patients with several different tumors. Further research using large cohort studies in different cancers is needed to clarify these issues. In addition, searching for novel classes of cholesterol-loweringdrugs withmore effects andless side effects couldprovide new therapeutic options for cancer prevention and therapy.
