**9. ABC transporters and cholesterol homeostasis**

In many neoplastic diseases, increased expression of proteins on which depends the multidrug resistance correlates with the presence of refractory disease. A small proportion of AML leukemia cells is responsible for the tumor proliferation and expansion. These are leukemic stem cells, primitive cells, which are frequently in a quiescent state. When they leave the quiescent state and progress along the cell cycle, these cells are characterized by the ability of self-renewal and express some ATP-binding cassette (ABC) transporters. It was observed that when some ABC transporters have a high expression in leukemia cells, the prognosis of patients with AML is reserved as the response to treatment is inadequate. (de Jonge-Peeters et al., 2007)

The most studied transporter is the P-glycoprotein transporter (P-gp) - an ABC transporter responsible for unidirectional transmembrane translocation of the substrate (Gayet et al., 2005). P-gp is frequently involved in the emergence of multidrug resistance during chemotherapy (Shu & Liu, 2007). The multidrug resistance gene encodes this membrane transporter. Not only P-gp occurs in CH homeostasis at the cellular level, but also the synthesis of CH and CH-esters affects ATP-ase (Bucher et al., 2007) and the transmembrane transport by P-gp (Bucher et al., 2007; Shu & Liu, 2007). The lipid structure of the cell membranes also depends on the P-gp function (Dos Santos et al., 2007).

The ATP-ase activity of P-gp is controlled linearly by CH of the membrane structure. On the other hand, the decrease of membrane CH correlates with the non-linear decrease of the daunorubicin efflux induced by P-gp. An effective way to raise awareness of ALL CEM resistant to chemotherapy cells consists of partial depletion of the CH from cell membrane structure, that lowers the daunorubicin efflux by P-gp. (Gayet et al., 2005)

CH is able to increase basal activity of P-gp ATP-ase and increase P-gp sensitivity to progesterone and verapamil, modulators of this transporter. (Bucher et al., 2007) LDLcholesterol can enlarge P-gp expression. In an experiment conducted in vitro, HMG-CoA reductase inhibitors were added to a primitive leukemia cells line (KG1a) and the observation was that lovastatin caused a decrease of 26% of P-gp expression, and pravastatin - a decrease of 16 %. (Connelly-Smith et al., 2007) But the CH derived from LDL was also able to restore sensitivity to chemotherapy of a human lymphoblastic leukemia cell line. It seems that the mechanism explaining this return is the restoration of the membrane CH and the reducing of the P-gp-associated ATPase to the same level. (Shu & Liu, 2007) The changes of the membrane CH quantity may be responsible for P-gp inhibition. It was observed that disassembly of lipid rafts can be produced both by the decrease of the CH content and by its increase. For a normal capacity of P-gp transport it is necessary to maintain accurate properties of membrane structures known as lipid raft. (Dos Santos et al., 2007)

In a clinical trial involving patients with CLL the P-gp expression of lymphocytes from peripheral blood was determined flowcytometricaly. Those patients whose lymphocytes expressed P-gp were treated for 6 days with 80 mg lovastatin daily, then a new sample of peripheral blood was examined flowcytometricaly. Lymphocytes of six of the 27 studied

and long-lasting lymphocyte subpopulation, which can infiltrate the atheromatous plaques, contributing to the their destabilization, which facilitates the instalation of major coronary

In many neoplastic diseases, increased expression of proteins on which depends the multidrug resistance correlates with the presence of refractory disease. A small proportion of AML leukemia cells is responsible for the tumor proliferation and expansion. These are leukemic stem cells, primitive cells, which are frequently in a quiescent state. When they leave the quiescent state and progress along the cell cycle, these cells are characterized by the ability of self-renewal and express some ATP-binding cassette (ABC) transporters. It was observed that when some ABC transporters have a high expression in leukemia cells, the prognosis of patients with AML is reserved as the response to treatment is inadequate. (de

The most studied transporter is the P-glycoprotein transporter (P-gp) - an ABC transporter responsible for unidirectional transmembrane translocation of the substrate (Gayet et al., 2005). P-gp is frequently involved in the emergence of multidrug resistance during chemotherapy (Shu & Liu, 2007). The multidrug resistance gene encodes this membrane transporter. Not only P-gp occurs in CH homeostasis at the cellular level, but also the synthesis of CH and CH-esters affects ATP-ase (Bucher et al., 2007) and the transmembrane transport by P-gp (Bucher et al., 2007; Shu & Liu, 2007). The lipid structure of the cell

The ATP-ase activity of P-gp is controlled linearly by CH of the membrane structure. On the other hand, the decrease of membrane CH correlates with the non-linear decrease of the daunorubicin efflux induced by P-gp. An effective way to raise awareness of ALL CEM resistant to chemotherapy cells consists of partial depletion of the CH from cell membrane

CH is able to increase basal activity of P-gp ATP-ase and increase P-gp sensitivity to progesterone and verapamil, modulators of this transporter. (Bucher et al., 2007) LDLcholesterol can enlarge P-gp expression. In an experiment conducted in vitro, HMG-CoA reductase inhibitors were added to a primitive leukemia cells line (KG1a) and the observation was that lovastatin caused a decrease of 26% of P-gp expression, and pravastatin - a decrease of 16 %. (Connelly-Smith et al., 2007) But the CH derived from LDL was also able to restore sensitivity to chemotherapy of a human lymphoblastic leukemia cell line. It seems that the mechanism explaining this return is the restoration of the membrane CH and the reducing of the P-gp-associated ATPase to the same level. (Shu & Liu, 2007) The changes of the membrane CH quantity may be responsible for P-gp inhibition. It was observed that disassembly of lipid rafts can be produced both by the decrease of the CH content and by its increase. For a normal capacity of P-gp transport it is necessary to maintain accurate properties of membrane structures known as lipid raft.

In a clinical trial involving patients with CLL the P-gp expression of lymphocytes from peripheral blood was determined flowcytometricaly. Those patients whose lymphocytes expressed P-gp were treated for 6 days with 80 mg lovastatin daily, then a new sample of peripheral blood was examined flowcytometricaly. Lymphocytes of six of the 27 studied

membranes also depends on the P-gp function (Dos Santos et al., 2007).

structure, that lowers the daunorubicin efflux by P-gp. (Gayet et al., 2005)

accidents. (Link et al., 2011)

Jonge-Peeters et al., 2007)

(Dos Santos et al., 2007)

**9. ABC transporters and cholesterol homeostasis** 

patients expressed P-gp; about 20% of them were positive. Following the administration of lovastatin only 7.33% of them also expressed P-gp (p = 0.016). Compared to the proportion of positive lymphocytes at baseline, the decline was of 63.35%. During the study, CH decreased statistically significantly, with 20.43%. There was no observation of the appearance of possible drug adverse effects. In conclusion, the 6 days therapy with lovastatin was able to reduce significantly the CH and the number of lymphocytes in the membranes where P-gp is expressed, so that this statin could contribute through its pleiotropic effects to reduce multidrug resistance, especially when it is followed by chemotherapy. (Mihăilă et al., 2010)

This drug efflux pump can be inhibited by verapamil, too, the research made in vitro proved that it can reduce multidrug resistance. In such a study conducted in two patients with leukemic lymphoma resistant to treatment, verapamil was able to increase the intracellular amount of doxorubicin (Tidefelt et al., 1994).

It was found that verapamil was able to overcome the P-gp - mediated resistance to doxorubicin and vincristine in a canine cell line of B cell lymphoma (GL-1) (Uozurmi et al., 2005, as cited in Mihăilă et al., 2008). These experimental findings were not confirmed by parallel administration of chemosensitizer verapamil to chemotherapy (cyclophosphamide, doxorubicin, vincristine, and dexamethasone) in patients with medium and high level NHL found in advanced stages. This drug combination did not increase the therapeutic response and did not extend the survival in these patients as compared to those treated only with the mentioned chemotherapy (without verapamil) (Gaynor et al., 2001, as cited in Mihăilă et al., 2008), but it cannot be excluded that it could be effective in some patients who develop multidrug resistance. But, in metastatic breast carcinoma that has become resistant to anthracyclines verapamil has showed that it is able to increase the survival of patients (Belpomme et al., 2000, as cited in Mihăilă et al., 2008). In the case of the patient described above with hypertension and primary mammary NHL, the evolution has been favorable under the combination of verapamil to chemo- and radiotherapy, that has resulted in a event-free survival of 6 years (up to the present day) (Mihăilă et al., 2008).

In another study, 45 patients with proliferative haematological disorders were included in one of the following two groups: A - those who had hypertension and who received verapamil + chemotherapy, and B - those with normal blood pressure, who received only chemotherapy. Group A included 7 patients with chronic lymphoproliferations and 2 with chronic myleoproliferations; under treatment, both systolic and diastolic pressure decreased significantly in all patients in the group. Initially the serum CH level was higher than in the patients in group B (p = 0.004), while other biological tests did not vary significantly between group A and B. No adverse effects were observed during the study. The fact that the initial blood CH was higher in in patients in group A suggests that malignant cells of patients in group B captured more blood CH that contributed to their proliferation. Although the average survival was not significantly different between group A and B, in group A there were more patients with stable disease (77.78% versus 44.44% - p<0.0001) while in group B more patients had progressive disease (30.56 % versus 11.11% - p<0.0001). The deaths due to progressive disease were significantly more numerous in group B. The authors consider that verapamil is useful not only because of its antihypertensive effect, but also due to its pleiotropic effects through which it could influence the evolution of neoplasia by inhibiting P-gp function and the efflux of drugs from lymphoma or leukemia cells. (Mihăilă et al., 2008)

Cholesterol and Triglycerides Metabolism Disorder in Malignant Hemopathies 403

By their pleiotropic effects, statins are useful not only for dyslipidemia treatment, but also in

I am deeply grateful to my teacher - Melinda Erzse - who was kind to check the English text

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This drug efflux pump can be inhibited by statins and verapamil.

antineoplastic therapy.

**11. Acknowledgment** 

**12. References** 

and make the appropriate corrections.

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Vol.4, No.3, (July-September 2009), pp.501-506.
