In this study, we have extended our initial findings with haloperidol and clozapine by demonstrating that antipsychotic drugs in general enhance the expression of SREBP-controlled lipid biosynthetic genes in a concentration-dependent manner in cultured glial cells, with less pronounced stimulation in neuroblastoma- and cortical neuronal cells. The degree of SREBP activation, however, varies considerably between the different antipsychotics.
The mechanisms by which the antipsychotic drugs activate the SREBP system could, in principle, involve receptor-dependent and -independent processes. All antipsychotic drugs block dopamine D2-like receptors (DRD2, DRD3 and DRD4), but with marked differences in their affinities [21
]. In addition, they also vary widely in their antagonistic binding to other neurotransmitter receptors, e.g., 5-HT and histamine H1 receptors [22
]. In the present dose-response comparison, haloperidol and clozapine were the most potent activators of SREBP-controlled gene expression on the basis of their molar concentrations in the cell cultures, whereas risperidone and ziprasidone were minimally effective. Chlorpromazine and olanzapine had intermediate effects. These data imply that the SREBP activation is neither linked to the receptor profiles nor the therapeutic classification (typical vs. atypical) of the drugs, since there is no apparent relationship between these properties and their SREBP-stimulating effect.
This assumption is supported by a recent study [23
], in which we demonstrated that several antidepressant drugs (especially tricyclic antidepressants), but not mood stabilizers (carbamazepine and valproate), activate the SREBP system in cultured glial cells in a similar manner to the antipsychotics. This fact makes is likely that the drug-induced SREBP activation is related to some shared chemical property of these psychotropic compounds. Due to the different receptor-binding properties of these drugs it is likely that their shared SREBP-activating effect is mediated via a receptor-independent mechanism of action. All of the antipsychotics and antidepressants that we have investigated are cationic amphiphiles. Such substances have previously been shown to increase the synthesis and accumulation of total cellular cholesterol levels via a mechanism involving a reduction of cholesterol levels in the endoplasmic reticulum (ER), which is the sterol-sensing compartment in the cell [20
]. SREBP activation is controlled by the sterol-sensitive SREBP-cleavage-activating-protein (SCAP) that is located in the ER [25
]. When ER-cholesterol is depleted, SCAP undergoes a conformational change that promotes translocation of the SREBP/SCAP complex to the Golgi, representing the inital step in SREBP activation. Both cholesterol and a hydroxylated derivative of cholesterol, 25-OH-cholesterol (25-HC), inhibit the SREBP system by affecting the function of the SCAP protein [26
]. It is therefore interesting to note that clozapine was able to counteract the SREBP-repressing effect of 25-HC, which indicates that the effect of antipsychotic drugs on SCAP/SREBP activation is the opposite of cholesterol and 25-HC. This contrasts in vitro
data from a recent study demonstrating that some cationic amphiphiles (including clozapine, haloperidol and chlorpromazine) mimic the effect of cholesterol on SCAP conformation [27
]. However, in line with our results, this study could not reproduce the cholesterol-mimicking effect of cationic amphiphiles when tested in cultured cells [27
We propose that drug-induced transcriptional activation of cholesterol biosynthesis in the brain could represent a new mechanism of action for some psychotropic drugs. Interestingly, the SREBP-stimulating effect is shared by several different antipsychotics and antidepressants, demonstrating a common molecular mode of action. This effect cannot be related to the group-specific antipsychotic- or antidepressant properties of the drugs. Instead, it might be linked to some common symptoms or deficits that are present in both schizophrenia and major affective disorders (e.g., cognitive dysfunctions). This possibility is further underscored by the therapeutic breadth of these classes of drugs, with antipsychotic drugs frequently used in treatment of depressed patients and vice versa.
Involvement of myelin- and oligodendrocyte (glial) abnormalities has been indicated in the etiology of both schizophrenia and bipolar disorder [1
]. The myelination process requires intact cholesterol biosynthesis [6
], and during CNS development, the expression of cholesterol biosynthesis genes is enhanced [29
]. Furthermore, malfunction of synaptic processes, including reduced dendritic spine density, has been proposed in the pathophysiology of schizophrenia [30
]. Cholesterol, together with ApoE, can act as a glia-derived growth factor supporting the formation of synapses in culture [7
]. In line with this, cholesterol has recently been demonstrated as essential for dendrite maturation, the rate-limiting step in glia-induced synaptogenesis, and is required for continuous synapse development in cultures [34
]. Interestingly, drug-induced SREBP activation was most evident in the glial-like GaMg glioma- and CCF-STTG1 astrocytoma cell lines, and less pronounced in the HCN2 cortical neuron cells and in a primary cell culture from rat hippocampus. Lipogenic activation was almost absent in SH-SY5Y neuroblastoma cells. These data are in agreement with the fact that in the CNS, the majority of cholesterol is produced de novo
by glial cells [6
]. Cultured neurons have a poorly developed machinery for cholesterol biosynthesis and rely on glia-derived cholesterol for synaptogenesis [35
]. It is thus conceivable that drug-induced SREBP-controlled activation of glial cholesterol biosynthesis in the brain represents a receptor-independent therapeutic mechanism of action and provides essential building blocks to the myelination process or synaptogenesis.
A critical issue is whether the SREBP-stimulating effect observed in cultured cells also occurs during drug treatment in the clinical setting. Although any interpretation of the clinical relevance of cell culture data is difficult, the relationship between in vitro
and in vivo
(therapeutically relevant) concentrations of the drugs requires special attention. The investigated drugs are clinically efficacious at highly different ranges of serum concentrations [36
]. In an attempt to highlight the possible clinical impact of their SREBP-activation, we transformed the molar concentrations of each drug (as used in the GaMg cell culture experiments) into multiples of the therapeutically relevant upper serum concentrations of the corresponding drug (as determined from the AGNP-TDM expert group consensus guidelines) [36
]. By this approach, clozapine and chlorpromazine appeared to significantly activate the SREBP system in the cultured cells at concentrations that were 5–10-fold above their therapeutic serum levels. In contrast, the concentration of haloperidol necessary to induce this lipogenic effect in the cultured cells was about 200 times higher than the clinically relevant serum level, and ziprasidone hardly activated the SREBP pathway at all. It is important to bear in mind that many psychotropic drugs are highly lipophilic, leading to enrichment in lipid-rich tissues. Levels of haloperidol and clozapine have been demonstrated to be 10–30 times higher in the CNS compared to the corresponding serum concentration [37
]. Taken together, these data indicate that SREBP-activation by clozapine and chlorpromazine might occur in the brain when serum levels are within their therapeutically relevant range, whereas the SREBP activation observed for haloperidol would be expected to be of minor clinical relevance. Interestingly, clozapine has been described as a drug with superior therapeutic efficacy [39
], with positive effects in otherwise treatment-refractory patients. However, a similar supremacy has not been shown for chlorpromazine, the other apparent potent SREBP-activating drug. Finally, it is uncertain how large an in vivo
response would need to be in order to be of any clinical relevance, since even minor changes in cellular lipid biosynthesis in vivo
could have marked clinical effects. Caution should always be taken when making inferences from cell cultures to the clinical setting, and further studies are indeed warranted to draw more reliable conclusions about the clinical impact of our results.