Post by Max on Jun 24, 2005 11:29:31 GMT -5
(Ro)accutane and formation of beta-amyloid, a possible role in the pathology of alzheimers disease, elderly cognitive decline and dementia
Introduction
In the studies of (Ro)accutane induced degenerative pathologies, a distinction can be made between:
1) a suggested direct degenerative process during (Ro)accutane exposure as seen in mice [3 and 4]
2) a suggested possible slowly continous degenerative process post (Ro)accutane exposure and a possible role of retinoids in Alzheimer´s disease [2, 27 and more]
Retinoids are suggested to be of importance for the maintenance of the central nervous system, and a loss of retinoid signaling may contribute to the pathogenesis of Alzheimers disease [2 and more]. After 1 year of a vitamin-A dietary deficiency in adult rats, there was a deposition of amyloid beta in the cerebral blood vessels, suggesting a clear role of vitamin-A in the formation of beta amyloid. A downregulation of retinoic acid receptor alpha in the forebrain neurons of the retinoid-deficient rats and a loss of choline acetyl transferase expression, which precedes amyloid beta deposition was found. In neocortex of pathology samples of patients with Alzheimer's disease, the same retinoic acid receptor alpha deficit in the surviving neurons was observed [2]. Abnormal retinoid metabolism may be involved in the downstream transcriptional regulation of phospholipase A2-mediated signal transduction in Alzheimers disease (AD) [27].
Suggested significant loss of hippocampal cells during exposure
Results from (Ro)accutane exposure in adult mice are suggesting a significantly decreased hippocampal cell survival, and loss of cells during exposure [3 and 4]. Little is known about long-term effects of a (Ro)accutane exposure in humans. One of the principal theories of the pathology of Alzheimers disease is an increased formation of beta-amyloid [5 and more]. AD (Alzheimer's disease) is a neurodegenerative disease associated with progressive memory loss and leading to dementia. Except for cell-loss, two histological characteristics are observed in AD patients after autopsy: extracellular plaques and intracellular tangles [17]. Several enzymes and receptors of importance for the formation of beta-amyloid are distinctively altered in association with exposure of retinoic acid [6, 7 and more], as well as receptors, such as megalin, that likely are involved in the clearance of beta amyloid [26 and more], as well as disruption of a number of signaling pathways, that are suggested to be involved in the pathology of Alzheimers disease [27 and more].
Formation of beta amyloid from APP
The principal component of amyloid fibrils is beta/A4 amyloid protein, which can be generated by the aberrant processing of a large membrane-bound glycoprotein, the beta/A4 amyloid protein precursor (APP)3 [1]. The amyloid beta-peptide (approximately 4 kDa-M(r)) is generated by the proteolytic cleavage of a larger beta-amyloid precursor protein (beta APP) encoded by a gene on chromosome 21 [9].
(Ro)accutane and disruptions in G-protein signaling; possible decreases in RGS protein expression
RGS proteins
A statistically significant downregulation of RGS4 was found in post-mortem examinations of brain samples from patients with Alzheimers Disease [31]. RGS proteins have been shown to control essential neurological functions by modulating G(i) and G(q) mediated signaling. More than 20 regulators of G-protein signaling, RGS proteins, have been identified, of which five are found to be expressed highly in the brain (RGS4, RGS7, RGS8, RGS11 and RGS17). During the G-protein activity cycle RGS reduces the GPCR signaling by accelerating the rate of GTP hydrolysis. RGS interacts specifically with the alpha subunit of the G-protein which leads to dimerization and an inactive heterotrimeric G-protein [31].
Retinoids and G-protein signaling
Significant interference between retinoids and G-protein signaling is well known. In the opthamologic field, retinoids are known to affect G(t) [32], but also a range of other G-proteins are found to be affected. In rat liver, during exposure RXR agonists were found to significantly decline mRNA levels of the G-protein subunit Galpha [33] - a subunit also known to be regulated by RGS proteins [31]. This observation and other observations are leading to the suggestion that (Ro)accutane alters RGS protein expression.
Transcription factor Phox2b
In embryos deficient for Phox2b, RGS4 expression is downregulated in the locus coeruleus, sympathetic ganglia, and cranial motor and sensory neurons [35].
The importance of Phox2b in adult brain is not yet fully understood.
In the developing rat heart, Retinoic Acid disrupts the differentiation of cardiac neural crest cells into ganglionic cells destined to contribute to the parasympathetic innervation of the heart, by regulating the expression of Phox2a and Phox2b [34].
(Ro)accutane induced increases in TGF-beta1 and significantly increased formation of beta amyloid from APP
In acne subjects, six weeks of isotretinoin exposure caused a statistically significant 19% increase in suction blister fluid TGF-beta1 [18]. TGF-b (transforming growth factor-b), which is overexpressed in AD patients, is capable of enhancing the synthesis of APP by astrocytes by a transcriptional mechanism leading to the accumulation of Ab. TGF-b1 induces the binding of nuclear factors to the APPtre sequence. The APPtre sequence not only responds to the Smad3 transcription factor, but also the Sp1 (signal protein 1) transcription factor co-operates with Smads to potentiate the TGF-b-dependent activation of APP [17].
In small doses, retinoic acid is found to upregulate APP in neuronal cell lines [19, 20 and more]. To test whether overexpression of APP generates abnormally processed derivatives that affect the viability of neurons, full-length human APP complementary DNA was stably transfected into murine embryonal carcinoma P19 cells. These cells differentiate into post-mitotic neurons and astrocytes after exposure to retinoic acid. When differentiation of the APP cDNA-transfected P19 cells was induced, all neurons showed severe degenerative changes and disappeared within a few days [19].
Binding and translocation of APP
The binding of amyloid beta precursor protein may be affected, since APBB1 is located in a subcellular area heavily affected by retinoic acid, resulting in a translocation of APP (11p15).
1) Alteration of APP cleavage (Bace 1 and 2)
2) Alteration of beta amyloid binding
APPBP1 and 2
APBA2
APBA3
APBB1
APBB2
3) Alteration in formation of APP
APBB1
amyloid beta (A4) precursor protein-binding, family B,
member 1 (Fe65)
Location 11p15.5
(Ro)accutane exposure and calpain redisribution
It is well documented that activation of calpain, a calcium-sensitive cysteine protease, marks the pathology of naturally and experimentally occuring neurodegenerative conditions. Calpain-mediated proteolysis of major membrane-skeletal protein, alphaII-spectrin, results in the appearance of two unique and highly stable breakdown products, which is an early event in neural cell pathology [23]. The calcium-dependent protease, calpain, cleaves the cytoplasmic domain of the integrin beta3 subunit [22].
Calpastatin distribution is affected by the intracellular increase in free Ca(2+), which results in calpastatin progressively becoming a soluble protein. However, calpain is distributed in the soluble cell fraction and, in activating conditions, partially accumulates on the plasma membrane. Similar behaviour has been observed in calpastatin localization in LAN-5 cells induced with retinoic acid, suggesting that the proteolytic system is activated during the differentiation process of these cells [24].
Alzheimer's beta-amyloid precursor protein (APP) is normally processed by an unidentified alpha-secretase. A unique feature of this protease is its high sensitivity to phorbol esters, yet the mechanism involved is unclear. We have previously reported that phorbol 12,13-dibutyrate (PDBu) activates calpain, a Ca2+-dependent protease, and PDBu-induced release of APPs (secreted APP) is sensitive to calpain inhibitors, suggesting that calpain is involved in APP alpha-processing. In the present study, we found that PDBu markedly promoted the expression of both mu- and m-calpains in cultured fibroblasts. Dose-response and time course studies revealed that mu-calpain was more sensitive to PDBu than m-calpain and the temporal course of the mu-calpain change coincides better with that of APPs release. Moreover, the stimulatory effect of PDBu on mu-calpain was selectively blocked by mu-calpain-specific siRNA (small interference RNA) and the blockage was accompanied by a concomitant decrease in APPs release. In contrast, m-calpain siRNA did not affect APPs release significantly. Measurement of amyloid beta protein (Abeta) release in the mu-calpain siRNA-treated cells indicated that Abeta40 and Abeta42 levels inversely changed in relation to APPs, and the changes in Abeta42 were more prominent than in Abeta40. Together, these data suggest that calpain, particularly mu-calpain, is a potential candidate for alpha-secretase in the regulated APP alpha-processing, and that changes in this protease can affect the outcome of the overall APP processing [21].
During exposure: Inhibition of LRP-2 mediated beta-amyloid clearance and P-gp-mediated clearance - vitamin A deficiency after exposure, suggested decrease of both LRP-2 and P-gp
The clearance mechanisms of beta amyloid are yet not fully understood, but based on observations in combinations with hypothesises. Two distinct patways of clearance are suggested: 1) beta amyloid couples to LRP-2 and 2) beta amyloid couples to P-gp. In addition PPARgamma has been suggested to, through an unknown pathway, influence beta amyloid clearance.
Critical in modulating beta-amyloid deposition in brain is the flux of Abeta across the blood brain barrier. The low-density lipoprotein receptor-related protein (LRP), is a large endocytic receptor that mediates the efflux of Abeta out of brain and into the periphery. The first step in the LRP-mediated clearance of Abeta involves the formation of a complex between Abeta and the LRP ligands apolipoprotein E (apoE) or alpha(2)-macroglobulin (alpha(2)M). The Abeta/chaperone complexes then bind to LRP via binding sites on apoE or alpha(2)M. The efflux of Abeta/chaperone complexes out of the neuropil and into the periphery may be attenuated by LRP-ligands that compete with apoE or alpha(2)M for LRP binding [26].
PPARgamma is suggested to be significantly affected during (Ro)accutane exposure. After exposure, when a vitamin A deficiency is present it may result in a downregulation of the PPARgamma receptor function.
(Ro)accutane and formation of ceramide and a possible role in Alzheimers disease, age-related cognitive decline and dementia
Retinoic acid in pro-apoptotic doses (doses associated with exposure in acne-subjects) is currently used in research models that possibly may approach an understanding of the pathology of alzheimers disease and age-related cognitive decline [10 and more]. A 2.5-fold increase of ceramide mass in the supernatant was detected after 48 h of treatment with RA.
In retinoic acid (RA)-induced neuronal apoptosis, RA slightly increased de novo synthesis of ceramide, but interestingly, RA dramatically inhibited conversion of [14C] ceramide to glucosylceramide (GlcCer), suggesting that the increase of ceramide mass is mainly due to inhibition of the ceramide-metabolizing enzyme GlcCer synthase. In addition, a significant increase of the [14C] ceramide level in the culture medium was detected by chasing and turnover experiments without alteration of extracellular [14C] sphingomyelin levels [10].
Introduction
In the studies of (Ro)accutane induced degenerative pathologies, a distinction can be made between:
1) a suggested direct degenerative process during (Ro)accutane exposure as seen in mice [3 and 4]
2) a suggested possible slowly continous degenerative process post (Ro)accutane exposure and a possible role of retinoids in Alzheimer´s disease [2, 27 and more]
Retinoids are suggested to be of importance for the maintenance of the central nervous system, and a loss of retinoid signaling may contribute to the pathogenesis of Alzheimers disease [2 and more]. After 1 year of a vitamin-A dietary deficiency in adult rats, there was a deposition of amyloid beta in the cerebral blood vessels, suggesting a clear role of vitamin-A in the formation of beta amyloid. A downregulation of retinoic acid receptor alpha in the forebrain neurons of the retinoid-deficient rats and a loss of choline acetyl transferase expression, which precedes amyloid beta deposition was found. In neocortex of pathology samples of patients with Alzheimer's disease, the same retinoic acid receptor alpha deficit in the surviving neurons was observed [2]. Abnormal retinoid metabolism may be involved in the downstream transcriptional regulation of phospholipase A2-mediated signal transduction in Alzheimers disease (AD) [27].
Suggested significant loss of hippocampal cells during exposure
Results from (Ro)accutane exposure in adult mice are suggesting a significantly decreased hippocampal cell survival, and loss of cells during exposure [3 and 4]. Little is known about long-term effects of a (Ro)accutane exposure in humans. One of the principal theories of the pathology of Alzheimers disease is an increased formation of beta-amyloid [5 and more]. AD (Alzheimer's disease) is a neurodegenerative disease associated with progressive memory loss and leading to dementia. Except for cell-loss, two histological characteristics are observed in AD patients after autopsy: extracellular plaques and intracellular tangles [17]. Several enzymes and receptors of importance for the formation of beta-amyloid are distinctively altered in association with exposure of retinoic acid [6, 7 and more], as well as receptors, such as megalin, that likely are involved in the clearance of beta amyloid [26 and more], as well as disruption of a number of signaling pathways, that are suggested to be involved in the pathology of Alzheimers disease [27 and more].
Formation of beta amyloid from APP
The principal component of amyloid fibrils is beta/A4 amyloid protein, which can be generated by the aberrant processing of a large membrane-bound glycoprotein, the beta/A4 amyloid protein precursor (APP)3 [1]. The amyloid beta-peptide (approximately 4 kDa-M(r)) is generated by the proteolytic cleavage of a larger beta-amyloid precursor protein (beta APP) encoded by a gene on chromosome 21 [9].
(Ro)accutane and disruptions in G-protein signaling; possible decreases in RGS protein expression
RGS proteins
A statistically significant downregulation of RGS4 was found in post-mortem examinations of brain samples from patients with Alzheimers Disease [31]. RGS proteins have been shown to control essential neurological functions by modulating G(i) and G(q) mediated signaling. More than 20 regulators of G-protein signaling, RGS proteins, have been identified, of which five are found to be expressed highly in the brain (RGS4, RGS7, RGS8, RGS11 and RGS17). During the G-protein activity cycle RGS reduces the GPCR signaling by accelerating the rate of GTP hydrolysis. RGS interacts specifically with the alpha subunit of the G-protein which leads to dimerization and an inactive heterotrimeric G-protein [31].
Retinoids and G-protein signaling
Significant interference between retinoids and G-protein signaling is well known. In the opthamologic field, retinoids are known to affect G(t) [32], but also a range of other G-proteins are found to be affected. In rat liver, during exposure RXR agonists were found to significantly decline mRNA levels of the G-protein subunit Galpha [33] - a subunit also known to be regulated by RGS proteins [31]. This observation and other observations are leading to the suggestion that (Ro)accutane alters RGS protein expression.
Transcription factor Phox2b
In embryos deficient for Phox2b, RGS4 expression is downregulated in the locus coeruleus, sympathetic ganglia, and cranial motor and sensory neurons [35].
The importance of Phox2b in adult brain is not yet fully understood.
In the developing rat heart, Retinoic Acid disrupts the differentiation of cardiac neural crest cells into ganglionic cells destined to contribute to the parasympathetic innervation of the heart, by regulating the expression of Phox2a and Phox2b [34].
(Ro)accutane induced increases in TGF-beta1 and significantly increased formation of beta amyloid from APP
In acne subjects, six weeks of isotretinoin exposure caused a statistically significant 19% increase in suction blister fluid TGF-beta1 [18]. TGF-b (transforming growth factor-b), which is overexpressed in AD patients, is capable of enhancing the synthesis of APP by astrocytes by a transcriptional mechanism leading to the accumulation of Ab. TGF-b1 induces the binding of nuclear factors to the APPtre sequence. The APPtre sequence not only responds to the Smad3 transcription factor, but also the Sp1 (signal protein 1) transcription factor co-operates with Smads to potentiate the TGF-b-dependent activation of APP [17].
In small doses, retinoic acid is found to upregulate APP in neuronal cell lines [19, 20 and more]. To test whether overexpression of APP generates abnormally processed derivatives that affect the viability of neurons, full-length human APP complementary DNA was stably transfected into murine embryonal carcinoma P19 cells. These cells differentiate into post-mitotic neurons and astrocytes after exposure to retinoic acid. When differentiation of the APP cDNA-transfected P19 cells was induced, all neurons showed severe degenerative changes and disappeared within a few days [19].
Binding and translocation of APP
The binding of amyloid beta precursor protein may be affected, since APBB1 is located in a subcellular area heavily affected by retinoic acid, resulting in a translocation of APP (11p15).
1) Alteration of APP cleavage (Bace 1 and 2)
2) Alteration of beta amyloid binding
APPBP1 and 2
APBA2
APBA3
APBB1
APBB2
3) Alteration in formation of APP
APBB1
amyloid beta (A4) precursor protein-binding, family B,
member 1 (Fe65)
Location 11p15.5
(Ro)accutane exposure and calpain redisribution
It is well documented that activation of calpain, a calcium-sensitive cysteine protease, marks the pathology of naturally and experimentally occuring neurodegenerative conditions. Calpain-mediated proteolysis of major membrane-skeletal protein, alphaII-spectrin, results in the appearance of two unique and highly stable breakdown products, which is an early event in neural cell pathology [23]. The calcium-dependent protease, calpain, cleaves the cytoplasmic domain of the integrin beta3 subunit [22].
Calpastatin distribution is affected by the intracellular increase in free Ca(2+), which results in calpastatin progressively becoming a soluble protein. However, calpain is distributed in the soluble cell fraction and, in activating conditions, partially accumulates on the plasma membrane. Similar behaviour has been observed in calpastatin localization in LAN-5 cells induced with retinoic acid, suggesting that the proteolytic system is activated during the differentiation process of these cells [24].
Alzheimer's beta-amyloid precursor protein (APP) is normally processed by an unidentified alpha-secretase. A unique feature of this protease is its high sensitivity to phorbol esters, yet the mechanism involved is unclear. We have previously reported that phorbol 12,13-dibutyrate (PDBu) activates calpain, a Ca2+-dependent protease, and PDBu-induced release of APPs (secreted APP) is sensitive to calpain inhibitors, suggesting that calpain is involved in APP alpha-processing. In the present study, we found that PDBu markedly promoted the expression of both mu- and m-calpains in cultured fibroblasts. Dose-response and time course studies revealed that mu-calpain was more sensitive to PDBu than m-calpain and the temporal course of the mu-calpain change coincides better with that of APPs release. Moreover, the stimulatory effect of PDBu on mu-calpain was selectively blocked by mu-calpain-specific siRNA (small interference RNA) and the blockage was accompanied by a concomitant decrease in APPs release. In contrast, m-calpain siRNA did not affect APPs release significantly. Measurement of amyloid beta protein (Abeta) release in the mu-calpain siRNA-treated cells indicated that Abeta40 and Abeta42 levels inversely changed in relation to APPs, and the changes in Abeta42 were more prominent than in Abeta40. Together, these data suggest that calpain, particularly mu-calpain, is a potential candidate for alpha-secretase in the regulated APP alpha-processing, and that changes in this protease can affect the outcome of the overall APP processing [21].
During exposure: Inhibition of LRP-2 mediated beta-amyloid clearance and P-gp-mediated clearance - vitamin A deficiency after exposure, suggested decrease of both LRP-2 and P-gp
The clearance mechanisms of beta amyloid are yet not fully understood, but based on observations in combinations with hypothesises. Two distinct patways of clearance are suggested: 1) beta amyloid couples to LRP-2 and 2) beta amyloid couples to P-gp. In addition PPARgamma has been suggested to, through an unknown pathway, influence beta amyloid clearance.
Critical in modulating beta-amyloid deposition in brain is the flux of Abeta across the blood brain barrier. The low-density lipoprotein receptor-related protein (LRP), is a large endocytic receptor that mediates the efflux of Abeta out of brain and into the periphery. The first step in the LRP-mediated clearance of Abeta involves the formation of a complex between Abeta and the LRP ligands apolipoprotein E (apoE) or alpha(2)-macroglobulin (alpha(2)M). The Abeta/chaperone complexes then bind to LRP via binding sites on apoE or alpha(2)M. The efflux of Abeta/chaperone complexes out of the neuropil and into the periphery may be attenuated by LRP-ligands that compete with apoE or alpha(2)M for LRP binding [26].
PPARgamma is suggested to be significantly affected during (Ro)accutane exposure. After exposure, when a vitamin A deficiency is present it may result in a downregulation of the PPARgamma receptor function.
(Ro)accutane and formation of ceramide and a possible role in Alzheimers disease, age-related cognitive decline and dementia
Retinoic acid in pro-apoptotic doses (doses associated with exposure in acne-subjects) is currently used in research models that possibly may approach an understanding of the pathology of alzheimers disease and age-related cognitive decline [10 and more]. A 2.5-fold increase of ceramide mass in the supernatant was detected after 48 h of treatment with RA.
In retinoic acid (RA)-induced neuronal apoptosis, RA slightly increased de novo synthesis of ceramide, but interestingly, RA dramatically inhibited conversion of [14C] ceramide to glucosylceramide (GlcCer), suggesting that the increase of ceramide mass is mainly due to inhibition of the ceramide-metabolizing enzyme GlcCer synthase. In addition, a significant increase of the [14C] ceramide level in the culture medium was detected by chasing and turnover experiments without alteration of extracellular [14C] sphingomyelin levels [10].