Post by Max on Jun 12, 2005 18:59:40 GMT -5
Found clinical hypothyroidism in acne-subjects exposed to (Ro)accutane
In human acne-subjects exposed to (Ro)accutane, levels of thyroxine and triiodothyronine were significantly lower after exposure (p less than 0.05), indicating a (Ro)accutane induced clinical thyroid deficiency (hypothyroidism) [5].
Decreased binding affinity of Sp1 to GC boxes here suggested to be one pathway to (Ro)accutane induced inhibition of transcriptional megalin expression
Hoffman la Roche itself has admitted to the findings of significantly raised homocysteine (Hcy) levels in human subjects exposed to (Ro)accutane [0]. These findings of statistically significant elevation of Hcy, have in acne-subjects been repeatedly verified by independent studies [13 and more]. Elevated homocysteine levels have frequently been associated with clinical hypothyroidism [12 and more]. This frequent association is likely due to the elevated homocysteine being a sign of lessened activity of the cystathionine beta-synthase (CBS) that catalyzes the condensation of serine and homocysteine to form cystathionine. CBS is promoted by Sp1 and Sp3 [17], and a dephorphorylation of Sp proteins reduces the promotion significantly by decreased binding of the Sp protein to the GC boxes (GC boxes are binding sites for Sp induced gene-promotion) [18]. These same Sp proteins are in rats found to promote megalin expression, and a dephosporylation of Sp proteins is highly likely to reduce megalin expression by a lessened gene promotion [19]. In small doses RARs/RXRs physically interact with Sp1, potentiate Sp1 binding to the GC box motifs [20]. It is here suggested that a supraphysiological dose of (Ro)accutane inhibits Sp1 binding partly through inhibited phoshorylation, and thereby reduces promoter activity of megalin.
The cellular retinoic acid binding protein I gene is induced by thyroid hormone (T3) through a T3 response element (TRE) approximately 1 kb upstream of the basal promoter. The upstream region is organized into a positioned nucleosomal array with the N1 nucleosome spanning the GC box region [26].
High-affinity agonists for the retinoic acid X receptors (RXR) and retinoic acid receptor (RAR) have pleotropic effects when administered to humans. These include induction of hypertriglyceridemia and hypothyroidism [6 and 11]. The RAR/RXR pan-agonist 9-cis-retinoic acid induce 2- to 3-fold higher levels of serum triglycerides than the RAR-selective agonists alone [11].
Inhibition of thyroxine and triiodthyronine release, likely partially through reduction of thyroglobulin due to downregulation of megalin mediated endocytosis
When thyroglobulin (Tg) is endocytosed by thyrocytes and transported to lysosomes, thyroid hormones (T4 and T3) are released. Megalin (gp330), a Tg receptor on thyroid cells, is found to be a mediator in Tg transcytosis [7, 14 and more]. Following incubation with exogenous rat Tg at 37 degrees C, Fisher rat thyroid (FRTL-5) cells, a differentiated thyroid cell line, released T3 into the medium [7]. A significant reduction of thyroglobulin was noted in isotretinoin exposed human subjects with thyroid carcinoma at doses comparable to what acne-subjects are exposed to (from 1,0 mg/kg/day), indicating that a significant fall in thyroglobulin may partially explain the significant fall in thyroxine and triiodthyronine seen in acne-subjects exposed to the toxin [8]. In F9 cells, small plasma concentrations of both retnoic acid (10^-6 M) and vitamin D, alone or in combination, have been found to upregulate thyroglubulin receptor megalin (gp330) during differentiation [9]. The gp330/Megalin/LRP-2 protein belongs to the low-density lipoprotein receptor gene family and is believed to function as an endocytic receptor for the uptake of lipoproteins and many other ligands. Other functions of this protein may include a role in calcium sensing in the parathyroid glands and other tissues [10].
Unfortunately no publicly available study shows how megalin (gp330) expression is affected in toxic (Ro)accutane exposure in human subjects. However, serum Tg levels were significantly reduced in homozygous (megalin(-/-)) mice, which, more importantly, were found to be hypothyroid, as demonstrated by significantly reduced serum free thyroxine and significantly increased serum thyroid stimulating hormone (TSH) levels [14]. In proximal-tubule-derived opossum kidney cells, TGF-beta1 was found to induce downregulation of megalin-cubilin-mediated endocytosis, sensitive to inhibition of translation and transcription and was preceded by Smad2 and 3 phosphorylation [15]. Six weeks of isotretinoin exposure caused a statistically significant 19% increase in suction blister fluid TGF-beta1 [16]. Type 3 iodothyronine deiodinase (D3) is the major inactivating pathway, preventing activation of the prohormone thyroxine (T4) and terminating the action of T3. TGF-beta stimulates transcription of the hDio3 gene via a Smad-dependent pathway [24].
The RAR/RXR pan-agonist 9-cis-retinoic acid effect included >50% inhibition of total heparin-releasable lipase activity in serum, consistent with inhibition of lipase-mediated triglyceride disposal [11].
Significant supression of thyroid receptors, likely through inhibition of Sp1 formation and binding
Ro(accutane) was found to suppress thyroid receptors in pituitary GH1 cells. Retinoic acid produced a time and dose-dependent depletion of thyroid hormone receptors in GH1 cells without modifying their affinity for triiodothyronine (T3) [3]. This is likely due to the thyroid receptor promoters contain a GC-box highly responsive to the transcription factor Sp1 [21]. In rats, TR beta1 promoter was regulated negatively by the proteins bound to the silencer sequence and the GC box, and positively by Oct-1. Adult brain extracts appear to contain more Oct-1 protein than do fetal extracts [25]. The effect on Oct-1 by a massive dose of retinoic acid, as seen in acne-subjects, is not known.
A maximal decrease of thyroid receptor expression (50-70%) was obtained after 24-48 h incubation with 5-10 microM retinoic acid [3]. The result is suggested to be hypothyroidism, or partial thyroid resistance.
The thyroid hormone receptor (TR) directly regulates the transcription of thyroid hormone-responsive genes in response to changing levels of thyroid hormone. Mechanistically TR utilizes a complex set of binding interactions, with hormone, response elements, and coregulatory proteins, to provide specific local control of patterns of transcriptional response that are partially responsible for inducing the tissue-selective responses to the circulating hormone. One of the apparently dominant phenomena in the regulation of thyroid hormone responses is the protein interactions between TR and its coregulators [2] .
Thyroid hormone receptors (TRs) often modulate transcriptional activity of target genes by heterodimerization with the 9-cis retinoic acid receptor (RXR). On positive thyroid response elements (TREs), RXR favors binding of the TR-RXR complex to DNA and stimulates transcription [1].
Expression of TR isoforms in several tissue including high expression in the adult brain
Thyroid hormone isoforms are derived from two separate genes to yield four major T3-binding isoforms: alpha1, beta1, beta2, and beta3 [4]. TRbeta is the predominant isoform in liver, whereas T3 effects on heart rate are mediated mostly by TRalpha [22]. TR alpha 1, alpha 2, beta 1, and beta 2 was studied in human consecutive sections of six hypothalami and pituitaries. Staining intensity showed strong interindividual variation but was consistently present in the infundibular nucleus, paraventricular nucleus, and supraoptic nucleus. In addition, strong TR immunoreactivity was observed in the anterior pituitary [23].
For further information about thyroid hormone receptor expression:
Thyroid receptor alpha isoforms
Genecards - THR A
Thyroid receptor beta isoforms
Genecards - THR B
(Genecards database)
In human acne-subjects exposed to (Ro)accutane, levels of thyroxine and triiodothyronine were significantly lower after exposure (p less than 0.05), indicating a (Ro)accutane induced clinical thyroid deficiency (hypothyroidism) [5].
Decreased binding affinity of Sp1 to GC boxes here suggested to be one pathway to (Ro)accutane induced inhibition of transcriptional megalin expression
Hoffman la Roche itself has admitted to the findings of significantly raised homocysteine (Hcy) levels in human subjects exposed to (Ro)accutane [0]. These findings of statistically significant elevation of Hcy, have in acne-subjects been repeatedly verified by independent studies [13 and more]. Elevated homocysteine levels have frequently been associated with clinical hypothyroidism [12 and more]. This frequent association is likely due to the elevated homocysteine being a sign of lessened activity of the cystathionine beta-synthase (CBS) that catalyzes the condensation of serine and homocysteine to form cystathionine. CBS is promoted by Sp1 and Sp3 [17], and a dephorphorylation of Sp proteins reduces the promotion significantly by decreased binding of the Sp protein to the GC boxes (GC boxes are binding sites for Sp induced gene-promotion) [18]. These same Sp proteins are in rats found to promote megalin expression, and a dephosporylation of Sp proteins is highly likely to reduce megalin expression by a lessened gene promotion [19]. In small doses RARs/RXRs physically interact with Sp1, potentiate Sp1 binding to the GC box motifs [20]. It is here suggested that a supraphysiological dose of (Ro)accutane inhibits Sp1 binding partly through inhibited phoshorylation, and thereby reduces promoter activity of megalin.
The cellular retinoic acid binding protein I gene is induced by thyroid hormone (T3) through a T3 response element (TRE) approximately 1 kb upstream of the basal promoter. The upstream region is organized into a positioned nucleosomal array with the N1 nucleosome spanning the GC box region [26].
High-affinity agonists for the retinoic acid X receptors (RXR) and retinoic acid receptor (RAR) have pleotropic effects when administered to humans. These include induction of hypertriglyceridemia and hypothyroidism [6 and 11]. The RAR/RXR pan-agonist 9-cis-retinoic acid induce 2- to 3-fold higher levels of serum triglycerides than the RAR-selective agonists alone [11].
Inhibition of thyroxine and triiodthyronine release, likely partially through reduction of thyroglobulin due to downregulation of megalin mediated endocytosis
When thyroglobulin (Tg) is endocytosed by thyrocytes and transported to lysosomes, thyroid hormones (T4 and T3) are released. Megalin (gp330), a Tg receptor on thyroid cells, is found to be a mediator in Tg transcytosis [7, 14 and more]. Following incubation with exogenous rat Tg at 37 degrees C, Fisher rat thyroid (FRTL-5) cells, a differentiated thyroid cell line, released T3 into the medium [7]. A significant reduction of thyroglobulin was noted in isotretinoin exposed human subjects with thyroid carcinoma at doses comparable to what acne-subjects are exposed to (from 1,0 mg/kg/day), indicating that a significant fall in thyroglobulin may partially explain the significant fall in thyroxine and triiodthyronine seen in acne-subjects exposed to the toxin [8]. In F9 cells, small plasma concentrations of both retnoic acid (10^-6 M) and vitamin D, alone or in combination, have been found to upregulate thyroglubulin receptor megalin (gp330) during differentiation [9]. The gp330/Megalin/LRP-2 protein belongs to the low-density lipoprotein receptor gene family and is believed to function as an endocytic receptor for the uptake of lipoproteins and many other ligands. Other functions of this protein may include a role in calcium sensing in the parathyroid glands and other tissues [10].
Unfortunately no publicly available study shows how megalin (gp330) expression is affected in toxic (Ro)accutane exposure in human subjects. However, serum Tg levels were significantly reduced in homozygous (megalin(-/-)) mice, which, more importantly, were found to be hypothyroid, as demonstrated by significantly reduced serum free thyroxine and significantly increased serum thyroid stimulating hormone (TSH) levels [14]. In proximal-tubule-derived opossum kidney cells, TGF-beta1 was found to induce downregulation of megalin-cubilin-mediated endocytosis, sensitive to inhibition of translation and transcription and was preceded by Smad2 and 3 phosphorylation [15]. Six weeks of isotretinoin exposure caused a statistically significant 19% increase in suction blister fluid TGF-beta1 [16]. Type 3 iodothyronine deiodinase (D3) is the major inactivating pathway, preventing activation of the prohormone thyroxine (T4) and terminating the action of T3. TGF-beta stimulates transcription of the hDio3 gene via a Smad-dependent pathway [24].
The RAR/RXR pan-agonist 9-cis-retinoic acid effect included >50% inhibition of total heparin-releasable lipase activity in serum, consistent with inhibition of lipase-mediated triglyceride disposal [11].
Significant supression of thyroid receptors, likely through inhibition of Sp1 formation and binding
Ro(accutane) was found to suppress thyroid receptors in pituitary GH1 cells. Retinoic acid produced a time and dose-dependent depletion of thyroid hormone receptors in GH1 cells without modifying their affinity for triiodothyronine (T3) [3]. This is likely due to the thyroid receptor promoters contain a GC-box highly responsive to the transcription factor Sp1 [21]. In rats, TR beta1 promoter was regulated negatively by the proteins bound to the silencer sequence and the GC box, and positively by Oct-1. Adult brain extracts appear to contain more Oct-1 protein than do fetal extracts [25]. The effect on Oct-1 by a massive dose of retinoic acid, as seen in acne-subjects, is not known.
A maximal decrease of thyroid receptor expression (50-70%) was obtained after 24-48 h incubation with 5-10 microM retinoic acid [3]. The result is suggested to be hypothyroidism, or partial thyroid resistance.
The thyroid hormone receptor (TR) directly regulates the transcription of thyroid hormone-responsive genes in response to changing levels of thyroid hormone. Mechanistically TR utilizes a complex set of binding interactions, with hormone, response elements, and coregulatory proteins, to provide specific local control of patterns of transcriptional response that are partially responsible for inducing the tissue-selective responses to the circulating hormone. One of the apparently dominant phenomena in the regulation of thyroid hormone responses is the protein interactions between TR and its coregulators [2] .
Thyroid hormone receptors (TRs) often modulate transcriptional activity of target genes by heterodimerization with the 9-cis retinoic acid receptor (RXR). On positive thyroid response elements (TREs), RXR favors binding of the TR-RXR complex to DNA and stimulates transcription [1].
Expression of TR isoforms in several tissue including high expression in the adult brain
Thyroid hormone isoforms are derived from two separate genes to yield four major T3-binding isoforms: alpha1, beta1, beta2, and beta3 [4]. TRbeta is the predominant isoform in liver, whereas T3 effects on heart rate are mediated mostly by TRalpha [22]. TR alpha 1, alpha 2, beta 1, and beta 2 was studied in human consecutive sections of six hypothalami and pituitaries. Staining intensity showed strong interindividual variation but was consistently present in the infundibular nucleus, paraventricular nucleus, and supraoptic nucleus. In addition, strong TR immunoreactivity was observed in the anterior pituitary [23].
For further information about thyroid hormone receptor expression:
Thyroid receptor alpha isoforms
Genecards - THR A
Thyroid receptor beta isoforms
Genecards - THR B
(Genecards database)
