| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| Megalin ( gp 330 ) : a putative endocytic receptor for thyroglobulin ( Tg ) . ^^^ Megalin ( gp 330 ) : a putative endocytic receptor for thyroglobulin ( Tg ) . ^^^ In the present study we have demonstrated that purified rat megalin binds rat thyroglobulin ( Tg ) in solid phase assays , with anestimated Kd of 9 . 2+ / 0 . 6 nM . ^^^ Binding was calcium dependent and was almost completely inhibited by excess Tg , by three megalin ligands lactoferrin , lipoprotein lipase and apolipoprotein J and by the receptor associated protein ( RAP ) , which inhibits binding of all megalin ligands . ^^^ Three anti megalin antibodies partially inhibited Tg binding to megalin . 125I labeled Tg bound to megalin was released by EDTA and heparin ; the released product was shown by SDS PAGE and autoradiography to be 660 kD ( dimeric ) Tg . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| We recently reported that megalin ( gp 330 ) , an endocytic receptor found on the apical surface of thyroid cells , binds thyroglobulin ( Tg ) with high affinity in solid phase assays . ^^^ Megalin bound Tg was releasable by heparin . ^^^ Here we show that Fisher rat thyroid ( FRTL 5 ) cells , a differentiated rat thyroid cell line , can bind and endocytose Tg via megalin . ^^^ Evidence of Tg binding to megalin on FRTL 5 cells and on an immortalized rat renal proximal tubule cell line ( IRPT cells ) , was obtained by incubating the cells with 125I Tg , followed by chemical cross linking and immunoprecipitation of 125I Tg with antibodies against megalin . ^^^ In solid phase experiments designed to illuminate the mechanism of heparin release , we demonstrated that Tg is a heparin binding protein , as are several megalin ligands . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| We recently showed that thyroglobulin ( Tg ) is a heparin binding protein and that heparin inhibits binding of Tg to its endocytic receptor megalin ( gp 330 ) . ^^^ Here we have identified a heparin binding region in the carboxyl terminal portion of rat Tg and have studied its involvement in megalin binding . ^^^ As assessed by enzyme linked immunoadsorbent assays and ligand blot binding assays , megalin bound to intact Tg ( 660 and 330 kDa ) and , to a even greater extent , to the 210 kDa Tg polypeptide . ^^^ Furthermore , the 210 kDa Tg polypeptide inhibited megalin binding to intact Tg by approximately 70 % . ^^^ Furthermore , the anti Tg peptide antibody almost completely inhibited binding of megalin to Tg , suggesting that the Tg region containing the peptide sequence is required for megalin binding . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| Here we show that megalin ( gp 330 ) , a Tg receptor on thyroid cells , plays a role in Tg transcytosis . ^^^ However , when cells were incubated with Tg plus either of two megalin competitors , T 3 release was increased , suggesting that Tg internalized by megalin bypassed the lysosomal pathway , possibly with release of undegraded Tg from cells . ^^^ Intact 330 kDa Tg was released into the medium , and the amount released was markedly reduced by megalin competitors . ^^^ The amount recovered was markedly reduced by megalin competitors , indicating that megalin mediates Tg transcytosis . ^^^ This was associated with increased megalin expression on thyrocytes and increased serum Tg levels , with reduced serum T 3 levels , supporting the conclusion that megalin mediates Tg transcytosis . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| Megalin , a multiligand receptor expressed on the apical surface of thyroid cells , mediates transepithelial transport ( transcytosis ) of thyroglobulin ( Tg ) across thyrocytes , resulting in diversion of Tg from the lysosomal pathway and reduction of the extent of thyroid hormone release from internalized Tg molecules . ^^^ Here we investigated the role of calmodulin in megalin mediated transcytosis of Tg by thyroid cells . ^^^ Furthermore , trifluoperazine increased the extent of triiodothyronine ( T 3 ) release from exogenously added Tg by FRTL 5 cells , indicating that Tg transported in the calmodulin dependent , megalin mediated pathway , bypasses the lysosomal pathway . . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| We previously showed that rat thyroglobulin ( Tg ) is a heparin binding protein and that heparin inhibits Tg binding to megalin ( gp 330 ) , an endocytic Tg receptor found on the apical surface of thyrocytes . ^^^ Based on our previous findings indicating that heparin and megalin binding sites of rat Tg are functionally related , here we investigated whether rat Tg binds to HSPGs , which are expressed by thyroid cells . ^^^ An antibody against a Tg heparin binding site functionally related to a major megalin binding site virtually abolished Tg binding to HS and to FRTL 5 cells , supporting the hypothesis that combined interactions of Tg with HSPGs and with megalin are involved in Tg binding to rat thyroid cells . . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| We previously showed that megalin ( gp 330 ) , an endocytic Tg receptor , mediates apical to basolateral Tg transcytosis . ^^^ Here we investigated whether a portion of megalin remains combined with Tg after its transcytosis , using studies with cultured thyroid cells and in vivo observations . ^^^ After the addition of Tg to surface biotinylated FRTL 5 cells , an anti Tg antibody and the two antimegalin ectodomain antibodies precipitated high molecular mass biotinylated material in fluids collected from the lower chamber , corresponding to much of the megalin ectodomain , as well as smaller amounts of lower molecular mass material . ^^^ The results indicate that Tg transcytosed across FRTL 5 cells remains complexed with megalin ectodomain components , which we refer to as megalin secretory components . ^^^ In aminotriazole treated rats , which develop increased megalin mediated Tg transcytosis , antimegalin antibodies precipitated some of the Tg in the serum . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| Megalin mediates Tg uptake by thyrocytes , especially under intense thyroid stimulating hormone stimulation , resulting in transcytosis of Tg from the colloid to the bloodstream , a function that prevents excessive hormone release . . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| In Tg rats , the renal expression pattern of megalin , which serves as an endocytotic receptor responsible for the reuptake of urinary proteins such as DBP and albumin , was identical to that of the wild type rats . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| TG internalized from the colloid by megalin , bypasses the lysosomal pathway and is transported across thyrocytes by transcytosis . ^^^ We first investigated the effect of pH on megalin binding to TG in solid phase assays and found that , although megalin bound to TG at various pH values ( ranging from 4 8 ) , optimal binding was seen at acidic pH ( ranging from 4 . 5 6 ) . ^^^ In conclusion , because TG binding to megalin is greatest at acidic pH , it is possible that TG does not dissociate from megalin in the lysosomal pathway . ^^^ However , the pH dependence of TG binding to megalin does not account for much of transcytosis , which probably occurs largely because of other mechanisms of targeting . . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| Thyroglobulin ( Tg ) , the major protein component of the colloid and the precursor of thyroid hormones , binds to megalin with high affinity and megalin mediates in part its uptake by thyrocytes . ^^^ Tg internalized by megalin avoids the lysosomal pathway and is delivered by transepithelial transport ( transcytosis ) to the basolateral membrane of thyrocytes , from which it is released into the bloodstream . ^^^ During transcytosis of Tg , a portion of megalin ( secretory component ) remains complexed with Tg and enters the circulation , where its detection may serve as a tool to identify the origin of serum Tg in patients with thyroid diseases . ^^^ Tg endocytosis via megalin is facilitated by the interaction of Tg with cell surface heparan sulfate proteoglycans , which occurs via a carboxyl terminal heparin binding site of Tg functionally related with a major megalin binding site . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| OBJECTIVE : To investigate the role of PI 3 K in transcytosis of thyroglobulin ( Tg ) across differentiated cultured Fisher rat thyroid cells ( FRTL 5 cells ) a process known to be mediated by megalin , a member of the low density lipoprotein receptor family . ^^^ In experiments in which a monoclonal antibody against megalin was used to reduce Tg transcytosis , pre incubation with wortmannin did not increase Tg transcytosis from its reduced levels , indicating that PI 3 K is involved in the megalin mediated pathway . ^^^ CONCLUSIONS : PI 3 K exerts an inhibitory role on megalin mediated Tg transcytosis across cultured thyroid cells . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| OBJECTIVE : Binding of thyroglobulin ( Tg ) to heparin allows efficient Tg interaction with its endocytic receptor , megalin . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| However , some Tg internalized by megalin bypasses lysosomes and is transcytosed across cells and released into the bloodstream . ^^^ We found that rat Tg with a low hormone content [ low hormonogenic rat Tg ( low horm rTg ) ] is transcytosed by megalin across thyroid FRTL 5 cells to a greater extent than rat Tg with a high hormone content [ hormonogenic rat Tg ( horm rTg ) ] . ^^^ In immunoprecipitation experiments , the Tg sequence Arg 2489 Lys 2503 ( required for binding to megalin and heparan sulfate proteoglycans ) was found to be more exposed in low horm rTg , which accounted for its preferential transcytosis . ^^^ Thus , the increase in hormone release from horm rTg in FRTL 5 cells determined by megalin blocking ( due to reduced transcytosis and enhanced Tg degradation ) was rescued by low horm rTg , suggesting that megalin is required for effective hormone release . ^^^ Reduced hormone formation within Tg in vivo , due to treatment of rats with aminotriazole or of patients with Graves ' disease with methimazole , resulted in increased Tg transcytosis via megalin , in confirmation of results with FRTL 5 cells . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| When thyroid follicles are intact , some colloidal thyroglobulin ( Tg ) reaches the circulation by megalin mediated transcytosis and is to various extents complexed with megalin secretory components . ^^^ In contrast , in papillary thyroid cancer ( PTC ) , serum Tg is not complexed with megalin because it is directly secreted by tumor cells . ^^^ Thus , we concluded that measurement of megalin secretory components is unlikely to be useful to identify the origin of serum Tg in PTC patients after thyroidectomy . . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| Megalin mediates transcytosis of thyroglobulin ( Tg ) , the thyroid hormone precursor , resulting in its passage into the bloodstream . ^^^ As expected from the knowledge that megalin mediates Tg transcytosis , 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 . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| The molecular chaperone receptor associated protein ( RAP ) is required for biosynthesis of megalin , an endocytic receptor for follicular thyroglobulin ( Tg ) , the thyroid hormone precursor . ^^^ Megalin was decreased in RAP KO mice , but this did not affect thyroid function , probably because of the concomitant reduction of follicular Tg . ^^^ |
|
| Interacting proteins: P01266 and P98164 |
Pubmed |
SVM Score :0.0 |
| NA |
|