Uncovering a novel mechanism: Butyrate induces estrogen receptor alpha activation independent of estrogen stimulation in MCF-7 breast cancer cells

Butyrate is a promising candidate for an antitumoral drug, as it promotes cancer cell apoptosis and reduces hormone receptor activity, while promoting differentiation and proliferation in normal cells. However, the effects of low-dose butyrate on breast cancer cell cultures are unclear. We explored the impact of sub-therapeutic doses of butyrate on estrogen receptor alpha (ERα) transcriptional activity in MCF-7 cells, using RT-qPCR, Western blot, wound-healing assays, and chromatin immunoprecipitation. Our results showed that sub-therapeutic doses of sodium butyrate (0.1 - 0.2 mM) increased the transcription of ESR1, TFF1, and CSTD genes, but did not affect ERα protein levels. Moreover, we observed an increase in cell migration in wound-healing assays. ChIP assays revealed that treatment with 0.1 mM of sodium butyrate resulted in estrogen-independent recruitment of ERα at the pS2 promoter and loss of NCoR. Appropriate therapeutic dosage of butyrate is essential to avoid potential adverse effects on patients' health, especially in the case of estrogen receptor-positive breast tumors. Sub-therapeutic doses of butyrate may induce undesirable cell processes, such as migration due to low-dose butyrate-mediated ERα activation. These findings shed light on the complex effects of butyrate in breast cancer and provide insights for research in the development of antitumoral drugs.

The inhibitory effect of trastuzumab on BT474 triple‑positive breast cancer cell viability is reversed by the combination of progesterone and estradiol

Breast cancer expressing the estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor-2 (HER2) is known as triple-positive (TPBC). TPBC represents 9-11% of breast cancer cases worldwide and is a heterogeneous subtype. Notably, TPBC presents a therapeutic challenge due to the crosstalk between the hormonal (ER and PR) and HER2 pathways. Patients with TPBC are treated with trastuzumab (TTZ); however, several patients treated with TTZ tend to relapse. The present study aimed to investigate the effect of the PR on inhibitory effect of TTZ on cell viability. BT474 cells (a model of TPBC) and BT474 PR-silenced cells were treated with either TTZ, progesterone (Pg), the PR antagonist mifepristone (RU486) or estradiol (E2) alone or in combination for 144 h (6 days). Cell viability assays and western blotting were subsequently performed. The results showed that Pg and E2 interfered with the inhibitory effect of TTZ on cell viability and this effect was potentiated when both hormones were combined. Pg was revealed to act through the PR, mainly activating the PR isoform B (PR-B) and inducing the protein expression levels of CDK4 and cyclin D1; however, it did not reactivate the HER2/Akt pathway. By contrast, E2 was able to increase PR isoform A (PR-A) expression, which was inhibited by Pg. Notably, in most of the experiments, RU486 did not antagonize the effects of Pg. In conclusion, Pg and E2 may interfere with the inhibitory effect of TTZ on cell viability through PR-B activation and PR-A inactivation.

Tristetraprolin: A cytosolic regulator of mRNA turnover moonlighting as transcriptional corepressor of gene expression

Tristetraprolin (TTP) is a nucleocytoplasmic 326 amino acid protein whose sequence is characterized by possessing two CCCH-type zinc finger domains. In the cytoplasm TTP function is to promote the degradation of mRNAs that contain adenylate/uridylate-rich elements (AREs). Mechanistically, TTP promotes the recruitment of poly(A)-specific deadenylases and exoribonucleases. By reducing the half-life of about 10% of all the transcripts in the cell TTP has been shown to participate in multiple cell processes that include regulation of gene expression, cell proliferation, metabolic homeostasis and control of inflammation and immune responses. However, beyond its role in mRNA decay, in the cell nucleus TTP acts as a transcriptional coregulator by interacting with chromatin modifying enzymes. TTP has been shown to repress the transactivation of NF-κB and estrogen receptor suggesting the possibility that it participates in the transcriptional regulation of hundreds of genes in human cells and its possible involvement in breast cancer progression. In this review, we discuss the cytoplasmic and nuclear functions of TTP and the effect of the dysregulation of its protein levels in the development of human diseases. We suggest that TTP be classified as a moonlighting tumor supressor protein that regulates gene expression through two different mechanims; the decay of ARE-mRNAs and a transcriptional coregulatory function.

IFI27/ISG12 Downregulates Estrogen Receptor α Transactivation by Facilitating Its Interaction With CRM1/XPO1 in Breast Cancer Cells

The estrogen receptor alpha (ERα) is a ligand-activated transcription factor whose activity is modulated by its interaction with multiple protein complexes. In this work, we have identified the protein interferon alpha inducible protein 27 (IFI27/ISG12) as a novel ERα-associated protein. IFI27/ISG12 transcription is regulated by interferon and estradiol and its overexpression is associated to reduced overall survival in ER+ breast cancer patients but its function in mammary gland tissue remains elusive. In this study we showed that overexpression of IFI27/ISG12 in breast cancer cells attenuates ERα transactivation activity and the expression of ERα-dependent genes. Our results demonstrated that IFI27/ISG12 overexpression in MCF-7 cells reduced their proliferation rate in 2-D and 3-D cell culture assays and impaired their ability to migrate in a wound-healing assay. We show that IFI27/ISG12 downregulation of ERα transactivation activity is mediated by its ability to facilitate the interaction between ERα and CRM1/XPO1 that mediates the nuclear export of large macromolecules to the cytoplasm. IFI27/ISG12 overexpression was shown to impair the estradiol-dependent proliferation and tamoxifen-induced apoptosis in breast cancer cells. Our results suggest that IFI27/ISG12 may be an important factor in regulating ERα activity in breast cancer cells by modifying its nuclear versus cytoplasmic protein levels. We propose that IFI27/ISG12 may be a potential target of future strategies to control the growth and proliferation of ERα-positive breast cancer tumors.

Biotin in metabolism, gene expression, and human disease

Biotin is a water-soluble vitamin that belongs to the vitamin B complex and which is an essential nutrient of all living organisms from bacteria to man. In eukaryotic cells biotin functions as a prosthetic group of enzymes, collectively known as biotin-dependent carboxylases that catalyze key reactions in gluconeogenesis, fatty acid synthesis, and amino acid catabolism. Enzyme-bound biotin acts as a vector to transfer a carboxyl group between donor and acceptor molecules during carboxylation reactions. In recent years, evidence has mounted that biotin also regulates gene expression through a mechanism beyond its role as a prosthetic group of carboxylases. These activities may offer a mechanistic background to a developing literature on the action of biotin in neurological disorders. This review summarizes the role of biotin in activating carboxylases and proposed mechanisms associated with a role in gene expression and in ameliorating neurological disease.

Jab1 is a T2-dependent coactivator or a T3-dependent corepressor of TRB1-mediated gene regulation

Thyroid hormones (THs) induce pleiotropic effects in vertebrates, mainly through the activation or repression of gene expression. These mechanisms involve thyroid hormone binding to thyroid hormone receptors, an event that is followed by the sequential recruitment of coactivator or corepressor proteins, which in turn modify the rate of transcription. In the present study, we looked for specific coregulators recruited by the long isoform of the teleostean thyroid hormone receptor beta 1 (L-Trb1) when bound to the bioactive TH, 3,5-T₂ (T₂). We found that jun activation domain-binding protein1 (Jab1) interacts with L-Trb1 + T₂ complex. Using both the teleostean and human TRB1 isoforms, we characterized the Jab1-TRB1 by yeast two-hybrid, pull-down and transactivation assays. Our results showed that the TRB1-Jab1 interaction was ligand dependent and involved the single Jab1 nuclear receptor box, as well as the ligand-binding and N-terminal domains of TRB1. We also provide evidence of ligand-dependent, dual coregulatory properties of Jab1. Indeed, when T₂ is bound to L-Trb1 or hTRB1, Jab1 acts as a coactivator of transcription, whereas it has corepressor activity when interacting with the T₃-bound S-Trb1 or hTRB1. These mechanisms could explain some of the pleiotropic actions exerted by THs to regulate diverse biological processes.

Two translation initiation codons direct the expression of annexin VI 64kDa and 68kDa isoforms

Annexin A6 is a multicompetent, multifunctional protein involved in several biological processes within and outside of the cell. Whereas HeLa cells express annexin A6 only as a 68/67-kDa doublet, indicating alternative splicing (Smith PD et al. (1994) Proc Natl Acad Sci USA 91, 2713-2717), the GMO2784 human fibroblast cell line expresses two additional isoforms at 64 and 58kDa. In both cell lines, annexin A6 is located intracellularly and on the plasma membrane. In vitro eukaryotic protein synthesis of pIRESneoAnxA6 cDNA and pIRESneoAnxA6/Met^1- or Met^33- using a reticulocyte lysate coupled transcription/translation system revealed that this gene contains two translation start codons, Met¹ and Met³³. Immunoprecipitation of the products obtained from the transcription/translation system using various anti-annexin A6 antibodies confirmed the presence of several isoforms and suggested that this protein might be present in different configurations.

Nuclear tristetraprolin acts as a corepressor of multiple steroid nuclear receptors in breast cancer cells

Tristetraprolin (TTP) is a 34-kDa, zinc finger-containing factor that in mammalian cells acts as a tumor suppressor protein through two different mechanisms. In the cytoplasm TTP promotes the decay of hundreds of mRNAs encoding cell factors involved in inflammation, tissue invasion, and metastasis. In the cell nucleus TTP has been identified as a transcriptional corepressor of the estrogen receptor alpha (ERα), which has been associated to the development and progression of the majority of breast cancer tumors. In this work we report that nuclear TTP modulates the transactivation activity of progesterone receptor (PR), glucocorticoid receptor (GR) and androgen receptor (AR). In recent years these steroid nuclear receptors have been shown to be of clinical and therapeutical relevance in breast cancer. The functional association between TTP and steroid nuclear receptors is supported by the finding that TTP physically interacts with ERα, PR, GR and AR in vivo. We also show that TTP overexpression attenuates the transactivation of all the steroid nuclear receptors tested. In contrast, siRNA-mediated reduction of endogenous TTP expression in MCF-7 cells produced an increase in the transcriptional activities of ERα, PR, GR and AR. Taken together, these results suggest that the function of nuclear TTP in breast cancer cells is to act as a corepressor of ERα, PR, GR and AR. We propose that the reduction of TTP expression observed in different types of breast cancer tumors may contribute to the development of this disease by producing a dysregulation of the transactivation activity of multiple steroid nuclear receptors.

SIP1/NHERF2 enhances estrogen receptor alpha transactivation in breast cancer cells

The estrogen receptor alpha (ERα) is a ligand-activated transcription factor that possesses two activating domains designated AF-1 and AF-2 that mediate its transcriptional activity. The role of AF-2 is to recruit coregulator protein complexes capable of modifying chromatin condensation status. In contrast, the mechanism responsible for the ligand-independent AF-1 activity and for its synergistic functional interaction with AF-2 is unclear. In this study, we have identified the protein Na+/H+ Exchanger RegulatoryFactor 2 (NHERF2) as an ERα-associated coactivator that interacts predominantly with the AF-1 domain of the nuclear receptor. Overexpression of NHERF2 in breast cancer MCF7 cells produced an increase in ERα transactivation. Interestingly, the presence of SRC-1 in NHERF2 stably overexpressing MCF7 cells produced a synergistic increase in ERα activity. We show further that NHERF2 interacts with ERα and SRC-1 in the promoter region of ERα target genes. The binding of NHERF2 to ERα in MCF7 cells increased cell proliferation and the ability of MCF7 cells to form tumors in a mouse model. We analyzed the expression of NHERF2 in breast cancer tumors finding a 2- to 17-fold increase in its mRNA levels in 50% of the tumor samples compared to normal breast tissue. These results indicate that NHERF2 is a coactivator of ERα that may participate in the development of estrogen-dependent breast cancer tumors.

Tristetraprolin represses estrogen receptor α transactivation in breast cancer cells

Estrogen receptor α (ERα) mediates the effects of 17β-estradiol (E2) in normal mammary gland, and it is a key participant in breast cancer tumor development. ERα transactivation activity is mediated by the synergistic interaction of two domains designated AF1 and AF2. The function of AF2 is to recruit coactivator and corepressor proteins that allow ERα to oscillate between the roles of transcriptional activator and repressor. In contrast, the mechanism responsible for AF-1 transcriptional activity is not completely understood. In this study, we identified tristetraproline (TTP) as a novel ERα-associated protein. TTP expression in MCF7 cells repressed ERα transactivation and reduced MCF7 cell proliferation and the ability of the cells to form tumors in a mouse model. We show that TTP transcriptional activity is mediated through its recruitment to the promoter region of ERα target genes and its interaction with histone deacetylases, in particular with HDAC1. TTP expression attenuates the coactivating activity of SRC-1, suggesting that exchange between TTP and other coactivators may play an important role in fine-tuning ERα transactivation. These results indicate that TTP acts as a bona fide ERα corepressor and suggest that this protein may be a contributing factor in the development of E2-dependent tumors in breast cancer.

Trafficking and chromatin dynamics of holocarboxylase synthetase during development of Drosophila melanogaster

This work examines the cellular localization of holocarboxylase synthetase (HCS) and its association to chromatin during different stages of development of Drosophila melanogaster. While HCS is well known for its role in the attachment of biotin to biotin-dependent carboxylase, it also regulates the transcription of HCS and carboxylases genes by triggering a cGMP-dependent signal transduction cascade. Further, its presence in the nucleus of cells suggests additional regulatory roles, but the mechanism involved has remained elusive. In this study, we show in D. melanogaster that HCS migrates to the nucleus at the gastrulation stage. In polytene chromosomes, it is associated to heterochromatin bands where it co-localizes with histone 3 trimethylated at lysine 9 (H3K9met3) but not with the euchromatin mark histone 3 acetylated at lysine 9 (H3K9ac). Further, we demonstrate the association of HCS with the hsp70 promoter by immunofluorescence and chromatin immuno-precipitation (ChIP) of associated DNA sequences. We demonstrate the occupancy of HCS to the core promoter region of the transcriptionally inactive hsp70 gene. On heat-shock activation of the hsp70 promoter, HCS is displaced and the promoter region becomes enriched with the TFIIH subunits XPD and XPB and elongating RNA pol II, the latter also demonstrated using ChIP assays. We suggest that HCS may have a role in the repression of gene expression through a mechanism involving its trafficking to the nucleus and interaction with heterochromatic sites coincident with H3K9met3.

Annexin VI is a mannose-6-phosphate-independent endocytic receptor for bovine β-glucuronidase

Endocytosis and transport of bovine liver β-glucuronidase to lysosomes in human fibroblasts are mediated by two receptors: the well-characterized cation-independent mannose 6-phosphate receptor (IGF-II/Man6PR) and an IGF-II/Man6PR-independent receptor, which recognizes a Ser-Trp*-Ser sequence present on the ligand. The latter receptor was detergent extracted from bovine liver membranes and purified. LC/ESI-MS/MS analysis revealed that this endocytic receptor was annexin VI (AnxA6). Several approaches were used to confirm this finding. First, the binding of bovine β-glucuronidase to the purified receptor from bovine liver membranes and His-tagged recombinant human AnxA6 protein was confirmed using ligand-blotting assays. Second, western blot analysis using antibodies raised against IGF-II/Man6PR-independent receptor as well as commercial antibodies against AnxA6 confirmed that the receptor and AnxA6 were indeed the same protein. Third, double immunofluorescence experiments in human fibroblasts confirmed a complete colocalization of the bovine β-glucuronidase and the AnxA6 receptor on the plasma membrane. Lastly, two cell lines were stably transfected with a plasmid containing the cDNA for human AnxA6. In both transfected cell lines, an increase in cell surface AnxA6 and in mannose 6-phosphate-independent endocytosis of bovine β-glucuronidase was detected. These results indicate that AnxA6 is a novel receptor that mediates the endocytosis of the bovine β-glucuronidase.

Functional and metabolic implications of biotin deficiency for the rat heart

The tricarboxylic acid (TCA) cycle is the main ATP provider for the heart. TCA carbons must be replenished by anaplerosis for normal cardiac function. Biotin is cofactor of the anaplerotic enzymes pyruvate and propionyl-CoA carboxylases. Here, we found that in biotin deficient rats, both carboxylases decreased 90% in adipose tissue, jejunum and spleen, but in heart they conserved about 60% residual activity. We then investigated if under biotin deficiency (BtDEF), the heart is able to maintain its function in vivo and in isolated conditions, and during ischemia and reperfusion, where metabolism drastically shifts from oxidative to mainly glycolytic. Neither glucose nor octanoate oxidation were severely affected in BtDEF hearts, as assessed by mechanical performance, oxygen uptake or high-energy metabolite content; however, myocardial hexokinase activity and lactate concentration were reduced in deficient hearts. When challenged by ischemia and reperfusion injury, BtDEF hearts did not suffer more damage than the controls, although they lowered significantly their performance, when changed to ischemic conditions, which may have clinical implications. Post-ischemic increase in ADP/ATP ratio was similar in both groups, but during reperfusion there was higher rhythm perturbation in BtDEF hearts. By being relatively insensitive to biotin deficiency, cardiac tissue seems to be able to replenish TCA cycle intermediates and to maintain ATP synthesis.

Management of a patient with holocarboxylase synthetase deficiency

We investigated in a patient with holocarboxylase synthetase deficiency, the relation between the biochemical and genetic factors of the mutant protein with the pharmacokinetic factors of successful biotin treatment. A girl exhibited abnormal skin at birth, and developed in the first days of life neonatal respiratory distress syndrome and metabolic abnormalities diagnostic of multiple carboxylase deficiency. Enzyme assays showed low carboxylase activities. Fibroblast analysis showed poor incorporation of biotin into the carboxylases, and low transfer of biotin by the holocarboxylase synthetase enzyme. Kinetic studies identified an increased Km but a preserved Vmax. Mutation analysis showed the child to be a compound heterozygote for a new nonsense mutation Q379X and for a novel missense mutation Y663H. This mutation affects a conserved amino acid, which is located the most 3' of all recorded missense mutations thus far described, and extends the region of functional biotin interaction. Treatment with biotin 100mg/day gradually improved the biochemical abnormalities in blood and in cerebrospinal fluid (CSF), corrected the carboxylase enzyme activities, and provided clinical stability and a normal neurodevelopmental outcome. Plasma concentrations of biotin were increased to more than 500 nM, thus exceeding the increased Km of the mutant enzyme. At these pharmacological concentrations, the CSF biotin concentration was half the concentration in blood. Measuring these pharmacokinetic variables can aid in optimizing treatment, as individual tailoring of dosing to the needs of the mutation may be required.

Impaired biotinidase activity disrupts holocarboxylase synthetase expression in late onset multiple carboxylase deficiency

Biotinidase catalyzes the hydrolysis of the vitamin biotin from proteolytically degraded biotin-dependent carboxylases. This key reaction makes the biotin available for reutilization in the biotinylation of newly synthesized apocarboxylases. This latter reaction is catalyzed by holocarboxylase synthetase (HCS) via synthesis of 5'-biotinyl-AMP (B-AMP) from biotin and ATP, followed by transfer of the biotin to a specific lysine residue of the apocarboxylase substrate. In addition to carboxylase activation, B-AMP is also a key regulatory molecule in the transcription of genes encoding apocarboxylases and HCS itself. In humans, genetic deficiency of HCS or biotinidase results in the life-threatening disorder biotin-responsive multiple carboxylase deficiency, characterized by a reduction in the activities of all biotin-dependent carboxylases. Although the clinical manifestations of both disorders are similar, they differ in some unique neurological characteristics whose origin is not fully understood. In this study, we show that biotinidase deficiency not only reduces net carboxylase biotinylation, but it also impairs the expression of carboxylases and HCS by interfering with the B-AMP-dependent mechanism of transcription control. We propose that biotinidase-deficient patients may develop a secondary HCS deficiency disrupting the altruistic tissue-specific biotin allocation mechanism that protects brain metabolism during biotin starvation.

Biotin availability regulates expression of the sodium-dependent multivitamin transporter and the rate of biotin uptake in HepG2 cells

In human cells, biotin is essential to maintain metabolic homeostasis and as regulator of gene expression. The enzyme holocarboxylase synthetase (HCS) transforms biotin into its active form 5'-biotinyl-AMP and this compound is used to biotinylate five biotin-dependent carboxylases or to activate a soluble guanylate cyclase (sGC) and a cGMP-dependent protein kinase (PKG). The HCS-sGC-PKG pathway is responsible for maintaining the mRNA levels of enzymes involved in biotin utilization including HCS, carboxylases, and a biotin carrier known as sodium-dependent multivitamin transporter (SMVT). To understand the role of SMVT in the control of biotin utilization, we have studied the effect of biotin availability on SMVT protein and mRNA expression levels in HepG2 cells by Western blot analysis and rtPCR, respectively; and their functional impact on the rate of [3H]biotin uptake in human cells. Our results showed that human HepG2 cells grown in a biotin-deficient medium have a lower rate of biotin uptake than normal cells. The impairment in biotin uptake is associated with a reduction in the amount of both SMVT protein mass and mRNA levels. Transfection of HepG2 cells with a vector containing a luciferase reporter gene under the control of the rat SMVT promoter demonstrated that its transcriptional activity is regulated by biotin availability through activation of the HCS-sGC-PKG pathway. Our results support the proposed role of SMVT in the altruistic regulation of biotin utilization in liver cells that has been associated with sparing biotin depletion of the brain.

Biotin-dependent regulation of gene expression in human cells

The role of biotin as cofactor of carboxylases and its importance in metabolic homeostasis are well known. In recent years, different researchers have suggested the participation of biotin as a regulator molecule in the control of gene expression. Biotin-dependent gene expression requires of the transformation of biotin into biotinyl-5'-AMP by holocarboxylase synthetase and the activation of soluble guanylate cyclase and a cGMP-dependent protein kinase. The regulatory role of biotin is responsible for the correct expression of enzymes involved in biotin utilization in human cells. We propose that this mechanism protects the brain from biotin deficiency.

Paradoxical Regulation of Biotin Utilization in Brain and Liver and Implications for Inherited Multiple Carboxylase Deficiency

Holocarboxylase synthetase (HCS) catalyzes the biotinylation of five carboxylases in human cells, and mutations of HCS cause multiple carboxylase deficiency (MCD). Although HCS also participates in the regulation of its own mRNA levels, the relevance of this mechanism to normal metabolism or to the MCD phenotype is not known. In this study, we show that mRNA levels of enzymes involved in biotin utilization, including HCS, are down-regulated during biotin deficiency in liver while remaining constitutively expressed in brain. We propose that this mechanism of regulation is aimed at sparing the essential function of biotin in the brain at the expense of organs such as liver and kidney during biotin deprivation. In MCD, it is possible that some of the manifestations of the disease may be associated with down-regulation of biotin utilization in liver because of the impaired activity of HCS and that high dose biotin therapy may in part be important to overcoming the adverse regulatory impact in such organs.

Holocarboxylase synthetase is an obligate participant in biotin-mediated regulation of its own expression and of biotin-dependent carboxylases mRNA levels in human cells

Holocarboxylase synthetase (HCS) catalyzes the covalent attachment of biotin to five biotin-dependent carboxylases in human cells. Multiple carboxylase deficiency (MCD) is a life-threatening disease characterized by the lack of carboxylase activities because of deficiency of HCS activity. Here, we report the obligatory participation of HCS in the biotin-dependent stimulation of the level of HCS mRNA and those of acetyl-CoA carboxylase and the alpha subunit of propionyl-CoA carboxylase in human cells. Fibroblasts from patients with MCD are unable to increase HCS mRNA in response to biotin unless the vitamin concentration is raised 100-fold, in keeping with mutations that cause a reduced affinity for biotin by the mutant enzyme. The outcome is deficient synthesis of biotinyl-5'-AMP, the active form of the vitamin in the biotinylation reaction. HCS and carboxylase mRNA levels in normal and MCD fibroblasts and HepG2 cells can be restored by the addition of the cGMP analogue, 8-Br-cGMP, and can be abolished by the addition of inhibitors of the soluble form of guanylate cyclase. We propose a regulatory role for biotin in the control of HCS and carboxylase mRNA levels through a signaling cascade that requires HCS, guanylate cyclase, and cGMP-dependent protein kinase.

Association of pancreatic biotinidase activity and intestinal uptake of biotin and biocytin in hamster and rat

Pancreatic biotinidase activity was higher in hamster than in rat; these results were reversed in plasma. Uptake was studied in everted intestinal rings. Saturation kinetics at 37 degrees C were observed for biotin in hamster and for biocytin in rat, with a Vmax of 1.83 and 1.05 nmol min-1 ml-1 and an apparent Kt of 25.14 and 40.7 microM, respectively. Biotin uptake by hamster intestine was reduced at 4 degrees C and when choline or potassium replaced sodium; it was inhibited by biocytin only at very high concentrations. Biocytin uptake in the rat was small compared to passive diffusion and was not influenced by sodium or temperature; it was not inhibited by biotin. We observed only passive diffusion of biotin in rat and of biocytin in hamster. Our results suggest that protein-bound biotin may be absorbed mainly in its free form in the hamster. In the rat, on the other hand, at least part of the dietary biotin may be absorbed lysine-bound, as biocytin.