Understanding the Dosage-Dependent Role of Dicer1 in Thyroid Tumorigenesis.

Tumors originating from thyroid follicular cells are the most common endocrine tumors, with rising incidence. Despite a generally good prognosis, up to 20% of patients experience recurrence and persistence, highlighting the need to identify the underlying molecular mechanisms. Dicer1 has been found to be altered in papillary thyroid cancer (PTC). Studies suggest that Dicer1 functions as a haploinsufficient tumor suppressor gene: partial loss promotes tumorigenesis, while complete loss prevents it. To investigate the effects of partial or total Dicer1 loss in PTC in vitro, we generated stable Dicer1 (+/-) cell lines from TPC1 using CRISPR-Cas9, though no Dicer1 (-/-) lines could be produced. Therefore, siRNA against Dicer1 was transfected into Dicer1 (+/-) cell lines to further decrease its expression. Transcriptomic analysis revealed changes in proliferation and cell locomotion. BrdU staining indicated a slow-down of the cell cycle, with fewer cells in S phase and more in G0-G1-phase. Additionally, transwell assays showed decreased invasion and migration after Dicer1 knockdown by siRNA. Moreover, Dicer1 overexpression led to decreased proliferation, invasion, and increased apoptosis. Our findings deepen the understanding of Dicer1's role in thyroid cancer, demonstrating that both complete elimination and overexpression of Dicer1 inhibit thyroid oncogenesis, highlighting Dicer1 as a promising target for novel therapeutic strategies.

CDK4 phosphorylation status and rational use for combining CDK4/6 and BRAF/MEK inhibition in advanced thyroid carcinomas.

Background: CDK4/6 inhibitors (CDK4/6i) have been established as standard treatment against advanced Estrogen Receptor-positive breast cancers. These drugs are being tested against several cancers, including in combinations with other therapies. We identified the T172-phosphorylation of CDK4 as the step determining its activity, retinoblastoma protein (RB) inactivation, cell cycle commitment and sensitivity to CDK4/6i. Poorly differentiated (PDTC) and anaplastic (ATC) thyroid carcinomas, the latter considered one of the most lethal human malignancies, represent major clinical challenges. Several molecular evidence suggest that CDK4/6i could be considered for treating these advanced thyroid cancers. Methods: We analyzed by two-dimensional gel electrophoresis the CDK4 modification profile and the presence of T172-phosphorylated CDK4 in a collection of 98 fresh-frozen tissues and in 21 cell lines. A sub-cohort of samples was characterized by RNA sequencing and immunohistochemistry. Sensitivity to CDK4/6i (palbociclib and abemaciclib) was assessed by BrdU incorporation/viability assays. Treatment of cell lines with CDK4/6i and combination with BRAF/MEK inhibitors (dabrafenib/trametinib) was comprehensively evaluated by western blot, characterization of immunoprecipitated CDK4 and CDK2 complexes and clonogenic assays. Results: CDK4 phosphorylation was detected in all well-differentiated thyroid carcinomas (n=29), 19/20 PDTC, 16/23 ATC and 18/21 thyroid cancer cell lines, including 11 ATC-derived ones. Tumors and cell lines without phosphorylated CDK4 presented very high p16CDKN2A levels, which were associated with proliferative activity. Absence of CDK4 phosphorylation in cell lines was associated with CDK4/6i insensitivity. RB1 defects (the primary cause of intrinsic CDK4/6i resistance) were not found in 5/7 tumors without detectable phosphorylated CDK4. A previously developed 11-gene expression signature identified the likely unresponsive tumors, lacking CDK4 phosphorylation. In cell lines, palbociclib synergized with dabrafenib/trametinib by completely and permanently arresting proliferation. These combinations prevented resistance mechanisms induced by palbociclib, most notably Cyclin E1-CDK2 activation and a paradoxical stabilization of phosphorylated CDK4 complexes. Conclusion: Our study supports further clinical evaluation of CDK4/6i and their combination with anti-BRAF/MEK therapies as a novel effective treatment against advanced thyroid tumors. Moreover, the complementary use of our 11 genes predictor with p16/KI67 evaluation could represent a prompt tool for recognizing the intrinsically CDK4/6i insensitive patients, who are potentially better candidates to immediate chemotherapy.

Dynamic Cancer Cell Heterogeneity: Diagnostic and Therapeutic Implications.

Though heterogeneity of cancers is recognized and has been much discussed in recent years, the concept often remains overlooked in different routine examinations. Indeed, in clinical or biological articles, reviews, and textbooks, cancers and cancer cells are generally presented as evolving distinct entities rather than as an independent heterogeneous cooperative cell population with its self-oriented biology. There are, therefore, conceptual gaps which can mislead the interpretations/diagnostic and therapeutic approaches. In this short review, we wish to summarize and discuss various aspects of this dynamic evolving heterogeneity and its biological, pathological, clinical, diagnostic, and therapeutic implications, using thyroid carcinoma as an illustrative example.

Transcriptomic Signature of Human Embryonic Thyroid Reveals Transition From Differentiation to Functional Maturation.

The human thyroid gland acquires a differentiation program as early as weeks 3-4 of embryonic development. The onset of functional differentiation, which manifests by the appearance of colloid in thyroid follicles, takes place during gestation weeks 10-11. By 12-13 weeks functional differentiation is accomplished and the thyroid is capable of producing thyroid hormones although at a low level. During maturation, thyroid hormones yield increases and physiological mechanisms of thyroid hormone synthesis regulation are established. In the present work we traced the process of thyroid functional differentiation and maturation in the course of human development by performing transcriptomic analysis of human thyroids covering the period of gestation weeks 7-11 and comparing it to adult human thyroid. We obtained specific transcriptomic signatures of embryonic and adult human thyroids by comparing them to non-thyroid tissues from human embryos and adults. We defined a non-TSH (thyroid stimulating hormone) dependent transition from differentiation to maturation of thyroid. The study also sought to shed light on possible factors that could replace TSH, which is absent in this window of gestational age, to trigger transition to the emergence of thyroid function. We propose a list of possible genes that may also be involved in abnormalities in thyroid differentiation and/or maturation, hence leading to congenital hypothyroidism. To our knowledge, this study represent the first transcriptomic analysis of human embryonic thyroid and its comparison to adult thyroid.

Reprogramming of Energy Metabolism: Increased Expression and Roles of Pyruvate Carboxylase in Papillary Thyroid Cancer.

Background: Energy metabolism is described to be deregulated in cancer, and the Warburg effect is considered to be a major hallmark. Recently, cellular heterogeneity in tumors and the tumor microenvironment has been recognized to play an important role in several metabolic pathways in cancer. However, its contribution to papillary thyroid cancer (PTC) development and metabolism is still poorly understood. Methods: A proteomic analysis of five PTC was performed, and the cellular distribution of several upregulated metabolic proteins was investigated in the cancerous and stromal cells of these tumors. Results: Tandem mass spectrometry analysis revealed the upregulation of many metabolism-related proteins, among them pyruvate carboxylase (PC). PC knockdown in thyroid cell lines alters their proliferative and motility capacities, and measurements of oxygen consumption rates show that this enzyme is involved in the replenishment of the tricarboxylic acid cycle. Immunostainings of several upregulated metabolic proteins show that thyroid cancer cells have an increased mitochondrial oxidative metabolism compared to stromal cells. Conclusions: PTC has a very active tricarboxylic acid cycle, continuously replenished by a PC-mediated anaplerosis. This is specifically observed in the tumor cells.

Thyroid follicular adenomas and carcinomas: molecular profiling provides evidence for a continuous evolution.

Non-autonomous thyroid nodules are common in the general population with a proportion found to be cancerous. A current challenge in the field is to be able to distinguish benign adenoma (FA) from preoperatively malignant thyroid follicular carcinoma (FTC), which are very similar both histologically and genetically. One controversial issue, which is currently not understood, is whether both tumor types represent different molecular entities or rather a biological continuum. To gain a better insight into FA and FTC tumorigenesis, we defined their molecular profiles by mRNA and miRNA microarray. Expression data were analyzed, validated by qRT-PCR and compared with previously published data sets. The majority of deregulated mRNAs were common between FA and FTC and were downregulated, however FTC showed additional deregulated mRNA. Both types of tumors share deregulated pathways, molecular functions and biological processes. The additional deregulations in FTC include the lipid transport process that may be involved in tumor progression. The strongest candidate genes which may be able to discriminate follicular adenomas and carcinomas, CRABP1, FABP4 and HMGA2, were validated in independent samples by qRT-PCR and immunohistochemistry. However, they were not able to adequately classify FA or FTC, supporting the notion of continuous evolving tumors, whereby FA and FTC appear to show quantitative rather than qualitative changes. Conversely, miRNA expression profiles showed few dysregulations in FTC, and even fewer in FA, suggesting that miRNA play a minor, if any, role in tumor progression.

miRNA expression and function in thyroid carcinomas: a comparative and critical analysis and a model for other cancers.

As in many cancer types, miRNA expression profiles and functions have become an important field of research on non-medullary thyroid carcinomas, the most common endocrine cancers. This could lead to the establishment of new diagnostic tests and new cancer therapies. However, different studies showed important variations in their research strategies and results. In addition, the action of miRNAs is poorly considered as a whole because of the use of underlying dogmatic truncated concepts. These lead to discrepancies and limits rarely considered. Recently, this field has been enlarged by new miRNA functional and expression studies. Moreover, studies using next generation sequencing give a new view of general miRNA differential expression profiles of papillary thyroid carcinoma. We analyzed in detail this literature from both physiological and differential expression points of view. Based on explicit examples, we reviewed the progresses but also the discrepancies and limits trying to provide a critical approach of where this literature may lead. We also provide recommendations for future studies. The conclusions of this systematic analysis could be extended to other cancer types.

Gene signature of the post-Chernobyl papillary thyroid cancer.

PURPOSE: Following the nuclear accidents in Chernobyl and later in Fukushima, the nuclear community has been faced with important issues concerning how to search for and diagnose biological consequences of low-dose internal radiation contamination. Although after the Chernobyl accident an increase in childhood papillary thyroid cancer (PTC) was observed, it is still not clear whether the molecular biology of PTCs associated with low-dose radiation exposure differs from that of sporadic PTC. METHODS: We investigated tissue samples from 65 children/young adults with PTC using DNA microarray (Affymetrix, Human Genome U133 2.0 Plus) with the aim of identifying molecular differences between radiation-induced (exposed to Chernobyl radiation, ECR) and sporadic PTC. All participants were resident in the same region so that confounding factors related to genetics or environment were minimized. RESULTS: There were small but significant differences in the gene expression profiles between ECR and non-ECR PTC (global test, p < 0.01), with 300 differently expressed probe sets (p < 0.001) corresponding to 239 genes. Multifactorial analysis of variance showed that besides radiation exposure history, the BRAF mutation exhibited independent effects on the PTC expression profile; the histological subset and patient age at diagnosis had negligible effects. Ten genes (PPME1, HDAC11, SOCS7, CIC, THRA, ERBB2, PPP1R9A, HDGF, RAD51AP1, and CDK1) from the 19 investigated with quantitative RT-PCR were confirmed as being associated with radiation exposure in an independent, validation set of samples. CONCLUSION: Significant, but subtle, differences in gene expression in the post-Chernobyl PTC are associated with previous low-dose radiation exposure.

Cancer stem cells, a fuzzy evolving concept: a cell population or a cell property?

The cancer stem cells (CSC) hypothesis represents a pathological extrapolation of the physiological concept of embryonic and somatic stem cells. In its initial definition, it encompassed the hypothesis of a qualitatively distinct population of immortal cancer cells originating from somatic stem cells, which generate in xenotransplants by a deterministic irreversible process, the hierarchy of more differentiated finite lifespan derived cells, which constitute, themselves, the bulk of the cancer. These CSC would express specific biomarkers and gene expressions related to chemo- and radioresistance, stemness, epithelial-mesenchymal transition, etc. No convincing congruence of several of these properties in one cell population has been demonstrated. The concept has greatly evolved with time and with different authors ("the plasticity of cancer stem cells"), leading to a minimal definition of cells generating a hierarchy of derived cells. In this article these concepts are analyzed. It is proposed that stemness is a property, more or less reversible, a hallmark of some cells at some time in a cancer cell population, as immortality, dormancy, chemo- or radioresistance, epithelial-mesenchymal transition etc. These phenotypic properties represent the result of independent, linked, or more or less congruent, genetic, epigenetic, or signaling programs.

5-aza-2'-deoxycytidine has minor effects on differentiation in human thyroid cancer cell lines, but modulates genes that are involved in adaptation in vitro.

BACKGROUND: In thyroid cancer, the lack of response to specific treatment, for example, radioactive iodine, can be caused by a loss of differentiation characteristics of tumor cells. It is hypothesized that this loss is due to epigenetic modifications. Therefore, drugs releasing epigenetic repression have been proposed to reverse this silencing. METHODS: We investigated which genes were reinduced in dedifferentiated human thyroid cancer cell lines when treated with the demethylating agent 5-aza-2'-deoxycytidine (5-AzadC) and the histone deacetylase inhibitors trichostatin A (TSA) and suberoylanilide hydroxamic acid, by using reverse transcriptase-polymerase chain reaction and microarrays. These results were compared to the expression patterns in in vitro human differentiated thyrocytes and in in vivo dedifferentiated thyroid cancers. In addition, the effects of 5-AzadC on DNA quantities and cell viability were investigated. RESULTS: Among the canonical thyroid differentiation markers, most were not, or only to a minor extent, re-expressed by 5-AzadC, whether or not combined with TSA or forskolin, an inducer of differentiation in normal thyrocytes. Furthermore, 5-AzadC-modulated overall mRNA expression profiles showed only few commonly regulated genes compared to differentiated cultured primary thyrocytes. In addition, most of the commonly strongly 5-AzadC-induced genes in cell lines were either not regulated or upregulated in anaplastic thyroid carcinomas. Further analysis of which genes were induced by 5-AzadC showed that they were involved in pathways such as apoptosis, antigen presentation, defense response, and cell migration. A number of these genes had similar expression responses in 5-AzadC-treated nonthyroid cell lines. CONCLUSIONS: Our results suggest that 5-AzadC is not a strong inducer of differentiation in thyroid cancer cell lines. Under the studied conditions and with the model used, 5-AzadC treatment does not appear to be a potential redifferentiation treatment for dedifferentiated thyroid cancer. However, this may reflect primarily the inadequacy of the model rather than that of the treatment. Moreover, the observation that 5-AzadC negatively affected cell viability in cell lines could still suggest a therapeutic opportunity. Some of the genes that were modulated by 5-AzadC were also induced in nonthyroid cancer cell lines, which might be explained by an epigenetic modification resulting in the adaptation of the cell lines to their culture conditions.

mRNA expression in papillary and anaplastic thyroid carcinoma: molecular anatomy of a killing switch.

Anaplastic thyroid carcinoma (ATC) is the most lethal form of thyroid neoplasia and represents the end stage of thyroid tumor progression. No effective treatment exists so far. ATC frequently derive from papillary thyroid carcinomas (PTC), which have a good prognosis. In this study, we analyzed the mRNA expression profiles of 59 thyroid tumors (11 ATC and 48 PTC) by microarrays. ATC and PTC showed largely overlapping mRNA expression profiles with most genes regulated in all ATC being also regulated in several PTC. 43% of the probes regulated in all the PTC are similarly regulated in all ATC. Many genes modulations observed in PTC are amplified in ATC. This illustrates the fact that ATC mostly derived from PTC. A molecular signature of aggressiveness composed of 9 genes clearly separates the two tumors. Moreover, this study demonstrates gene regulations corresponding to the ATC or PTC phenotypes like inflammatory reaction, epithelial to mesenchymal transition (EMT) and invasion, high proliferation rate, dedifferentiation, calcification and fibrosis processes, high glucose metabolism and glycolysis, lactate generation and chemoresistance. The main qualitative differences between the two tumor types bear on the much stronger EMT, dedifferentiation and glycolytic phenotypes showed by the ATC.

Thyroid cancer cell lines: an overview.

Human thyroid cancer cell lines are the most used models for thyroid cancer studies. They must be used with detailed knowledge of their characteristics. These in vitro cell lines originate from differentiated and dedifferentiated in vivo human thyroid tumors. However, it has been shown that mRNA expression profiles of these cell lines were closer to dedifferentiated in vivo thyroid tumors (anaplastic thyroid carcinoma, ATC) than to differentiated ones. Here an overview of the knowledge of these models was made. The mutational status of six human thyroid cancer cell lines (WRO, FTC133, BCPAP, TPC1, K1, and 8505C) was in line with previously reported findings for 10 genes frequently mutated in thyroid cancer. However, the presence of a BRAF mutation (T1799A: V600E) in WRO questions the use of this cell line as a model for follicular thyroid carcinoma (FTC). Next, to investigate the biological meaning of the modulated mRNAs in these cells, a pathway analysis on previously obtained mRNA profiles was performed on five cell lines. In five cell lines, the MHC class II pathway was down-regulated and in four of them, ribosome biosynthesis and translation pathways were up-regulated. mRNA expression profiles of the cell lines were also compared to those of the different types of thyroid cancers. Three datasets originating from different microarray platforms and derived from distinct laboratories were used. This meta-analysis showed a significant higher correlation between the profiles of the thyroid cancer cell lines and ATC, than to differentiated thyroid tumors (i.e., PTC or FTC) specifically for DNA replication. This already observed higher correlation was obtained here with an increased number of in vivo tumors and using different platforms. In summary, this would suggest that some papillary thyroid carcinoma or follicular thyroid carcinoma (PTC or FTC) cell lines (i.e., TPC-1) might have partially lost their original DNA synthesis/replication regulation mechanisms during their in vitro cell adaptation/evolution.

Role of Epac and protein kinase A in thyrotropin-induced gene expression in primary thyrocytes.

cAMP pathway activation by thyrotropin (TSH) induces differentiation and gene expression in thyrocytes. We investigated which partners of the cAMP cascade regulate gene expression modulations: protein kinase A and/or the exchange proteins directly activated by cAMP (Epac). Human primary cultured thyrocytes were analysed by microarrays after treatment with the adenylate cyclase activator forskolin, the protein kinase A (PKA) activator 6-MB-cAMP and the Epac-selective cAMP analog 8-pCPT-2'-O-Me-cAMP (007) alone or combined with 6-MB-cAMP. Profiles were compared to those of TSH. Cultures treated with the adenylate cyclase- or the PKA activator alone or the latter combined with 007 had profiles similar to those induced by TSH. mRNA profiles of 007-treated cultures were highly distinct from TSH-treated cells, suggesting that TSH-modulated gene expressions are mainly modulated by cAMP and PKA and not through Epac in cultured human thyroid cells. To investigate whether the Epac-Rap-RapGAP pathway could play a potential role in thyroid tumorigenesis, the mRNA expressions of its constituent proteins were investigated in two malignant thyroid tumor types. Modulations of this pathway suggest an increased Rap pathway activity in these cancers independent from cAMP activation.

Endocytosis provides a major alternative pathway for lysosomal biogenesis in kidney proximal tubular cells.

Recruitment of acid hydrolases to lysosomes generally occurs by intracellular sorting based on recognition of a common mannose 6-phosphate signal in the transGolgi network and selective transport to late endosomes/lysosomes. Here we provide evidence for an alternative, efficient secretion-recapture pathway mediated by megalin and exemplified by cathepsin B in kidney proximal convoluted tubules (PCT). We found that in mouse kidneys with defective megalin expression [megalin knockout (KO)] or apical PCT trafficking (ClC-5 KO), the (pro)cathepsin B mRNA level was essentially preserved, but the protein content was greatly decreased and the enzyme was excreted in the urine as mannose 6-phosphate-devoid species. In polarized PCT-derived cells, purified cathepsin B was avidly and selectively taken up at the apical membrane, and uptake was abolished by the megalin competitor, receptor-associated protein. Direct interaction of cathepsin B with megalin was demonstrated by surface plasmon resonance. Procathepsin B was detected in normal mouse serum. Purified cathepsin B injected into mice was efficiently taken up by kidneys (approximately 10% of injection) and targeted to lysosomes where it remained active, as shown by autoradiography and subcellular fractionation. A single cathepsin B injection into cathepsin B KO mice could reconstitute full lysosomal enzyme activity in the kidneys. These findings demonstrate a pathway whereby circulating lysosomal enzymes are continuously filtered in glomeruli, reabsorbed by megalin-mediated endocytosis, and transferred into lysosomes to exert their function, providing a major source of enzymes to PCT. These results also extend the significance of megalin in PCT and have several physiopathological and clinical implications.

Cystic fibrosis is associated with a defect in apical receptor-mediated endocytosis in mouse and human kidney.

Inactivation of the chloride channel cystic fibrosis transmembrane conductance regulator (CFTR) causes cystic fibrosis (CF). Although CFTR is expressed in the kidney, no overwhelming renal phenotype has been documented in patients with CF. This study investigated the expression, subcellular distribution, and processing of CFTR in the kidney; used various mouse models to assess the role of CFTR in proximal tubule (PT) endocytosis; and tested the relevance of these findings in patients with CF. The level of CFTR mRNA in mouse kidney approached that found in lung. CFTR was located in the apical area of PT cells, with a maximal intensity in the straight part (S3) of the PT. Fractionation showed that CFTR co-distributed with the chloride/proton exchanger ClC-5 in PT endosomes. Cftr(-/-) mice showed impaired (125)I-beta(2)-microglobulin uptake, together with a decreased amount of the multiligand receptor cubilin in the S3 segment and a significant loss of cubilin and its low molecular weight (LMW) ligands into the urine. Defective receptor-mediated endocytosis was found less consistently in Cftr(DeltaF/DeltaF) mice, characterized by a large phenotypic heterogeneity and moderate versus mice that lacked ClC-5. A significant LMW proteinuria (and particularly transferrinuria) also was documented in a cohort of patients with CF but not in patients with asthma and chronic lung inflammation. In conclusion, CFTR inactivation leads to a moderate defect in receptor-mediated PT endocytosis, associated with a cubilin defect and a significant LMW proteinuria in mouse and human. The magnitude of the endocytosis defect that is caused by CFTR versus ClC-5 loss likely reflects functional heterogeneity along the PT.

Loss of chloride channel ClC-5 impairs endocytosis by defective trafficking of megalin and cubilin in kidney proximal tubules.

Loss of the renal endosome-associated chloride channel, ClC-5, in Dent's disease and knockout (KO) mice strongly inhibits endocytosis of filtered proteins by kidney proximal tubular cells (PTC). The underlying mechanism remains unknown. We therefore tested whether this endocytic failure could primarily reflect a loss of reabsorption by the multiligand receptors, megalin, and cubilin, caused by a trafficking defect. Impaired protein endocytosis in PTC of ClC-5 KO mice was demonstrated by (i) a major decreased uptake of injected 125I-beta 2-microglobulin, but not of the fluid-phase tracer, FITC-dextran, (ii) reduced labeling of endosomes by injected peroxidase and for the endogenous megalin/cubilin ligands, vitamin D- and retinol-binding proteins, and (iii) urinary appearance of low-molecular-weight proteins and the selective cubilin ligand, transferrin. Contrasting with preserved mRNA levels, megalin and cubilin abundance was significantly decreased in kidney extracts of KO mice. Percoll gradients resolving early and late endosomes (Rab5a, Rab7), brush border (villin, aminopeptidase M), and a dense peak comprising lysosomes (acid hydrolases) showed a disappearance of the brush border component for megalin and cubilin in KO mice. Quantitative ultrastructural immunogold labeling confirmed the overall decrease of megalin and cubilin in PTC and their selective loss at the brush border. In contrast, total contents of the rate-limiting endocytic catalysts, Rab5a and Rab7, were unaffected. Thus, impaired protein endocytosis caused by invalidation of ClC-5 primarily reflects a trafficking defect of megalin and cubilin in PTC.

Cellular uptake of Antennapedia Penetratin peptides is a two-step process in which phase transfer precedes a tryptophan-dependent translocation.

Several homeodomains and homeodomain-containing proteins enter live cells through a receptor- and energy-independent mechanism. Translocation through biological membranes is conferred by the third alpha-helix of the homeodomain, also known as Penetratin. Biophysical studies demonstrate that entry of Penetratin into cells requires its binding to surface lipids but that binding and translocation are differentially affected by modifications of some physico-chemical properties of the peptide, like helical amphipathicity or net charge. This suggests that the plasma membrane lipid composition affects the internalization of Penetratin and that internalization requires both lipid binding and other specific properties. Using a phase transfer assay, it is shown that negatively charged lipids promote the transfer of Penetratin from a hydrophilic into a hydrophobic environment, probably through charge neutralization. Accordingly, transfer into a hydrophobic milieu can also be obtained in the absence of negatively charged lipids, by the addition of DNA oligonucleotides. Strikingly, phase transfer by charge neutralization was also observed with a variant peptide of same charge and hydrophobicity in which the tryptophan at position 6 was replaced by a phenylalanine. However, Penetratin, but not its mutant version, is internalized by live cells. This underscores that charge neutralization and phase transfer represent only a first step in the internalization process and that further crossing of a biological membrane necessitates the critical tryptophan residue at position 6.