Quantitative analysis of the anti-proliferative activity of combinations of selected iron-chelating agents and clinically used anti-neoplastic drugs

Isosteric Replacement of Sulfur to Selenium in a Thiosemicarbazone: Promotion of Zn(II) Complex Dissociation and Transmetalation to Augment Anticancer Efficacy

We implemented isosteric replacement of sulfur to selenium in a novel thiosemicarbazone (PPTP4c4mT) to create a selenosemicarbazone (PPTP4c4mSe) that demonstrates potentiated anticancer efficacy and selectivity. Their design specifically incorporated cyclohexyl and styryl moieties to sterically inhibit the approach of their Fe(III) complexes to the oxy-myoglobin heme plane. Importantly, in contrast to the Fe(III) complexes of the clinically trialed thiosemicarbazones Triapine, COTI-2, and DpC, the Fe(III) complexes of PPTP4c4mT and PPTP4c4mSe did not induce detrimental oxy-myoglobin oxidation. Furthermore, PPTP4c4mSe demonstrated more potent antiproliferative activity than the homologous thiosemicarbazone, PPTP4c4mT, with their selectivity being superior or similar, respectively, to the clinically trialed thiosemicarbazone, COTI-2. An advantageous property of the selenosemicarbazone Zn(II) complexes relative to their thiosemicarbazone analogues was their greater transmetalation to Cu(II) complexes in lysosomes. This latter effect probably promoted their antiproliferative activity. Both ligands down-regulated multiple key receptors that display inter-receptor cooperation that leads to aggressive and resistant breast cancer.

Innovative Thiosemicarbazones that Induce Multi-Modal Mechanisms to Down-Regulate Estrogen-, Progesterone-, Androgen- and Prolactin-Receptors in Breast Cancer

The estrogen receptor-α (ER-α) is a key driver of breast cancer (BC) and the ER-antagonist, tamoxifen, is a central pillar of BC treatment. However, cross-talk between ER-α, other hormone and growth factor receptors enables development of de novo resistance to tamoxifen. Herein, we mechanistically dissect the activity of a new class of anti-cancer agents that inhibit multiple growth factor receptors and down-stream signaling for the treatment of ER-positive BC. Using RNA sequencing and comprehensive protein expression analysis, we examined the activity of di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT) and di-2-pyridylketone-4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC), on the expression and activation of hormone and growth factor receptors, co-factors, and key resistance pathways in ER-α-positive BC. DpC differentially regulated 106 estrogen-response genes, and this was linked to decreased mRNA levels of 4 central hormone receptors involved in BC pathogenesis, namely ER, progesterone receptor (PR), androgen receptor (AR), and prolactin receptor (PRL-R). Mechanistic investigation demonstrated that due to DpC and Dp44mT binding metal ions, these agents caused a pronounced decrease in ER-α, AR, PR, and PRL-R protein expression. DpC and Dp44mT also inhibited activation and down-stream signaling of the epidermal growth factor (EGF) family receptors, and expression of co-factors that promote ER-α transcriptional activity, including SRC3, NF-κB p65, and SP1. In vivo, DpC was highly tolerable and effectively inhibited ER-α-positive BC growth. Through bespoke, non-hormonal, multi-modal mechanisms, Dp44mT and DpC decrease the expression of PR, AR, PRL-R, and tyrosine kinases that act with ER-α to promote BC, constituting an innovative therapeutic approach.

Developing an Anticancer Platinum(II) Compound Based on the Uniqueness of Human Serum Albumin

To develop the next-generation Pt drug with remarkable activity and low toxicity to maximally inhibit tumor growth, we optimized a Pt(II) thiosemicarbazone compound (C4) with remarkable cytotoxicity to SK-N-MC cells and then constructed a new human serum albumin-C4 (HSA-C4) complex delivery system. The in vivo results showed that C4 and the HSA-C4 complex have remarkable therapeutic efficiency and almost no toxicity; they induced apoptosis and inhibited tumor angiogenesis. This system showed potential as a practical Pt drug. This study could pave the way for developing next-generation dual-targeted Pt drugs and achieving their targeting therapy for cancer.

Targeting critical pathways in ferroptosis and enhancing antitumor therapy of Platinum drugs for colorectal cancer

Colorectal cancer (CRC) can be resistant to platinum drugs, possibly through ferroptosis suppression, albeit the need for further work to completely understand this mechanism. This work aimed to sum up current findings pertaining to oxaliplatin resistance (OR) or resistance to ascertain the potential of ferroptosis to regulate oxaliplatin effects. In this review, tumor development relating to iron homeostasis, which includes levels of iron that ascertain cells' sensitivity to ferroptosis, oxidative stress, or lipid peroxidation in colorectal tumor cells that are connected with ferroptosis initiation, especially the role of c-Myc/NRF2 signaling in regulating iron homeostasis, coupled with NRF2/GPX4-mediated ferroptosis are discussed. Importantly, ferroptosis plays a key role in OR and ferroptotic induction may substantially reverse OR in CRC cells, which in turn could inhibit the imbalance of intracellular redox induced by oxaliplatin and ferroptosis, as well as cause chemotherapeutic resistance in CRC. Furthermore, fundamental research of small molecules, ferroptosis inducers, GPX4 inhibitors, or natural products for OR coupled with their clinical applications in CRC have also been summarized. Also, potential molecular targets and mechanisms of small molecules or drugs are discussed as well. Suggestively, OR of CRC cells could significantly be reversed by ferroptosis induction, wherein this result is discussed in the current review. Prospectively, the existing literature discussed in this review will provide a solid foundation for scientists to research the potential use of combined anticancer drugs which can overcome OR via targeting various mechanisms of ferroptosis. Especially, promising therapeutic strategies, challenges ,and opportunities for CRC therapy will be discussed.

Thiosemicarbazones and selected tyrosine kinase inhibitors synergize in pediatric solid tumors: NDRG1 upregulation and impaired prosurvival signaling in neuroblastoma cells

Tyrosine kinase inhibitors (TKIs) are frequently used in combined therapy to enhance treatment efficacy and overcome drug resistance. The present study analyzed the effects of three inhibitors, sunitinib, gefitinib, and lapatinib, combined with iron-chelating agents, di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT) or di-2-pyridylketone-4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC). Simultaneous administration of the drugs consistently resulted in synergistic and/or additive activities against the cell lines derived from the most frequent types of pediatric solid tumors. The results of a detailed analysis of cell signaling in the neuroblastoma cell lines revealed that TKIs inhibited the phosphorylation of the corresponding receptor tyrosine kinases, and thiosemicarbazones downregulated the expression of epidermal growth factor receptor, platelet-derived growth factor receptor, and insulin-like growth factor-1 receptor, leading to a strong induction of apoptosis. Marked upregulation of the metastasis suppressor N-myc downstream regulated gene-1 (NDRG1), which is known to be activated and upregulated by thiosemicarbazones in adult cancers, was also detected in thiosemicarbazone-treated neuroblastoma cells. Importantly, these effects were more pronounced in the cells treated with drug combinations, especially with the combinations of lapatinib with thiosemicarbazones. Therefore, these results provide a rationale for novel strategies combining iron-chelating agents with TKIs in therapy of pediatric solid tumors.

Puzzling Out Iron Complications in Cancer Drug Resistance

Iron metabolism are frequently disrupted in cancer. Patients with cancer are prone to anemia and receive transfusions frequently; the condition which results in iron overload, contributing to serious therapeutic complications. Iron is introduced as a carcinogen that may increase tumor growth. However, investigations regarding its impact on response to chemotherapy, particularly the induction of drug resistance are still limited. Here, iron contribution to cell signaling and various molecular mechanisms underlying iron-mediated drug resistance are described. A dual role of this vital element in cancer treatment is also addressed. On one hand, the need to administer iron chelators to surmount iron overload and improve the sensitivity of tumor cells to chemotherapy is discussed. On the other hand, the necessary application of iron as a therapeutic option by iron-oxide nanoparticles or ferroptosis inducers is explained. Authors hope that this paper can help unravel the clinical complications related to iron in cancer therapy.

Targeting iron metabolism in cancer therapy

Iron is a critical component of many cellular functions including DNA replication and repair, and it is essential for cell vitality. As an essential element, iron is critical for maintaining human health. However, excess iron can be highly toxic, resulting in oxidative DNA damage. Many studies have observed significant associations between iron and cancer, and the association appears to be more than just coincidental. The chief characteristic of cancers, hyper-proliferation, makes them even more dependent on iron than normal cells. Cancer therapeutics are becoming as diverse as the disease itself. Targeting iron metabolism in cancer cells is an emerging, formidable field of therapeutics. It is a strategy that is highly diverse with regard to specific targets and the various ways to reach them. This review will discuss the importance of iron metabolism in cancer and highlight the ways in which it is being explored as the medicine of tomorrow.

Building up PtII -Thiosemicarbazone-Lysine-sC18 Conjugates

Three chiral tridentate N^N^S coordinating pyridine-carbaldehyde (S)-N4-(α-methylbenzyl)thiosemicarbazones (HTSCmB) were synthesised along with lysine-modified derivatives. One of them was selected and covalently conjugated to the cell-penetrating peptide sC18 by solid-phase peptide synthesis. The HTSCmB model ligands, the HTSCLp derivatives and the peptide conjugate rapidly and quantitatively form very stable PtII chlorido complexes [Pt(TSC)Cl] when treated with K2 PtCl4 in solution. The Pt(CN) derivatives were obtained from one TSCmB model complex and the peptide conjugate complex through Cl- →CN- exchange. Ligands and complexes were characterised by NMR, IR spectroscopy, HR-ESI-MS and single-crystal XRD. Intriguingly, no decrease in cell viability was observed when testing the biological activity of the lysine-tagged HdpyTSCLp, its sC18 conjugate HdpyTSCL-sC18 or the PtCl and Pt(CN) conjugate complexes in three different cell lines. Thus, given the facile and effective preparation of such Pt-TSC-peptide conjugates, these systems might pave the way for future use in late-stage labelling with Pt radionuclides and application in nuclear medicine.

Novel Thiosemicarbazones Sensitize Pediatric Solid Tumor Cell-Types to Conventional Chemotherapeutics through Multiple Molecular Mechanisms

Simple Summary

Combination of chemotherapeutics for the treatment of childhood cancer can lead to the use of lower cytotoxic drug doses and better therapeutic tolerability (i.e., lower side effects) for patients. We discovered novel molecular targets of two lead thiosemicarbazone agents of the di-2-pyridylketone thiosemicarbazone class. These molecular targets include: cyclooxygenase, the DNA repair protein, O6-methylguanine DNA methyltransferase, mismatch repair proteins, and topoisomerase 2α. This research also identifies promising synergistic interactions of these thiosemicarbazones particularly with the standard chemotherapeutic, celecoxib.

Abstract

Combining low-dose chemotherapies is a strategy for designing less toxic and more potent childhood cancer treatments. We examined the effects of combining the novel thiosemicarbazones, di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC), or its analog, di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT), with the standard chemotherapies, celecoxib (CX), etoposide (ETO), or temozolomide (TMZ). These combinations were analyzed for synergism to inhibit proliferation of three pediatric tumor cell-types, namely osteosarcoma (Saos-2), medulloblastoma (Daoy) and neuroblastoma (SH-SY5Y). In terms of mechanistic dissection, this study discovered novel thiosemicarbazone targets not previously identified and which are important for considering possible drug combinations. In this case, DpC and Dp44mT caused: (1) up-regulation of a major protein target of CX, namely cyclooxygenase-2 (COX-2); (2) down-regulation of the DNA repair protein, O⁶-methylguanine DNA methyltransferase (MGMT), which is known to affect TMZ resistance; (3) down-regulation of mismatch repair (MMR) proteins, MSH2 and MSH6, in Daoy and SH-SY5Y cells; and (4) down-regulation in all three cell-types of the MMR repair protein, MLH1, and also topoisomerase 2α (Topo2α), the latter of which is an ETO target. While thiosemicarbazones up-regulate the metastasis suppressor, NDRG1, in adult cancers, it is demonstrated herein for the first time that they induce NDRG1 in all three pediatric tumor cell-types, validating its role as a potential target. In fact, siRNA studies indicated that NDRG1 was responsible for MGMT down-regulation that may prevent TMZ resistance. Examining the effects of combining thiosemicarbazones with CX, ETO, or TMZ, the most promising synergism was obtained using CX. Of interest, a positive relationship was observed between NDRG1 expression of the cell-type and the synergistic activity observed in the combination of thiosemicarbazones and CX. These studies identify novel thiosemicarbazone targets relevant to childhood cancer combination chemotherapy.

Patterns of cell death induced by metformin in human MCF-7 breast cancer cells

The ability to evade apoptosis is an important mechanism of drug resistance and tumor progression in breast cancer. The induction of different pathways of cell death could be an important strategy to limit tumor progression. Metformin, a drug used to treat type two diabetes, has demonstrated promising results in breast cancer experiments. However, little is known about the patterns of cell death induced by this drug. We analyzed the involvement of apoptosis, necroptosis and ferroptosis in the toxicity of metformin in MCF-7 cells, evaluating proliferation, viability and oxidative stress. It was used different inhibitors of cell death: Z-VAD, a pan-caspase inhibitor that blocks apoptosis; Necrostatin-1, which inhibits RIPK1 activity and blocks necroptosis; and the iron chelator, deferoxamine, that chelates iron and prevents ferroptosis. The participation of oxidative stress was analyzed through the evaluation of total thiols, reduced glutathione (GSH) and malondialdehyde (MDA). Our results showed that metformin increased cell death, reduced proliferation, thiol and GSH and increased MDA in cells. After the association between metformin and Z-VAD or Necrostatin-1, the drug toxicity was abolished. Ferroptosis did not significantly enrolled in metformin action against MCF-7 cells. The preservation of cellular antioxidants was found in all situations that cell death was blocked. Together, these results reveals that metformin induces necroptosis and apoptosis in MCF-7 cells and oxidative stress generation play a role in these two pathways of cell death. This information could help future studies to improve strategies to breast cancer treatment.

Altered Iron Metabolism and Impact in Cancer Biology, Metastasis, and Immunology

Iron is an essential nutrient that plays a complex role in cancer biology. Iron metabolism must be tightly controlled within cells. Whilst fundamental to many cellular processes and required for cell survival, excess labile iron is toxic to cells. Increased iron metabolism is associated with malignant transformation, cancer progression, drug resistance and immune evasion. Depleting intracellular iron stores, either with the use of iron chelating agents or mimicking endogenous regulation mechanisms, such as microRNAs, present attractive therapeutic opportunities, some of which are currently under clinical investigation. Alternatively, iron overload can result in a form of regulated cell death, ferroptosis, which can be activated in cancer cells presenting an alternative anti-cancer strategy. This review focuses on alterations in iron metabolism that enable cancer cells to meet metabolic demands required during different stages of tumorigenesis in relation to metastasis and immune response. The strength of current evidence is considered, gaps in knowledge are highlighted and controversies relating to the role of iron and therapeutic targeting potential are discussed. The key question we address within this review is whether iron modulation represents a useful approach for treating metastatic disease and whether it could be employed in combination with existing targeted drugs and immune-based therapies to enhance their efficacy.

The iron chelator deferasirox synergises with chemotherapy to treat triple-negative breast cancers

To ensure their high proliferation rate, tumor cells have an iron metabolic disorder causing them to have increased iron needs, making them more susceptible to iron deprivation. This vulnerability could be a therapeutic target. In breast cancers, the development of new therapeutic approaches is urgently needed for patients with triple-negative tumors, which frequently relapse after chemotherapy and suffer from a lack of targeted therapies. In this study, we demonstrated that deferasirox (DFX) synergises with standard chemotherapeutic agents such as doxorubicin, cisplatin and carboplatin to inhibit cell proliferation and induce apoptosis and autophagy in triple-negative breast cancer (TNBC) cells. Moreover, the combination of DFX with doxorubicin and cyclophosphamide delayed recurrences in breast cancer patient-derived xenografts without increasing the side-effects of chemotherapies alone or altering the global iron storage of mice. Antitumor synergy of DFX and doxorubicin seems to involve downregulation of the phosphoinositide 3-kinase and nuclear factor-κB pathways. Iron deprivation in combination with chemotherapy could thus help to improve the effectiveness of chemotherapy in TNBC patients without increasing toxicity. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Effect of the FE2+ chelation by 2,2'-dipyridyl in the doxorubicin-induced lethality in breast tumor cell lines

Breast cancer cells may exhibit changes in iron homeostasis, which results in increased labile iron pool (LIP) levels. Several studies highlight the crucial role of high LIP levels in the maintenance of tumor cell physiology. Iron chelators have been tested in anticancer therapy in combination with chemotherapeutic agents, to improve drug efficacy. Thus, the aim of this study was to evaluate the effect of 2,2'-dipyridyl (DIP), a Fe²⁺ chelator, in combination with doxorubicin (DOX) in breast tumor cells. The maximum concentration of DIP that did not significantly reduce the viability of MDA-MB-231 cells was 10μM and for MCF-7 cells was 50μM. We observed that MCF-7 had higher LIP levels than MDA-MB-231 cells. DIP alone increased ROS generation in MCF-7 cells, and DIP pretreatment reduced ROS generation induced by DOX treatment. In conclusion, the increase in MCF-7 cell viability induced by DIP pretreatment in DOX-treated cells seems to be related to an increase in the cellular antioxidant capacity and the iron chelator did not improve drug efficacy in the two breast tumor cell lines analyzed.

Targeting cancer by binding iron: Dissecting cellular signaling pathways

Newer and more potent therapies are urgently needed to effectively treat advanced cancers that have developed resistance and metastasized. One such strategy is to target cancer cell iron metabolism, which is altered compared to normal cells and may facilitate their rapid proliferation. This is supported by studies reporting the anti-neoplastic activities of the clinically available iron chelators, desferrioxamine and deferasirox. More recently, ligands of the di-2-pyridylketone thiosemicarbazone (DpT) class have demonstrated potent and selective anti-proliferative activity across multiple cancer-types in vivo, fueling studies aimed at dissecting their molecular mechanisms of action. In the past five years alone, significant advances have been made in understanding how chelators not only modulate cellular iron metabolism, but also multiple signaling pathways implicated in tumor progression and metastasis. Herein, we discuss recent research on the targeting of iron in cancer cells, with a focus on the novel and potent DpT ligands. Several key studies have revealed that iron chelation can target the AKT, ERK, JNK, p38, STAT3, TGF-β, Wnt and autophagic pathways to subsequently inhibit cellular proliferation, the epithelial-mesenchymal transition (EMT) and metastasis. These developments emphasize that these novel therapies could be utilized clinically to effectively target cancer.

Deferoxamine-induced increase in the intracellular iron levels in highly aggressive breast cancer cells leads to increased cell migration by enhancing TNF-α-dependent NF-κB signaling and TGF-β signaling

Recent studies have suggested that excess iron accumulation may be a risk factor for breast cancer. However the role of iron in breast cancer metastasis has remained unclear. The major goal of our study is to investigate the roles of iron in breast cancer metastasis. We modulated the intracellular iron levels of human breast cancer cells, including the aggressive MDA-MB-231 cells and non-aggressive MCF-7 cells, by using Deferoxamine (DFO) - a most widely used iron chelator. We found that DFO treatment could deplete intracellular iron in MCF-7 cells. In contrast, DFO treatment led to a significant increase in the intracellular iron level in MDA-MB-231 cells. The MDA-MB-231 cells with the increased intracellular iron level exhibited increases in both mesenchymal markers and cell migration. Furthermore, the DFO-treated MDA-MB-231 cells showed increases in both tumor necrosis factor α (TNF-α)-induced nuclear factor kappa B (NF-κB) signaling and transforming growth factor-β (TGF-β) signaling, which could contribute to the enhanced cell migration. Collectively, our study has provided the first evidence suggesting that increased intracellular iron levels could lead to enhanced migration of aggressive breast cancer cells by increasing TNF-α-dependent NF-κB signaling and TGF-β signaling. Our study has also suggested that caution should be taken when DFO is applied for treating breast cancer cells, since DFO could produce differential effects on the intracellular iron levels for aggressive breast cancer cells and non-aggressive breast cancer cells.

A sensitive liquid chromatography-mass spectrometry bioanalytical assay for a novel anticancer candidate--ZMC1

ZMC1 {azetidinecarbothioic acid, [1-(2-pyridinyl) ethylidene] hydrazide} is a lead compound being developed as one of the first mutant p53 targeted anti-cancer drugs. Establishing a precise quantitative method is an integral component of this development. The aim of this study was to develop a sensitive LC/MS/MS assay suitable for assessing purity, stability and preclinical pharmacokinetic studies of ZMC1. Acetonitrile protein precipitation extraction was chosen for plasma sample preparation with satisfactory recovery (84.2-92.8%) for ZMC1. Chromatographic separation was achieved on an Xterra C18 column (50 × 4.6 mm, 3.5 µm) using a gradient elution with mobile phase of 0.1% formic acid in water and acetonitrile. ZMC1 and internal standard 2-amino-6-bromobenzothiazole were identified using selected-ion monitoring mode at m/z 235.2/178.2 and m/z 231.0/150.0 at retention times of 5.2 and 6.3 min, respectively. The method was validated with a linearity range of 3.9-500.0 ng/mL in human plasma and showed acceptable reproducibility with intra- and interday precisions <5.9 and 10.5%, and accuracy within ±5.4% of nominal values. This analytical method together with basic stability data in plasma and plasma binding experiments provides a reliable protocol for the study of ZMC1 pharmacokinetics. This will greatly facilitate the pre-clinical development of this novel anti-cancer drug.

In Vitro Characterization of the Pharmacological Properties of the Anti-Cancer Chelator, Bp4eT, and Its Phase I Metabolites

Cancer cells have a high iron requirement and many experimental studies, as well as clinical trials, have demonstrated that iron chelators are potential anti-cancer agents. The ligand, 2-benzoylpyridine 4-ethyl-3-thiosemicarbazone (Bp4eT), demonstrates both potent anti-neoplastic and anti-retroviral properties. In this study, Bp4eT and its recently identified amidrazone and semicarbazone metabolites were examined and compared with respect to their anti-proliferative activity towards cancer cells (HL-60 human promyelocytic leukemia, MCF-7 human breast adenocarcinoma, HCT116 human colon carcinoma and A549 human lung adenocarcinoma), non-cancerous cells (H9c2 neonatal rat-derived cardiomyoblasts and 3T3 mouse embryo fibroblasts) and their interaction with intracellular iron pools. Bp4eT was demonstrated to be a highly potent and selective anti-neoplastic agent that induces S phase cell cycle arrest, mitochondrial depolarization and apoptosis in MCF-7 cells. Both semicarbazone and amidrazone metabolites showed at least a 300-fold decrease in cytotoxic activity than Bp4eT towards both cancer and normal cell lines. The metabolites also lost the ability to: (1) promote the redox cycling of iron; (2) bind and mobilize iron from labile intracellular pools; and (3) prevent ⁵⁹Fe uptake from ⁵⁹Fe-labeled transferrin by MCF-7 cells. Hence, this study demonstrates that the highly active ligand, Bp4eT, is metabolized to non-toxic and pharmacologically inactive analogs, which most likely contribute to its favorable pharmacological profile. These findings are important for the further development of this drug candidate and contribute to the understanding of the structure-activity relationships of these agents.

Iron depletion results in Src kinase inhibition with associated cell cycle arrest in neuroblastoma cells

Iron is required for cellular proliferation. Recently, using systematic time studies of neuroblastoma cell growth, we better defined the G1 arrest caused by iron chelation to a point in mid-G1, where cyclin E protein is present, but the cyclin E/CDK2 complex kinase activity is inhibited. In this study, we again used the neuroblastoma SKNSH cells lines to pinpoint the mechanism responsible for this G1 block. Initial studies showed in the presence of DFO, these cells have high levels of p27 and after reversal of iron chelation p27 is degraded allowing for CDK2 kinase activity. The initial activation of CDK2 kinase allows cells to exit G1 and enter S phase. Furthermore, we found that inhibition of p27 degradation by DFO is directly associated with inhibition of Src kinase activity measured by lack of phosphorylation of Src at the 416 residue. Activation of Src kinase occurs very early after reversal from the DFO G1 block and is temporally associated with initiation of cellular proliferation associated with entry into S phase. For the first time therefore we show that iron chelation inhibits Src kinase activity and this activity is a requirement for cellular proliferation.