Type I insulin-like growth factor receptor activation regulates apoptotic proteins

Increased Sensitivity of Acute Myeloid Leukemias to Lovastatin-Induced Apoptosis: A Potential Therapeutic Approach

We recently demonstrated that 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme of de novo cholesterol synthesis, was a potential mediator of the biological effects of retinoic acid on human neuroblastoma cells. The HMG-CoA reductase inhibitor, lovastatin, which is used extensively in the treatment of hypercholesterolemia, induced a potent apoptotic response in human neuroblastoma cells. This apoptotic response was triggered at lower concentrations and occurred more rapidly than had been previously reported in other tumor-derived cell lines, including breast and prostate carcinomas. Because of the increased sensitivity of neuroblastoma cells to lovastatin-induced apoptosis, we examined the effect of this agent on a variety of tumor cells, including leukemic cell lines and primary patient samples. Based on a variety of cytotoxicity and apoptosis assays, the 6 acute lymphocytic leukemia cell lines tested displayed a weak apoptotic response to lovastatin. In contrast, the majority of the acute myeloid leukemic cell lines (6/7) and primary cell cultures (13/22) showed significant sensitivity to lovastatin-induced apoptosis, similar to the neuroblastoma cell response. Of significance, in the acute myeloid leukemia, but not the acute lymphocytic leukemia cell lines, lovastatin-induced cytotoxicity was pronounced even at the physiological relevant concentrations of this agent. Therefore, our study suggests the evaluation of HMG-CoA reductase inhibitors as a therapeutic approach in the treatment of acute myeloid leukemia.

Increased Sensitivity of Acute Myeloid Leukemias to Lovastatin-Induced Apoptosis: A Potential Therapeutic Approach

We recently demonstrated that 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme of de novo cholesterol synthesis, was a potential mediator of the biological effects of retinoic acid on human neuroblastoma cells. The HMG-CoA reductase inhibitor, lovastatin, which is used extensively in the treatment of hypercholesterolemia, induced a potent apoptotic response in human neuroblastoma cells. This apoptotic response was triggered at lower concentrations and occurred more rapidly than had been previously reported in other tumor-derived cell lines, including breast and prostate carcinomas. Because of the increased sensitivity of neuroblastoma cells to lovastatin-induced apoptosis, we examined the effect of this agent on a variety of tumor cells, including leukemic cell lines and primary patient samples. Based on a variety of cytotoxicity and apoptosis assays, the 6 acute lymphocytic leukemia cell lines tested displayed a weak apoptotic response to lovastatin. In contrast, the majority of the acute myeloid leukemic cell lines (6/7) and primary cell cultures (13/22) showed significant sensitivity to lovastatin-induced apoptosis, similar to the neuroblastoma cell response. Of significance, in the acute myeloid leukemia, but not the acute lymphocytic leukemia cell lines, lovastatin-induced cytotoxicity was pronounced even at the physiological relevant concentrations of this agent. Therefore, our study suggests the evaluation of HMG-CoA reductase inhibitors as a therapeutic approach in the treatment of acute myeloid leukemia.

Insulin-like growth factor I-mediated activation of the transcription factor cAMP response element-binding protein in PC12 cells. Involvement of p38 mitogen-activated protein kinase-mediated pathway

IGF-I is known to support growth and to prevent apoptosis in neuronal cells. Activation of the nuclear transcription factor cAMP response element-binding protein (CREB) has emerged as a central determinant in neuronal functions. In the present investigation, we examined the IGF-I-mediated phosphorylation and transcriptional activation of CREB in rat pheochromocytoma (PC12) cells, a cellular model for neuronal differentiation, and defined three distinct postreceptor signaling pathways important for this effect including the p38 mitogen-activated protein kinase (MAPK) pathway. CREB phosphorylation at serine 133 and its transcriptional activation as measured by a CREB-specific Gal4-CREB reporter and the neuroendocrine-specific gene chromogranin A was induced 2-3.3-fold by insulin-like growth factor (IGF)-I. This activation was significantly blocked (p < 0.001) by the dominant negative K-CREB or by mutation of the CRE site. IGF-I stimulated chromogranin A gene expression by Northern blot analysis 3.7-fold. Inhibition of MAPK kinase with PD98059, PI 3-kinase with wortmannin, and p38 MAPK with SB203580 blocked IGF-I-mediated phosphorylation and transcriptional activation of CREB by 30-50% (p < 0.001). Constitutively active and dominant negative regulators of the Ras and PI 3-kinase pathways confirmed the contribution of these pathways for CREB regulation by IGF-I. Cotransfection of PC12 cells with p38beta and constitutively active MAPK kinase 6 resulted in enhanced basal as well as IGF-I-stimulated chromogranin A promoter. IGF-I activated p38 MAPK, which was blocked by the inhibitor SB203580. This is the first description of a p38 MAPK-mediated nuclear signaling pathway for IGF-I leading to CREB-dependent neuronal specific gene expression.

The immune-endocrine loop during aging: role of growth hormone and insulin-like growth factor-I

Why a primary lymphoid organ such as the thymus involutes during aging remains a fundamental question in immunology. Aging is associated with a decrease in plasma growth hormone (somatotropin) and IGF-I, and this somatopause of aging suggests a connection between the neuroendocrine and immune systems. Several investigators have demonstrated that treatment with either growth hormone or IGF-I restores architecture of the involuted thymus gland by reversing the loss of immature cortical thymocytes and preventing the decline in thymulin synthesis that occurs in old or GH-deficient animals and humans. The proliferation, differentiation and functions of other components of the immune system, including T and B cells, macrophages and neutrophils, also demonstrate age-associated decrements that can be restored by IGF-I. Knowledge of the mechanism by which cytokines and hormones influence hematopoietic cells is critical to improving the health of aged individuals. Our laboratory has recently demonstrated that IGF-I prevents apoptosis in promyeloid cells, which subsequently permits these cells to differentiate into neutrophils. We also demonstrated that IL-4 acts much like IGF-I to promote survival of promyeloid cells and to activate the enzyme phosphatidylinositol 3'-kinase (PI 3-kinase). However, the receptors for IGF-I and IL-4 are completely different, with the intracellular beta chains of the IGF receptor possessing intrinsic tyrosine kinase activity and the alpha and gammac subunit of the heterodimeric IL-4 receptor utilizing the Janus kinase family of nonreceptor protein kinases to tyrosine phosphorylate downstream targets. Both receptors share many of the components of the PI 3-kinase signal transduction pathway, converging at the level of insulin receptor substrate-1 or insulin receptor subtrate-2 (formally known as 4PS, or IL-4 Phosphorylated Substrate). Our investigations with IGF-I and IL-4 suggest that PI 3-kinase inhibits apoptosis by maintaining high levels of the anti-apoptotic protein Bcl-2. The sharing of common activation molecules, despite vastly different protein structures of their receptors, forms a molecular explanation for the possibility of cross talk between IL-4 and IGF-I in regulating many of the events associated with hematopoietic differentiation, proliferation and survival.

Insulin receptor substrate 2 and Shc play different roles in insulin-like growth factor I signaling

The major substrates for the type I insulin-like growth factor (IGF-I) receptor are Shc and insulin receptor substrate (IRS) proteins. In the current study, we report that IGF-I induces a sustained tyrosine phosphorylation of Shc and its association with Grb2 in SH-SY5Y human neuroblastoma cells. The time course of Shc tyrosine phosphorylation parallels the time course of IGF-I-stimulated activation of extracellular signal-regulated kinase (ERK). Transfection of SH-SY5Y cells with a p52 Shc mutant decreases Shc tyrosine phosphorylation and Shc-Grb2 association. This results in the inhibition of IGF-I-mediated ERK tyrosine phosphorylation and neurite outgrowth. In contrast, IGF-I induces a transient tyrosine phosphorylation of IRS-2 and an association of IRS-2 with Grb2. The time course of IRS-2 tyrosine phosphorylation and IRS-2-Grb2 and IRS-2-p85 association closely resembles the time course of IGF-I-mediated membrane ruffling. Treating cells with the phosphatidylinositol 3'-kinase inhibitors wortmannin and LY294002 blocks IGF-I-induced membrane ruffling. The ERK kinase inhibitor PD98059, as well as transfection with the p52 Shc mutant, has no effect on IGF-I-mediated membrane ruffling. Immunolocalization studies show IRS-2 and Grb2, but not Shc, concentrated at the tip of the extending growth cone where membrane ruffling is most active. Collectively, these results suggest that the association of Shc with Grb2 is essential for IGF-I-mediated neurite outgrowth, whereas the IRS-2-Grb2-phosphatidylinositol 3'-kinase complex may regulate growth cone extension and membrane ruffling.

Differential regulation of insulin receptor substrate-2 and mitogen-activated protein kinase tyrosine phosphorylation by phosphatidylinositol 3-kinase inhibitors in SH-SY5Y human neuroblastoma cells

Insulin-like growth factor I (IGF-I) is a potent neurotropic factor promoting the differentiation and survival of neuronal cells. SH-SY5Y human neuroblastoma cells are a well characterized in vitro model of nervous system growth. We report here that IGF-I stimulated the tyrosine phosphorylation of the type I IGF receptor (IGF-IR) and insulin receptor substrate-2 (IRS-2) in a time- and concentration-dependent manner. These cells lacked IRS-1. After being tyrosine phosphorylated, IRS-2 associated transiently with downstream signaling molecules, including phosphatidylinositol 3-kinase (PI 3-K) and Grb2. Treatment of the cells with PI 3-K inhibitors (wortmannin and LY294002) increased IGF-I-induced tyrosine phosphorylation of IRS-2. We also observed a concomitant increase in the mobility of IRS-2, suggesting that PI 3-K mediates or is required for IRS-2 serine/threonine phosphorylation, and that this phosphorylation inhibits IRS-2 tyrosine phosphorylation. Treatment with PI 3-K inhibitors induced an increased association of IRS-2 with Grb2, probably as a result of the increased IRS-2 tyrosine phosphorylation. However, even though the PI 3-K inhibitors enhanced the association of Grb2 with IRS-2, these compounds suppressed IGF-I-induced mitogen-activated protein kinase activation and neurite outgrowth. Together, these results indicate that although PI 3-K participates in a negative regulation of IRS-2 tyrosine phosphorylation, its activity is required for IGF-IR-mediated mitogen-activated protein kinase activation and neurite outgrowth.

Insulin and insulin-like growth factor-I promote rabbit blastocyst development and prevent apoptosis

Insulin as well as insulin-like growth factor-I (IGF-I) promote early embryo development, and IGF-I binds to the coats of preimplantation rabbit embryos. As the IGF-I receptor is expressed from the morula stage onwards, the embryos are capable of responding to insulin and IGF-I, which is present in the oviductal and uterine secretions that surround them. The embryonic coats were removed to exclude any influence by IGF-I bound to the coats. The in vitro development of such embryos under classical conditions appears to be retarded. Addition of IGF-I (68 pM-6.8 nM) or insulin (68 nM-6.8 microM), however, promotes blastocyst formation. Embryo development under such conditions is not significantly different from that of embryos cultured with intact coats. In contrast, coat-free embryos cultured without IGF-I or insulin supplementation show apoptosis. Because IGF-I stimulates cell proliferation and prevents apoptosis, we investigated whether insulin or IGF-I may act as "survival factors" in preimplantation development. Therefore, apoptosis was induced by slight UV irradiation (254 nm wave length; 11.8 W/m2). Compared to the untreated controls, embryos displaying retarded development or degeneration were increased by 22% and 14%, respectively. Addition of IGF-I or insulin to the culture medium of UV-irradiated embryos improved [3H]thymidine incorporation and blastocyst formation significantly. By immunohistochemistry we could show that addition of insulin (0.68-68 nM) decreased apoptosis and increased cell proliferation in a dose-dependent manner, supporting blastocyst development significantly.

Synergistic action of R-Ras and IGF-1 on Bcl-xL expression and caspase-3 inhibition in BaF3 cells: R-Ras and IGF-1 control distinct anti-apoptotic kinase pathways

R-Ras and insulin-like growth factor-1 (IGF-1) synergistically inhibit apoptosis of BaF3 cells upon interleukin-3 deprivation. To characterize the mechanism of this synergistic inhibition, we examined the effect of R-Ras and IGF-1 on several apoptosis-related proteins. Extracellular signal-regulated kinase (ERK) was activated by IGF-1, but not by R-Ras. In contrast, Akt was activated strongly by R-Ras, but weakly by IGF-1. It was also found that R-Ras and IGF-1 cooperatively induced Bcl-xL expression and inhibited caspase-3 activation.

Insulin-like growth factor-I inhibits the stress-activated protein kinase/c-Jun N-terminal kinase

The pathways involved in the cellular responses to the insulin-like growth factors (IGFs) are numerous and vary according to cell type. Following activation of the IGF-I receptor, the mitogen-activated protein kinase and phosphatidylinositide 3'-kinase (PI3'K) pathways are activated and result in cellular proliferation and inhibition of apoptosis. In this study, we analyzed the IGF-I effect on the stress-activated protein kinase/c-Jun N-terminal kinase (JNK) activity using human embryonic kidney 293 cells, 293 cells transiently expressing hemagglutinin-JNK, and 293 cells stably expressing a hemagglutinin-JNK transgene. In all cell types, endogenous or transfected JNK activity was strongly stimulated by anisomycin or tumor necrosis factor-alpha, and 10 nM IGF-I pretreatment suppressed the induced JNK activity. To determine whether the effect of IGF-I on JNK activity involves the mitogen-activated protein kinase or PI3'K pathway, we used the specific MEK1 inhibitor PD098059 and the PI3'K inhibitor LY 294002. PD098059 did not alter the IGF-I suppressive effect on stressor-induced JNK activity, but LY 294002 suppressed the IGF-I effect. Moreover, in transiently transfected parental 293 cells expressing dominant-negative Akt, anisomycin-increased JNK activity was not suppressed by pretreatment with IGF-I. Our results demonstrate that the action of IGF-I on JNK in these cells is via PI3'K and Akt.

Regulation of cardiomyocyte apoptotic signaling by insulin-like growth factor I

Apoptosis is regulated by specific intracellular signaling pathways. The development of cardiomyopathy involves the apoptosis of cardiomyocytes; however, the details of their apoptotic signaling are not yet known. Insulin-like growth factor I (IGF I) is an important survival growth factor for myocardium and other tissues, but the effects of IGF I on apoptotic signaling remain largely unknown. To study apoptotic signaling pathways in cardiomyocytes and to understand IGF I actions on the apoptotic signaling of cardiac muscle cells, we have defined the effects of IGF I on Bcl-2, Bax, caspase 3, DNA fragmentation, and cell survival in primary cardiomyocytes. Compared with Bax levels, the levels of Bcl-2 were found to be quite low in these cells. Serum withdrawal and doxorubicin reduced cell viability, increased fragmentation of DNA, increased cellular contents of Bax, and activated caspase 3. IGF I enhanced cell viability, suppressed DNA fragmentation, attenuated Bax induction, and suppressed caspase 3 activation. The levels of Bcl-2-associated Bax were increased after serum withdrawal and incubation with doxorubicin and were reduced by IGF I. Thus, cardiomyocyte apoptosis induced by serum withdrawal and doxorubicin likely results, in part, from the induction of Bax and activation of caspase 3, but IGF I may inhibit cardiomyocyte apoptosis by attenuating Bax induction and caspase 3 activation. These findings provide new insight into the mechanisms of cardiomyocytes apoptosis and may help elucidate how IGF I modulates apoptotic signaling in cardiac muscle.

Co-ordinated and cellular specific induction of the components of the IGF/IGFBP axis in the rat brain following hypoxic-ischemic injury

Insulin-like growth factor 1 (IGF-1) is induced after hypoxic-ischemic (HI) brain injury, and therapeutic studies suggest that IGF-1 may restrict delayed neuronal and glial cell loss. We have used a well-characterised rat model of HI injury to extend our understanding of the modes of action of the IGF system after injury. The induction of the IGF system by injury was examined by in situ hybridization, immunohistochemistry, Northern blot analysis, RNase protection assay and reverse transcriptase-polymerase chain reaction (RT-PCR). IGF-1 accumulated in blood vessels of the damaged hemisphere within 5 h after a severe injury. By 3 days, IGF-1 mRNA was expressed by reactive microglia in regions of delayed neuronal death, and immunoreactive IGF-1 was associated with these microglia and reactive astrocytes juxtaposed to surviving neurones surrounding the infarct. Total IGF-1 receptor mRNA was unchanged by the injury. IGFBP-2 mRNA was strongly induced in reactive astrocytes throughout the injured hemisphere, and IGFBP-3 and IGFBP-5 mRNA were moderately induced in reactive microglia and neurones of the injured hippocampus, respectively. IGFBP-6 mRNA was induced in the damaged hemisphere by 3 days and increased protein was seen on the choroid plexus, ependyma and reactive glia. In contrast, insulin II was not induced. These results indicate cell type-specific expression for IGF-1, IGFBP-2,3,5 and 6 after injury. Our findings suggest that the IGF-1 produced by microglia after injury is transferred to perineuronal reactive astrocytes expressing IGFBP-2. Thus, modulation of IGF-1 action by IGFBP-2 might represent a key mechanism that restricts neuronal cell loss following HI brain injury.

Bidirectional regulation of p38 kinase and c-Jun N-terminal protein kinase by insulin-like growth factor-I

We have previously shown that insulin-like growth factor I (IGF-I) activation of the IGF-I receptor rescues SH-SY5Y human neuroblastoma cells from high glucose-mediated programmed cell death (PCD). In the current study, we further explored the potential points in the cell death cascade where IGF-I receptor activation may afford neuroprotection. As an initial step, we examined the effects of the PCD stimulus, high glucose, on stress-activated protein kinases, specifically the two mitogen-activated protein kinases p38 kinase and c-Jun N-terminal kinase (JNK). High glucose treatment activated the tyrosine phosphorylation of both p38 kinase and JNK in a dose- and time-dependent fashion. We next examined the effects of IGF-I on JNK and p38 kinase under normoglycemic and hyperglycemic conditions. IGF-I activated p38 kinase alone and had additive effects on glucose-induced p38 kinase phosphorylation. In contrast, IGF-I inhibited glucose activation of JNK phosphorylation and JNK activity. IGF-I also inhibited the glucose-induced nuclear translocation of JNK, but did not effect glucose-induced translocation of p38 kinase. Finally, IGF-I inhibition of JNK phosphorylation was blocked by the mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor, PD98059. Collectively, these data imply cross-talk between the mitogen-activated protein kinase pathway and JNK and suggest that IGF-I activation of mitogen-activated protein kinases interferes with JNK activation and protects cells from PCD.

A combination of NaCl and urea enhances survival of IMCD cells to hyperosmolality

Physiological adaptation to the hyperosmolar milieu of the renal medulla involves a complex series of signaling and gene expression events in which NaCl and urea activate different cellular processes. In the present study, we evaluated the effects of NaCl and urea, individually and in combination, on the viability of murine inner medullary collecting duct (mIMCD3) cells. Exposure to hyperosmolar NaCl or urea caused comparable dose- and time-dependent decreases in cell viability, such that 700 mosmol/kgH2O killed >90% of the cells within 24 h. In both cases, cell death was an apoptotic event. For NaCl, loss of viability at 24 h paralleled decreases in RNA and protein synthesis at 4h, whereas lethal doses of urea had little or no effect on these biosynthetic processes. Cell cycle analysis showed both solutes caused a slowing of the G2/M phase. Surprisingly, cells exposed to a combination of NaCl + urea were significantly more osmotolerant such that 40% survived 900 mosmol/kgH2O. Madin-Darby canine kidney cells but not human umbilical vein endothelial cells also exhibited a similar osmotolerance response. Enhanced survival was not associated with a restoration of normal biosynthetic rates or cell cycle progression. However, the combination of NaCl + urea resulted in a shift in Hsp70 expression that appeared to correlate with survival. In conclusion, hyperosmolar NaCl and urea activate independent and complementary cellular programs that confer enhanced osmotolerance to renal medullary epithelial cells.

The baculovirus anti-apoptotic p35 protein promotes transformation of mouse embryo fibroblasts

The baculovirus p35 protein is a potent inhibitor of programmed cell death induced by a variety of stimuli in insects, nematodes, and mammalian cell lines. The broad ability of p35 in preventing apoptosis has led us to investigate its effect on mouse embryo fibroblasts in vitro and in vivo. For this purpose, we have used R- cells (3T3-like fibroblasts derived from mouse embryos with a targeted disruption of the insulin-like growth factor I receptor (IGF-IR) genes) and R508 cells (derived from R- and with 15 x 10(3) IGF-IRs per cell). Both cell lines grow normally in monolayer, but they do not form colonies in soft agar, and they are non-tumorigenic in nude mice. We show here that, in addition to its anti-apoptotic effect, p35 causes transformation of R508 cells, as evidenced by the following: 1) decreased growth factor requirements, 2) ability to form foci in monolayer and colonies in soft agar, and 3) ability to form tumors in nude mice. Since R- cells stably transfected with p35 do not transform, our observations suggest that in addition to its effect as an inhibitor of apoptosis, the baculovirus p35 protein has transforming potential that requires the presence of the IGF-IR. The possibility that these two properties could be separated was confirmed by demonstrating that R508 cells expressing another anti-apoptotic protein, Bcl-2, could not form tumors in nude mice.

Paracrine/autocrine regulation of breast cancer by the insulin-like growth factors

Local environmental signals regulate the growth and development of both normal and malignant breast epithelium. Members of the insulin-like growth factor (IGF) family likely influence both of these processes. The localization of IGF2 to stroma specifically surrounding malignant breast epithelium indicates that this growth factor may play a critical role in the genesis or maintenance of this transformed phenotype. Recent studies have sought to understand the mechanism by which IGF2 expressing fibroblasts are localized to the periphery of malignant breast cancer cells. In addition, the consequences of the expression of IGF-signaling components likely expand beyond their direct effects on mitogenesis. Indirect effects predominantly associated with the IGF2 receptor could also influence the invasive potential of breast tumor cells.

Cellular survival pathways and resistance to cancer therapy

Chemotherapy and irradiation induce programmed cell death (apoptosis) in their target cells. Dysregulated apoptosis is a feature that is selected for during tumor formation and contributes to therapeutic resistance. Cell survival in the face of cytotoxic therapy is dictated by both internal properties of the cell, such as status of components of the apoptotic machinery, and its extracellular milieu, such as extracellular matrix (ECM) and growth factor receptor expression and signaling. The importance of extracellular survival signals as key regulators of apoptosis is now being recognized by the ability of growth factors (GFs), GF receptors (GFRs), and GFR signaling to promote cellular survival. GFs can mitigate or abrogate the effectiveness of cancer therapy and protect against other cellular insults. Because survival signals generated by different extracellular sources converge at key molecules, new molecular targets have been identified which could be exploited to maximize the effectiveness of cytotoxic cancer therapy.

Insulin-like growth factors regulate neuronal differentiation and survival

Insulin-like growth factor I (IGF-I) and IGF-II are potent trophic factors for motor and sensory neurons and glial cells. The actions of IGF-I and IGF-II are mediated via the IGF-I receptor (IGF-IR). IGF:IGF-IR binding activates distinct signaling cascades, which in turn mediate the trophic effects of the IGFs. We discuss three main IGF coupled events: growth cone motility, long-term neurite outgrowth, and neuroprotection. Our data suggest that IGF-I enhances growth cone motility by promoting reorganization of actin and activation of focal adhesion proteins via the phosphatidylinositol-3 kinase (Pl-3K) pathway. Long-term treatment with IGF-I activates the mitogen-activated protein (MAP) kinase cascade and promotes neurite outgrowth. A separable, but likely linked, action of the IGFs via Pl-3K is protection of neurons from apoptosis. These pleotrophic effects of IGFs suggest that this family of growth factors may have potential clinical utility in the treatment of neurological disorders.

TNF-alpha induces apoptosis in rat fetal brown adipocytes in primary culture

The effect of TNF-alpha on cell death in rat fetal brown adipocytes maintained in primary culture was determined. TNF-alpha inhibited proliferation and induced apoptosis in these cells. Most of the cells undergoing apoptosis after TNF-alpha treatment did not express PCNA, suggesting an induction of apoptosis by TNF-alpha in non-proliferative cells. IGF-I but not EGF prevented TNF-alpha-induced apoptosis.

Type-1 insulin-like growth factor receptor reexpression in the malignant phenotype of SV40-T-immortalized human prostate epithelial cells enhances apoptosis

The authors have previously shown that the type 1 insulin-like growth factor receptor (IGF-1R) is decreased in the transformation from benign to malignant human prostate epithelial cells in vivo. Further, in a well-described human SV40-T immortalized human epithelial cell system beginning with the immortalized, but rarely tumorigenic P69SV40-T cell line, to the highly tumorigenic and metastatic M12 subline, there is a similar decrease in IGF-1R number from 2.0 x 10(4) receptors per cell to 1.1 x 10(3) receptors per cell. When the IGF-1R was reexpressed in the M12 subline using a retroviral expression vector, M12-LISN, to a receptor number similar to that of the P69SV40-T parental cell line, the authors demonstrated a marked decrease in colony formation in soft agar in the M12-LISN cells vs the M12 control cells (p < or = 0.01), and a decrease in vivo tumor growth and metastases when injected either subcutaneously or an intraprostatic location (p < or = 0.01). This decrease in tumor volume was not because of a decrease in proliferative capacity, but was associated with an increase in apoptosis in baseline cultures and in response to the apoptotic-inducing agents 6-hydroxyurea, retinoic acid, and transforming growth factor beta 1.