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The Journal of Clinical Endocrinology & Metabolism Vol. 88, No. 7 3354-3359
Copyright © 2003 by The Endocrine Society

Insulin-Like Growth Factor I Stimulates Telomerase Activity in Prostate Cancer Cells

Lawrence A. Wetterau, Malik J. Francis, Liqun Ma and Pinchas Cohen

Columbus Children’s Hospital, Ohio State University College of Medicine (L.A.W.), Columbus, Ohio 43205; Mattel Children’s Hospital, University of California (M.J.F., L.M., P.C.), Los Angeles, California 90095

Address all correspondence and requests for reprints to: Lawrence A. Wetterau, M.D., Section of Endocrinology, Columbus Children’s Hospital, Ohio State University College of Medicine, 700 Children’s Drive, Columbus, Ohio 43205-2696. E-mail: lwetterau{at}chi.osu.edu.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
IGF-I has been implicated in the pathogenesis of human cancer. We sought to establish a role for IGF-I in the regulation of telomerase, an enzyme critically involved in cancer cell immortalization. Telomerase activity was assayed in LAPC-4, PC-3, and DU-145 prostate cancer cell lines treated with and without IGF-I/IGF-I analogs. Relative expression of human telomerase reverse transcriptase (hTERT) mRNA and protein was determined by quantitative RT-PCR and Western immunoblot, respectively. IGF-I stimulated baseline telomerase activity in all three cell lines, ranging from 2- to 10-fold (P < 0.05). Enhancement was noted at IGF concentrations as low as 10 ng/ml and was maximal at 100 ng/ml. Stimulation was noted by 0.5 h, was maximal by 8 h, and persisted to 48 h. A similar 3-fold enhancement (P < 0.01) was noted in response to Long-R3 IGF-I, but not in response to [Ala31,Leu60]IGF-I. Pretreatment with the Akt kinase inhibitor wortmannin abolished the stimulatory IGF effect, whereas blockade of MAPK activity did not. Lastly, IGF-I provoked a 2-fold increase in hTERT mRNA and protein expression (P < 0.01). In summary, IGF-I clearly stimulates telomerase activity in prostate cancer cells through a dual mode of action, including early rapid effects probably involving phosphorylation of hTERT by Akt and later up-regulation of hTERT expression.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
GROWTH HORMONE, THE IGFs, and the IGF-binding proteins (IGFBPs) comprise a complex, powerful axis integral to the regulation of cellular and somatic growth in vitro and in vivo. Emerging evidence suggests that this axis plays an important role in the mechanisms underlying human cancer. The IGFs are potent mitogens whose cellular actions are mediated through interaction with the IGF receptor (IGFR) (1). The IGFs also bind with high affinity to a family of six extracellular IGFBPs that carry IGFs in the circulation, regulate IGF availability to the IGFR, and have both stimulatory and inhibitory effects on IGF actions (2, 3). The importance of the IGF-IGFBP axis in prostate cancer pathophysiology is well appreciated, and serum IGF-I levels may be linked to prostate cancer risk (4). IGFBP-3, the most abundant IGFBP in human serum, is central to the regulation of IGF action on cell growth and proliferation (5, 6). In addition to effects mediated through interactions with IGFs, IGFBP-3 has been shown to directly modulate growth, independent of IGFs.

Telomerase is a nuclear ribonucleoprotein enzyme complex whose activity may be linked to the processes governing cellular senescence and cellular immortalization (7, 8, 9). Telomerase is an RNA-dependent DNA polymerase comprised of three main components: 1) an RNA component [human telomerase RNA (hTR)] that serves as a template; 2) the catalytic subunit, human telomerase reverse transcriptase (hTERT); and 3) telomerase-associated protein, an association protein of unknown function (7, 8). Telomerase uses its RNA template to catalyze the addition of TTAGGG repeats to the ends of vertebrate chromosomes (10). In the absence of telomerase, the telomere will shorten with each successive cell division. This occurs because DNA polymerase {alpha} is unable to replicate the very ends of linear DNA. This leads to the progressive shortening of telomeric ends in normal somatic cells (11, 12) and appears to be linked to the limited proliferative capacity of normal cells (13, 14, 15).

Telomerase activity is typically absent in most normal human cells, but is nearly universally expressed in human cancer cells (7). Primary prostate cancer tissue samples as well as human prostate cancer cell lines have been demonstrated to exhibit high levels of telomerase activity (16). Recently, inhibition of telomerase in actively dividing tumor cells was shown to lead to massive cell death (17, 18). The regulatory processes governing the activation and level of telomerase expression remain under active research.

Preliminary evidence suggests that the GH-IGF-IGFBP axis may play a regulatory role in determining telomerase activity. A study conducted in nude mice bearing xenografts of U-87MG human glioblastomas demonstrated that the tumor inhibition by a GHRH antagonist was associated with down-regulation of the hTERT gene and a decrease in telomerase activity (19). Although the mechanism of this effect is not known, it may involve suppression of IGF-I and/or IGF-II signaling, but could potentially involve antagonism of direct GH effects. Studies in human mononuclear cells suggested a stimulatory role for IGF-I in the modulation of telomerase activity stimulated by phytohemagglutinin (PHA) (20). There is also recent evidence to suggest that cell survival pathways initiated by growth factor receptor signaling (including the IGFR) regulate telomerase activity (21, 22). Our hypothesis, leading to the following studies, is that IGF-I stimulates telomerase activity in prostate cancer cells.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Materials

Pharmacia Biotech (Uppsala, Sweden) provided recombinant human IGF-I and GH. IGF-II and the IGF-I analogs Long-R3 IGF-I and [Leu60]IGF-I were purchased from GroPep (Adelaide, Australia). Insulin, wortmannin, and PD-98059 were purchased from Sigma-Aldrich Corp. (St. Louis, MO). IGFBP-3 was a gift from Protigen (Mountain View, CA). Protein assay reagent was purchased from Bio-Rad Laboratories, Inc. (Hercules, CA). Rabbit anti-hTERT antibody was purchased from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA).

Cell culture

The human, androgen-independent prostate cancer cell lines PC-3 and DU-145 were purchased from American Type Culture Collection (Manassas, VA). The androgen-dependent prostate cancer cell line LAPC-4 was obtained from Dr. Charles Sawyers (University of California, Los Angeles, CA). DU-145 cells were grown in DMEM (American Type Culture Collection) containing 10% fetal bovine serum (FBS) and 1% amphotericin/penicillin/streptomycin (A/P/S). PC-3 cells were grown in F12K medium (Life Technologies, Inc., Gaithersburg, MD) supplemented with 10% FBS and 1% A/P/S. LAPC-4 cells were cultured in Isocove’s modified Dulbecco’s medium supplemented with 10% FBS, 1% A/P/S, 2 mM L-glutamine, and 10 nM R1881 (NEN Life Science Products, Boston, MA).

Telomerase assay

The commercially available telomerase PCR ELISA kit (Roche Molecular Biochemicals, Indianapolis, IN) was used to perform the telomerase assays (23). The preparation of cell lysates and the assay procedure were performed according to the manufacturer’s protocol. Absorbance at 450 nm was measured for each sample using a microplate reader (Bio-Rad Laboratories, Inc.). All precautionary measures against ribonuclease activity were observed. For each experiment, 2 x 105 cells were plated per well using 6-well plates, and cells were grown to 70% confluence. They were subsequently placed in serum-free medium for 12 h before treatment with IGFs or IGFBPs. Protein concentrations of cell lysates were determined by Bradford assay, and 2 µg protein were used for each telomerase PCR reaction. Telomerase assay was similarly performed using positive (HeLa cell extracts) and negative controls as specified in the kit and according to the manufacturer’s protocol.

RT-PCR

Total cell RNA was first extracted using the RNeasy Mini Kit (QIAGEN, Valencia, CA). RNA content and quality were assessed by spectrophotometry at 260 and 280 nm. The 260/280 nm ratio was consistently between 1.8–2.0. RT-PCR was performed with 0.5 µg total RNA using an Ez rTth RNA PCR kit (PerkinElmer, Branchburg, NJ) according to the manufacturer’s protocol. All primers used were purchased from Sigma Genosys (Pittsburgh, PA). Primer sequences for the control, ß-actin, were 5'-CCAAGGCCAACCGCGCGAGAAGA-3' for the upstream primer and 5'-AGGGTACATGGTGGTGCCGCCAGAC-3' for the downstream primer (24). Primer sequences for hTERT mRNA were 5'-CGGAAGAGTGTCTGGAGCAA-3' for the upstream primer and 5'-GGATGAAGCGGAGTCTGGA-3' for the downstream primer (25). The linear portion of the cycle amplification curve and the optimal cycle number were determined for both hTERT and ß-actin by employing the following RT-PCR program: 60 C for 35 min at room temperature, 94 C for 1 min for denaturation, and cycles of 94 C for 15 sec, 60 C for 30 sec, and 60 C for 7 min for final annealing and extension. For hTERT, the linear portion of the amplification curve was empirically determined to be 26, 28, 30, 32, 34, and 36 cycles. The optimal number of cycles was 32, and this was used for all RT-PCR experiments. For ß-actin the linear portion of the amplification curve included 18, 20, 22, and 24 cycles. The optimal number of cycles was 22, and this was used for all subsequent RT-PCR experiments. RT-PCR products were separated on a 2.0% agarose gel stained with ethidium bromide. Scanning densitometry using Quantity One software (Bio-Rad Laboratories, Inc., Hercules, CA) was employed to quantify relative expression. The relative hTERT mRNA levels were normalized to the corresponding levels of ß-actin.

Western immunoblot

DU-145 cells were grown to 80% confluence on 10-cm plates before treatment. Nuclear extracts of DU-145 cells were prepared using the Nu-CLEAR Extraction Kit (Sigma Genosys) according to the manufacturer’s protocol. One hundred microliters of nuclear extract were separated by SDS-PAGE (8%) at constant voltage overnight, then transferred to nitrocellulose for 1 h at 18 V. The nitrocellulose was immersed in blocking solution (5% nonfat milk/TBS) for 45 min, washed with PBS/0.1% Tween, and incubated with primary rabbit anti-hTERT antibody (1:1000) for 2 h. After washing off any unbound antibodies, the nitrocellulose was incubated with a general secondary (goat antirabbit) antibody (1:10,000) for 1 h. The membrane was washed four times with Tris-buffered saline/0.1% Tween and Tris-buffered saline. Bands were visualized using the peroxidase-linked enhanced chemiluminescence detection system (ECL, Amersham Pharmacia Biotech, Arlington Heights, IL).

Statistical analyses

Data are shown as the means of quadruplicate determinations, and each experiment was performed three times. Data are expressed as the mean ± SEM. Statistical analyses were performed using an unpaired, two-tailed t test. The time-course experiment (Fig. 3Go) was analyzed using one-way ANOVA. Differences were considered statistically significant at P < 0.05 and very significant at P < 0.01.



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  		FIG. 3. IGF-I stimulation is dose and time dependent. A, The dose response was assessed in DU-145 cells using IGF-I concentrations ranging from 10–1000 ng/ml (1.31–131 nM) for 48 h. We observed enhancement of telomerase at concentrations as low as 10 ng/ml (1.31 nM), and the maximal effect was produced by 100 ng/ml (13.1 nM). B, DU-145 cells were treated with IGF-I at 1 µg/ml (131 nM) for 0, 0.5, 2.0, 8.0, 24.0, and 48.0 h. Enhancement of telomerase was noted by 0.5 h, peaked by 8 h, and was sustained to 48 h.

 

   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Modulation of telomerase activity by the GH-IGF axis in DU-145 cells

Telomerase activity was assayed in DU-145 prostate cancer cells in response to 100 ng/ml (4.52 nM) GH, 100 nM insulin, 100 ng/ml (13.4 nM) IGF-II, 100 ng/ml (13.1 nM) IGF-I, and 1000 ng/ml (34.5 nM) IGFBP-3 treatment for 48 h in DU-145 prostate cancer cells. No change in baseline telomerase activity was observed after treatment with GH, IGFBP-3, IGF-II, or insulin (Fig. 1Go). However, a marked increase in telomerase activity was noted in response to IGF-I.



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  		FIG. 1. Modulation of telomerase activity by the GH-IGF axis. Telomerase activity was assayed in response to 100 ng/ml (4.52 nM) GH, 100 nM insulin, 100 ng/ml (13.4 nM) IGF-II, 100 ng/ml (13.1 nM) IGF-I, and 1000 ng/ml (34.5 nM) IGFBP-3 treatment for 48 h in DU-145 prostate cancer cells. A marked increase in telomerase activity was noted only in response to IGF-I. *, P < 0.05; **, P < 0.01.

 
IGF-I stimulates telomerase activity in several prostate cancer cell lines

We measured telomerase activity in three human prostate cancer cell lines, including two androgen-independent cell lines, DU-145 and PC-3, and the androgen-dependent cell line LAPC-4. We detected high levels of telomerase activity in all three cell lines, similar to that in the positive control (data not shown). Cells were subsequently treated with and without IGF-I at 1000 ng/ml (131 nM) for 48 h. Marked stimulation of telomerase activity was noted in all three cell lines (Fig. 2Go, A–C), ranging from 2- to 10-fold. This was most marked in PC-3 cells (Fig. 2BGo). In the androgen-sensitive cell line LAPC-4 (Fig. 2CGo), IGF-I stimulated telomerase activity by nearly 3-fold compared with baseline activity in serum-free, androgen-free conditions.



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  		FIG. 2. IGF-I stimulates telomerase activity in prostate cancer cells. Telomerase activity was assayed in response to IGF-I treatment at 1 µg/ml (131 nM) for 48 h in three prostate cancer cell lines. A marked increase was noted in all three cell lines.

 
IGF-I stimulation is dose and time dependent

The dose response was assessed in DU-145 cells (Fig. 3AGo). Cells were treated for 48 h with IGF-I concentrations of 10 ng/ml (1.31 nM), 100 ng/ml (13.1 nM), 500 ng/ml (6.54 nM), and 1000 ng/ml (131 nM). We observed statistically significant stimulation of telomerase activity at all concentrations. IGF-I at 10 ng/ml (1.31 nM) stimulated telomerase by 1.5-fold. This effect was maximal by 100 ng/ml (13.1 nM), with a 2-fold increase observed. Notably, although initial experiments employed higher doses (1000 ng/ml or 131 nM) of IGF-I, the stimulatory effect on telomerase occurred at physiological concentrations. The time course of this effect in DU-145 cells was assessed (Fig. 3BGo). Cells were treated with IGF-I at 1 µg/ml for 0, 0.5, 2.0, 8.0, 24.0, and 48.0 h. Enhancement of telomerase was noted at all time points (P < 0.01). This included early activation by 30 min. This effect peaked by 8 h and was sustained to 48 h.

IGF effect is likely mediated through type I IGFR

DU-145 cells were treated with IGF-I or IGF analogs for 48 h at 1000 ng/ml (Fig. 4Go). Long-R3 IGF is an analog that activates the type I, IGFR, but does not bind to the IGFBPs. A 3-fold enhancement (P < 0.01) was noted in DU-145 cells in response to Long-R3 IGF-I, an analog that does not bind to the IGFBPs. This was similar to the 2.6-fold increase evoked by IGF-I (P < 0.01). Treatment with [Leu60]IGF-I, an analog that binds only IGFBPs and not the IGFR, had no effect on telomerase activity. This suggests that the stimulatory effect on telomerase activity is mediated through the type I IGFR.



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  		FIG. 4. IGF effect probably mediated through type I IGFR. Stimulation of telomerase activity by IGF-I (131 nM) was also observed in response to its analog Long-R3 (110 nM), which binds the type I IGFR, but not IGFBPs, in DU-145 cells. Leu60 (132 nM), an IGF analog that only binds IGFBPs and not the type I IGFR, had no effect.

 
Stimulation of telomerase is likely mediated through the phosphoinositol 3-kinase (PI3 kinase)-Akt kinase pathway

Considering that Akt protein kinase had been previously shown to activate telomerase activity (22), we postulated that IGF activation of the PI3-kinase-Akt kinase pathway was required for IGF’s stimulatory effect on telomerase activation. DU-145 cells were pretreated with or without 40 µM wortmannin for 2 h before treatment with IGF-I at 100 ng/ml (13.1 nM) for 24 h. Blockade of the PI3-kinase-Akt kinase pathway with wortmannin has been demonstrated to reduce baseline telomerase activity in breast cancer cells (22). Here, we replicated the inhibitory effect of wortmannin on baseline telomerase activity in DU-145 prostate cancer cells. Furthermore, pretreatment with wortmannin abolished the stimulatory effect of IGF, suggesting that Akt activation is necessary for the activation of telomerase by IGF-I (Fig. 5Go). Blockade of the MAPK pathway with PD-98059 had no effect.



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  		FIG. 5. Stimulation of telomerase is probably mediated through the PI3-kinase-Akt kinase pathway. DU-145 cells were pretreated with wortmannin at 40 µM for 2 h before treatment with IGF-I at 100 ng/ml (13.1 nM) for 24 h. Wortmannin treatment reduced baseline activity and abolished the IGF effect. Enhancement by IGF was unaffected by pretreatment with the MAPK inhibitor PD98059.

 
IGF-I stimulates hTERT mRNA expression

Considering our observation that IGF stimulation of telomerase was sustained to 48 h, we postulated that enhancement may have been partly secondary to up-regulation of telomerase mRNA. Others have previously noted that telomerase activity most closely correlates with the expression of hTERT, the catalytic reverse transcriptase subunit of telomerase. DU-145 cells were treated for 48 h with IGF-I at 1000 ng/ml (131 nM). Total RNA was extracted, and RT-PCR was performed as described in Materials and Methods. RT-PCR products were separated on a 2.0% agarose gel (Fig 6AGo). Mean densitometry data (from four experiments performed in triplicate) revealed a 2-fold increase in hTERT expression (P < 0.01) in response to IGF-I (Fig. 6BGo). This suggests that activation of telomerase by IGF-I may at least in part be secondary to stimulatory effects on hTERT expression.



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  		FIG. 6. IGF-I stimulates hTERT mRNA expression. IGF-I treatment induced a 2-fold increase in hTERT mRNA expression. DU-145 cells were treated for 48 h with IGF-I at 1 µg/ml (131 nM).

 
IGF-I stimulates hTERT protein

DU-145 cells were treated for 48 h with IGF-I at 1000 ng/ml (131 nM). The relative hTERT protein content was determined by performing a Western immunoblot on DU-145 cell nuclear extracts using an antibody specific for hTERT. A high intensity band at 134 kDa was detected (Fig. 7AGo) in untreated serum-free controls as well as in the positive control (HeLa cell nuclear extracts). Mean densitometry data revealed that IGF-I treatment induced a 2.4-fold relative increase in nuclear hTERT protein content (P < 0.01).



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  		FIG. 7. IGF-I stimulates hTERT protein. DU-145 cells were treated for 48 h with IGF-I at 1 µg/ml (131 nM). Western immunoblot was performed on nuclear extracts using an antibody specific for hTERT. IGF-I treatment induced a 2-fold increase in the relative hTERT protein level.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The mechanisms governing telomerase activation are incompletely understood. Ample evidence suggests that the regulation of telomerase in mammalian cells is multifactorial, involving telomerase gene expression, posttranslational protein-protein interactions, and protein phosphorylation. Several protooncogenes and tumor suppressor genes have been implicated to play a role in this process, including c-myc, bcl-2, c-abl, p21WAF1, Rb, p53, protein kinase C, Akt/protein kinase B, and protein phosphatase 2A (8, 26, 27, 28, 29, 30, 31). Furthermore, important hormonal factors influencing telomerase regulation continue to emerge, including vitamin D (32), retinoic acid (33), estrogen (34), and androgens (35).

Evidence for important regulatory roles for various growth factors and cytokines is emerging. Such agents include epidermal growth factor, fibroblast growth factor, TGFß, and interferon-{alpha}. A potential role for the GH-IGF axis has been suggested (19, 20). In the initial study to directly investigate the role of IGF-I, treatment with IGF-I was demonstrated to modulate telomerase activity in human cord blood mononuclear cells (20). IGF-I alone had no effect on telomerase activity or hTERT mRNA expression, but did stimulate telomerase-associated protein mRNA expression. IGF-I potentiated the PHA-induced increase in telomerase activity as well as the PHA-induced increase in hTERT mRNA expression. An in vivo study conducted in nude mice bearing xenografts of U-87MG human glioblastomas demonstrated that the tumor inhibition by a GHRH antagonist was associated with down-regulation of the hTERT gene and a decrease in telomerase activity (19). Although the mechanism of this effect was not established, it appeared likely to involve suppression of IGF-I and/or IGF-II signaling. Similar to our findings in prostate cancer cells, another group noted stimulatory effects on telomerase activity by IGF-I in a multiple myeloma cell line (36). They also found evidence that the effect is mediated through PI3-kinase/Akt signaling.

In this report we demonstrate that addition of IGF-I to the medium of prostate cancer cells in culture markedly stimulates telomerase activity. We have shown that IGF-I treatment induces a marked stimulation of already substantial telomerase activity in both androgen-dependent (LAPC-4) and androgen-independent (PC-3 and DU-145) prostate cancer cells. This effect appears to be mediated through the type I IGF receptor. This effect was initially observed at a substantial concentration of IGF-I (1000 ng/ml or 131 nM); however, significant stimulation was also observed at more physiological concentrations of IGF-I. The onset of this effect is rapid, increases over time, and is sustained to at least 48 h. We have also shown that IGF-I stimulates mRNA expression as well as protein levels of hTERT, the catalytic subunit of telomerase.

There appears to be two distinct modes of action inherent to the mechanism of telomerase activation induced by IGF-I. The rapid onset of the IGF stimulatory effect may reflect a phosphorylation cascade initiated by type I IGF receptor signaling. Similar to the findings of others, wortmannin abolished the enhancement of telomerase activity (22, 36), suggesting that activation of the PI3-kinase-Akt kinase signaling pathway may be involved. The more sustained effects probably reflect up-regulation of hTERT expression.

We did not observe any effect on baseline telomerase activity with the addition of IGFBP-3, GH, insulin, or IGF-II. Considering that one would expect some cross-reactivity with the type I IGF receptor, it is somewhat surprising that 100 nM insulin did not at least partially replicate the stimulatory effect of IGF-I on telomerase activity. The lack of effect by IGF-II is even more difficult to explain, as IGF-II is a potent mitogen that acts through type I IGF receptor signaling. We did not perform these experiments in any other cell lines, and this may merely represent a peculiarity intrinsic to the IGF signaling cascade in DU-145 cells. Additional studies with insulin and IGF-II will be necessary at wider concentration ranges, at multiple time points, and in multiple prostate cancer cell lines before the possibility of a similar effect by IGF-II can be excluded.

A potential relationship between the IGF axis and telomerase activation may have relevance to the pathogenesis of human cancer. The potent mitogenic and potent cell survival-enhancing properties of IGF-I are well recognized, and ample in vitro data have suggested a potential role in human cancer pathogenesis (37, 38, 39). Additionally, several case-control studies have suggested a link between serum IGF-I levels and risk of several cancers, including prostate, breast, colorectal, and lung carcinoma (40, 41, 42, 43, 44). In the context of our findings, it appears that IGF-I could potentially contribute to the immortalization process of malignancy by up-regulating telomerase activation, leading to telomere lengthening and extension of the cellular life span.

On the other hand, the significance of IGF-I up-regulating telomerase in cell lines that already possess high levels of telomerase and are already immortalized could be questioned. However, further elevation of telomerase in tumor cells is significant and in several tumor types has been associated with a more aggressive tumor phenotype (7, 45). Telomerase is not merely an on/off switch, and the relative level of activity is important. Inhibition of telomerase activity to a critical level has been demonstrated in immortalized tumor cells in culture as well as in tumor implants in vivo to inhibit tumor cell proliferation and induce massive apoptosis (17, 18, 46). Specifically in prostate cancer cell lines, a relationship between telomerase inhibition and relative inhibitory effects on cell life span, cell viability, and tumorigenicity has been demonstrated (47, 48).

Up-regulation of telomerase by IGF-I may have additional relevance to the ongoing investigation of the processes underlying human aging. Telomere attrition is a central component of aging, and its prevention by the introduction of telomerase activity has been demonstrated to extend the cellular life span indefinitely (7). Compelling in vivo and clinical models, such as the telomerase-deficient mouse (mTERC-/-) and the human conditions dyskeratosis congenita and Werner syndrome illustrate the potentially critical role telomerase may play in aging (49, 50, 51, 52). A role for the GH-IGF axis in aging has been suggested, considering the association between declining GH levels and the catabolic processes inherent to aging (53). In light of our findings, up-regulation of telomerase is one potential mechanism by which the GH-IGF axis could mediate antiaging effects.

In summary, the cellular properties of IGF-I now appear to include the up-regulation of telomerase activity. Establishing a relationship between telomerase activation and the IGF axis may serve as an important conceptual model of immortalization in hormonally mediated cancers. Up-regulation of telomerase could represent a mechanism by which the GH-IGF axis could mediate antiaging effects. Our observations remain incomplete. The mechanism of telomerase activation by IGF-I needs to be further elucidated. The potential role of IGFBPs in modulating this effect or in impacting telomerase activation directly, independent of IGFs, also needs to be explored. Lastly, investigation of the potential effects of IGF-I on telomerase in normal somatic cells will be of considerable value.


   Footnotes
 
This work was supported in part by Grants 2R01-DK-47591 and 1RO1-AI-40203; by awards from the Department of Defense, the American Cancer Society, and the Juvenile Diabetes Foundations (to P.C.); and by National Research Scientist Award and Lawson Wilkins Pediatric Endocrine Society grants (to L.W.).

Abbreviations: A/P/S, Amphotericin/penicillin/streptomycin; FBS, fetal bovine serum; hTERT, human telomerase reverse transcriptase; hTR, human telomerase RNA; IGFBP, IGF-binding protein; IGFR, IGF receptor; PHA, phytohemagglutinin; PI3-kinase, phosphoinositol 3-kinase.

Received August 19, 2002.

Accepted March 24, 2003.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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