Age-associated increased interleukin-6 gene expression, late-life diseases, and frailty

Interleukin-6 (IL-6) is a proinflammatory cytokine that is normally tightly regulated and expressed at low levels, except during infection, trauma, or other stress. Among several factors that down-regulate IL-6 gene expression are estrogen and testosterone. After menopause or andropause, IL-6 levels are elevated, even in the absence of infection, trauma, or stress. IL-6 is a potent mediator of inflammatory processes, and it has been proposed that the age-associated increase in IL-6 accounts for certain of the phenotypic changes of advanced age, particularly those that resemble chronic inflammatory disease [decreased lean body mass, osteopenia, low-grade anemia, decreased serum albumin and cholesterol, and increased inflammatory proteins such as C-reactive protein (CRP) and serum amyloid A]. Furthermore, the age-associated rise in IL-6 has been linked to lymphoproliferative disorders, multiple myeloma, osteoporosis, and Alzheimer's disease. This overview discusses the data relating IL-6 to age-associated diseases and to frailty. Like the syndrome of inappropriate antidiuretic hormone, it is possible that certain clinically important late-life changes are due to an inappropriate presence of IL-6.

Interleukin-6: a cytokine for gerontologists

Interleukin-6 (IL-6) is a multifunctional cytokine that presumably plays its major role as a mediator of several of the acute phase inflammatory responses. These include inflammatory cell and lymphocyte activation and hepatocellular stimulation of acute phase protein synthesis. IL-6 expression is normally low, and serum levels are usually non-detectable in the absence of inflammation. However, with advancing age, serum levels become detectable, and it is proposed that this reflects an age-associated loss in the normal regulation of gene expression for this molecule. The cause of this is most likely multi-factorial, but there is evidence that it relates to an age-associated loss of T cell immunoregulatory functions as well as menopausal loss of estrogen. In any event, the "inappropriate" presence of IL-6 results in many changes typical of chronic inflammation. There is also speculation that IL-6 may contribute to the pathogenesis of several diseases of late-life including lymphoma, osteoporosis, and Alzheimer's disease. In this review the biology of this important cytokine is presented and its relevance to gerontology is highlighted.

Dietary restriction of adult male rhesus monkeys: design, methodology, and preliminary findings from the first year of study

Dietary restriction (DR) retards aging processes and extends maximum life span in rodents and in simpler animals. We initiated a study in 30 adults (8-14 years old) male rhesus monkeys to determine whether or not aging processes are retarded by adult-onset DR in a primate species and herein report results from the experiment's first year. Following a 3-6 month period when baseline data were obtained, 15 animals were assigned to a control group and given free access to a semipurified diet for 6-8 hours per day. The other 15 monkeys were fed the same diet but at 70% of their baseline intake levels predetermined individually. The animals are being evaluated semi-annually for body size and composition, physical activity, metabolic rate, glucose tolerance and insulin sensitivity, hematologic indices, immunologic function, and fingernail growth. Ocular function is assessed annually. The preliminary observations after one year are: (a) all monkeys appear to be in excellent health; (b) average body weights for controls increased by 9% while monkeys on DR did not gain weight; (c) monkeys on DR have less body fat than do control monkeys, whereas the amount of lean body mass has not been significantly influenced by DR; (d) there was a small but statistically significant reduction in physical activity for monkeys on DR relative to controls; and (e) DR has not overtly influenced the other measures. Control monkeys gradually reduced their voluntary levels of food intake during the first year of study, and food allotments to DR monkeys are being adjusted accordingly in order to reinstate the intended 30% difference between groups. These early data indicate that DR can be safely instituted in adult monkeys, but that longer term and/or more severe DR is required to determine if it is capable of influencing age-sensitive indices in long-lived primates.

In vivo cytokine gene transfer by gene gun reduces tumor growth in mice

Implantation of tumor cells modified by in vitro cytokine gene transfer has been shown by many investigators to result in potent in vivo antitumor activities in mice. Here we describe an approach to tumor immunotherapy utilizing direct transfection of cytokine genes into tumor-bearing animals by particle-mediated gene transfer. In vivo transfection of the human interleukin 6 gene into the tumor site reduced methylcholanthrene-induced fibrosarcoma growth, and a combination of murine tumor necrosis factor alpha and interferon gamma genes inhibited growth of a renal carcinoma tumor model (Renca). In addition, treatment with murine interleukin 2 and interferon gamma genes prolonged the survival of Renca tumor-bearing mice and resulted in tumor eradication in 25% of the test animals. Transgene expression was demonstrated in treated tissues by ELISA and immunohistochemical analysis. Significant serum levels of interleukin 6 and interferon gamma were detected, demonstrating effective secretion of transgenic proteins from treated skin into the bloodstream. This in vivo cytokine gene therapy approach provides a system for evaluating the antitumor properties of various cytokines in different tumor models and has potential utility for human cancer gene therapy.

Molecular and cellular biology of interleukin-6 and its receptor

Interleukin-6 (IL-6) is a member of the family of cytokines collectively termed "the interleukin-6 type cytokines." Among its many functions, IL-6 plays an active role in immunology, bone metabolism, reproduction, arthritis, neoplasia, and aging. IL-6 expression is regulated by a variety of factors, including steroidal hormones, at both the transcriptional and post-transcriptional levels. IL-6 achieves its effects through the ligand-specific IL-6 receptor (IL-6R). Unlike most other cytokine receptors, the IL-6R is active in both membrane bound and soluble forms. Defining mechanisms to control IL-6 or IL-6R expression may prove useful for therapy of the many clinical disorders in IL-6 plays a role.

Inhibition of NFkappaB activity through maintenance of IkappaBalpha levels contributes to dihydrotestosterone-mediated repression of the interleukin-6 promoter

Androgens repress expression of many genes, yet the mechanism of this activity has remained elusive. The cytokine, interleukin-6, is active in a variety of biological systems, and its expression is repressed by androgens. Accordingly we dissected the mechanism of androgen's ability to inhibit interleukin-6 expression at the molecular level. In a series of co-transfection assays, we found that 5alpha-dihydrotestosterone, through the androgen receptor, repressed activation of the interleukin-6 promoter, in part, by inhibiting NFkappaB activity. It did not appear that 5alpha-dihydrotestosterone inhibited NFkappaB by activating the androgen receptor to compete for the NFkappaB response element as we could not detect androgen receptor binding to the IL-6 promoter by DNase I footprinting assay. However, by electrophoretic mobility shift assay we found that 5alpha-dihydrotestosterone repressed formation of NFkappaB middle dotNFkappaB response element complex formation. In LNCaP prostate carcinoma cells, 5alpha-dihydrotestosterone achieved this effect through maintenance of IkappaBalpha protein levels in the face of phorbol ester, a stimulus that results in IkappaBalpha degradation. Finally, we confirmed that IkappaBalpha inhibits NFkappaB-mediated activation of the interleukin-6 promoter. These data suggest that maintenance of IkappaBalpha levels may represent the first identified mechanism for androgen-mediated repression of a natural androgen-regulated gene.

Aging and cancer: issues of basic and clinical science

The majority of patients with cancer in the United States are more than 70 years old. Despite the increased understanding of the molecular bases for both oncogenesis and aging, the overlap of cancer and aging at that level remains a wide-open research domain. Similarly, at the clinical level, there is also an increased awareness of the need for more information about the influence of host age on the development of tumors, on the growth and spread of the disease, and on treatment expectations. In this review, we have attempted to frame questions regarding cancer and aging from the perspective of biogerontology and geriatric medicine. An increased effort to address the issues of aging is of paramount importance at all levels of cancer investigation.

Frontal brain asymmetry and immune function

The relation between brain activity and the immune system was evaluated by assessing immune responses in 20 healthy women who manifested extreme differences in the asymmetry of frontal cortex activation. One group showed extreme and stable left frontal activation; the other group showed extreme and stable right frontal activation. As predicted, women with extreme right frontal activation had significantly lower levels of natural killer cell activity (at effector:target cell ratios of 33:1 and 11:1) than did left frontally activated individuals. This difference did not extend to two other immune measures, lymphocyte proliferation and T-cell subsets. However, higher immunoglobulin levels of the M class were observed in the right frontal group. In this study, the immune patterns could not be accounted for by plasma cortisol levels, anxiety- and depression-related symptomatology, or recent health histories. These findings support the hypothesis that there is a specific association between frontal brain asymmetry and certain immune responses.

Efficacy of an influenza hemagglutinin-diphtheria toxoid conjugate vaccine in elderly nursing home subjects during an influenza outbreak

OBJECTIVE

To compare the efficacy of an influenza hemagglutinin-diphtheria toxoid conjugate vaccine with the commercially available influenza hemagglutinin-subunit vaccine in preventing influenza in older adults living in a nursing home.

DESIGN

A prospective, randomized, double-blind vaccine trial with 5 months of follow-up after vaccination.

SETTING

Fourteen Wisconsin nursing homes.

PARTICIPANTS

Nursing home residents at least 65 years old who were able to give informed consent and were free of malignancy and not receiving immunosuppressive therapy.

INTERVENTIONS

Participants received, by intramuscular injection, 0.5 mL of a trivalent influenza vaccine containing 15 micrograms each of A/Leningrad/360/86 (H3N2), A/Taiwan/1/86 (H1N1), and B/Ann Arbor/1/86 (HA) or 0.5 mL of an influenza vaccine containing the same antigens conjugated to diphtheria toxoid (HA-D).

MEASUREMENTS

Blood was obtained pre- and 1 month post-vaccination to assess for any vaccine-induced antibody titer change. Clinical surveillance for respiratory illness was performed twice weekly for 5 months. A record was kept of all signs and symptoms of new respiratory illness, and a viral culture and acute and convalescent sera were obtained.

RESULTS

204 participants received HA and 204 received HA-D. Both groups had similar baseline antibody levels to all influenza antigens. HA-D recipients seroconverted more frequently based on serum neutralizing activity (P < 0.05), had a greater increase in geometric mean titer (GMT), and sustained the increase in antibody titer longer than HA recipients. Vaccine hemagglutinin recall was greater in a subset of HA-D recipients as measured by lymphocyte proliferative assays (P < 0.05). During an outbreak of influenza A (H3N2 A/Shanghai/11/87-like and A/Victoria/7/87-like), fewer HA-D (29/195) than HA (43/204) recipients had laboratory-confirmed infection (P = 0.053), and, of these, fewer HA-D-treated subjects had lower respiratory tract involvement (5/29 HA-D and 17/43 HA) (P = 0.022).

CONCLUSIONS

HA-D was more immunogenic in institutionalized elderly recipients and produced greater protection from influenza infection. Superior protection may be due to HA-D's ability to stimulate and recruit antigen-presenting cells, thus enabling the recipient to achieve and maintain functional antibody titers.

Effects of early rearing environment on immune responses of infant rhesus monkeys

In animals, perturbations of the rearing environment have been shown to alter behavior, cognition, and physiology, including immune responses. In order to evaluate the effect of early rearing conditions on the development of immune responses in the infant primate, several immunological measures were assessed in rhesus monkey infants, nursery-reared (NR) or mother-reared (MR), from birth to 2 years of age. Rearing in the absence of the mother affected several aspects of cellular immunity. NR monkeys had significantly lower proportions of CD8 cells and lower natural killer cell activity than did MR monkeys. In contrast, their lymphocyte proliferation responses to mitogen stimulation were higher than those of MR monkeys. An attempt to behaviorally rehabilitate the NR infants at 1 year of age did not result in a recovery of normal immune responses. This study indicates that abnormal early rearing may have long-lasting effects on the immune system, which could have health consequences later in life.

Duration of immunity after hepatitis B vaccination: efficacy of low-dose booster vaccine

Although the efficacy of hepatitis vaccine is well documented, the duration of immunity of healthy adults after vaccination is unknown. We studied 245 hospital employees 3 years after primary vaccination with hepatitis B vaccine to determine the prevalence of immunity indicated by levels of antibody to hepatitis B surface antigen of 10 mIU/mL or greater; and to compare the immunogenicity of low-dose intradermal vaccine with standard-dose intramuscular vaccine in persons found to be seronegative. Thirty-eight percent of employees studied had antibody levels less than 10 mIU/mL. Low levels were associated with smoking, older age, and higher body-mass index. Seventy-eight percent of persons with low antibody levels responded to a single booster vaccine. Two micrograms of intradermal vaccine was as effective as 20 micrograms of intramuscular vaccine in inducing an antibody response; however, intradermal vaccine was associated with more local reactions (42% compared with 17%). Many healthy adults will need periodic boosters of hepatitis B vaccine to maintain production of antibody to hepatitis B surface antigen; low-dose intradermal booster schedules may be feasible.

Chinese hamster ovary cell adhesion to human platelet thrombospondin is dependent on cell surface heparan sulfate proteoglycan

Thrombospondin is a 420-kD platelet alpha-granule glycoprotein that binds specifically to heparin. We examined adhesion to thrombospondin of CHO K1 cells and three mutant CHO lines with varying deficiencies in glycosaminoglycan (GAG) synthesis. In an experiment in which the parent line (K1) had 78% adherence to thrombospondin adsorbed to tissue culture plastic, CHO S745 cells, with less than 6% normal GAG synthesis had 11% adherence. CHO S677 cells, with decreased heparan sulfate proteoglycan but increased chondroitin sulfate proteoglycan, had 42% adherence. CHO S803 cells, with decreased heparan sulfate proteoglycan and normal chondroitin sulfate proteoglycan, had 31% adherence. Heparin inhibited K1 cell adhesion to thrombospondin, but not fibronectin, in a concentration-dependent manner. Dermatan sulfate but not chondroitin sulfate was also inhibitory. There was markedly decreased K1 cell adhesion to a thrombospondin core fragment that lacked the heparin binding NH2-terminal domain. Purified heparin binding domain, although poorly adhesive when adsorbed to substratum, inhibited cell adhesion to intact thrombospondin. Adhesion was better for all cell lines tested, including three human tumor cell lines, when thrombospondin was adsorbed at pH 4.0 compared with pH 7.4. When adsorption of thrombospondin was done at pH 7.4, cell adhesion was better when thrombospondin was adsorbed in the presence of greater than or equal to 0.6 mM calcium, compared to 0.1 mM calcium or EDTA. These findings suggest that thrombospondin can adsorb to plastic with varying degrees of exposure of a cell adhesion domain. We conclude that the thrombospondin cell adhesion receptor on CHO cells is a heparan sulfate proteoglycan, and that cell adhesion to thrombospondin depends on conformation of adsorbed thrombospondin.

Influence of early rearing on lymphocyte proliferation responses in juvenile rhesus monkeys

Lymphocyte proliferation responses and natural killer cell activity were evaluated in 35 juvenile rhesus monkeys derived from five different rearing conditions. Nursery-reared monkeys had proliferation responses which were significantly higher than those of mother-reared subjects. Reexamination of the nursery-reared monkeys 1.5 years later indicated that an abnormally high response to concanavalin A was still evident at 2.5 years of age, but both PHA and PWM responses had shown an age-appropriate decrease into the normal range for this species. Proliferation responses in monkeys that had been weaned early from their mothers at 6 months of age were also higher than values for control monkeys that remained with their mothers, but below those of the nursery-reared monkeys. In contrast, monkeys that had received multiple separations from the mother between 3 and 7 months of age showed lymphocyte proliferation responses that were below normal. These results indicate that early rearing conditions can have a lasting effect on certain immune responses in the developing primate.

Elevated plasma levels of interleukin-6 in postmenopausal women do not correlate with bone density

OBJECTIVE

To determine if plasma levels of Interleukin-6 (IL-6) across the lifespan correlate with bone density or plasma osteocalcin.

DESIGN

Cross-sectional study.

PARTICIPANTS

Forty-five healthy community-dwelling volunteers aged 25-74 years. Exclusion criteria were smoking use of medications known to affect bone metabolism (corticosteroids, heparin, thyroxine, thiazides, and anticonvulsants), and presence of chronic inflammatory disease.

MEASUREMENTS

Bone density was measured by dual-energy X-ray absorptiometry at the femoral neck and lumbar spine. Plasma levels of IL-6 and osteocalcin were determined by ELISA and RIA, respectively.

RESULTS

Plasma levels of IL-6 increased with advancing age (P < .0001) and correlated with postmenopausal status (P < .0001). No correlation was observed between plasma IL-6 level and bone mineral density at either the lumbar spine or femoral neck, and none was observed with plasma osteocalcin.

CONCLUSIONS

The elevation of plasma IL-6 observed following menopause is consistent with the proposed importance of estrogen in the regulation of IL-6. These findings do not provide support for a role of IL-6 in determination of peak bone density or subsequent development of osteoporosis. However, it is possible that plasma levels of IL-6 differ from those in the bone microenvironment.

Estrogen inhibits phorbol ester-induced I kappa B alpha transcription and protein degradation

Estrogen (E2) is known to prevent bone loss and the mechanism is, at least in part, mediated by inhibition of expression of cytokines such as interleukin-6 (IL-6). Expression of IL-6 is tightly regulated and the transcription factor NF kappa B can upregulate IL-6 gene expression by binding to its promoter region. NF kappa B is kept in an inactive state by associating with its cytoplasmic inhibitor I kappa B alpha. Upon mitogenic stimulation, I kappa B alpha becomes phosphorylated, followed by a rapid protein degradation. As a result, NF kappa B is released and translocate to the nucleus where DNA binding occurs. It has been shown that E2 treatment downregulates mitogen-induced IL-6 expression by inhibiting NF kappa B activity. Here, we sought to determine whether E2 regulates IL-6 gene expression by modulating the levels of I kappa B alpha. Our results show that E2 treatment almost completely inhibits phorbol ester-induced I kappa B alpha protein degradation. In addition, E2 inhibits phorbol ester-stimulated I kappa B alpha gene expression. Taken together, our results suggest that E2 maintains steady state levels of I kappa B alpha upon mitogen stimulation, resulting in inhibition of NF kappa B activation and IL-6 gene expression. This may explain the protective effect of E2 on bone loss.

The role of interleukin-6 in certain age-related diseases

Interleukin-6 (IL-6) is a pro-inflammatory cytokine with a wide range of functions. Perhaps the most important physiologically is its role as a mediator of the acute phase inflammatory response. Normally, there is little measurable IL-6 in the circulation, but levels increase abruptly to nanogram amounts during an inflammatory process. During aging, it has been proposed that the tight regulation of IL-6 gene expression becomes less effective and levels are measurable even when there is no evidence for inflammation. Several investigators have identified this cytokine as being involved in the pathogenesis of various disease processes and we have suggested that certain age-associated diseases are directly related. Among these are late-life lymphoma and myeloma, osteoporosis and possibly Alzheimer's disease.

Bone-marrow microenvironment defect in congenital hypoplastic anemia

An abnormal bone-marrow microenvironment has been thought to have an important role in the pathogenesis of aplastic and hypoplastic anemia in some patients, but direct evidence of such a defect has not been found. We have investigated the pathogenesis of chronic anemia in a young woman. Her bone-marrow cells, obtained by means of aspiration, showed exuberant erythroid growth in methylcellulose despite marked erythroid hypoplasia. The erythroid nature of the colony growth was further confirmed through measurement of heme synthesis and messenger RNA-globin accumulation in a liquid-culture system. In contrast, when whole bone fragments were similarly cultured, no appreciable hemoglobin synthesis was observed. The experimental evidence suggests that, in this patient, hypoplastic anemia resulted from an unfavorable microenvironment.

B16 murine melanoma and aging: slower growth and longer survival in old mice

The growth characteristics and colonization potential of a transplantable melanoma administered to young (3 mo) and old (24 mo) C57BL/6 mice were investigated. After sc injection of B16-F10 melanoma cells, tumor growth was slower, and final tumor volume was less in the older mice. Furthermore, after iv injection of B16-F1 melanoma cells, the number of pulmonary colonies was also less, and the survival was greater in the older mice. These findings indicate an age advantage in this experimental tumor model that may be attributed to either physical or immunologic factors.

Influenza and aging: age-related changes and the effects of thymosin on the antibody response to influenza vaccine

Despite massive immunization programs, influenza remains a major cause of morbidity and mortality for elderly people. This may occur because immune senescent recipients may respond to vaccination with inadequate antibody production. We measured antibody response to the trivalent 1983-1984 influenza vaccine in young and elderly volunteers and found a significantly reduced response in the latter. The age-associated decreased antibody production was also observed in lymphocyte cultures in which specific antiinfluenza antibody synthesis was measured. In these cultures, however, the addition of a thymic hormone preparation (either thymosin fraction 5 or thymosin alpha 1) was shown to enhance specific antibody synthesis to a greater extent in the cultures established from the elderly volunteers. If this in vitro observation of thymosin induced increased antibody production reflects what might occur in a clinical trial in which elderly subjects receive thymosin coincident with vaccine, greater protection against influenza infection may result.

Prenatal endocrine activation alters postnatal cellular immunity in infant monkeys

Cellular immune responses were evaluated in 35 infant rhesus monkeys generated from two types of pregnancy conditions. Pregnant females were administered either saline or adrenocorticotropic hormone (ACTH) for 2 weeks between Days 120 and 133 postconception, approximately 1 month before parturition. After birth, lymphocytes obtained from infants in the ACTH condition failed to respond as readily to allogeneic cells in mixed lymphocyte cultures, proliferated less to Con A, exhibited lower suppressor function following stimulation with Con A, and showed lower cytolytic activity against target cells. For some measures, the prenatal effect was observed more consistently in male infants. Differences were evident with these in vitro immune assays through 6 months of age, indicating that acute disturbance during the prenatal period can have lingering effects on postnatal immunity.

The influence of an aging immune system on cancer incidence and progression

The majority of cancers occur in patients over the age of 65 years, and yet the important interactions of normal aging upon the neoplastic process remain to be thoroughly explored. For example, there is no unifying and conclusive explanation for the increased risk of cancer with advancing age. The age-associated decline in immune function has previously been implicated in this regard. However, it is likely that other factors, such as the time it takes to proceed through the multistep neoplastic process, and the combined influence of age-associated increased susceptibility to carcinogen and decreased DNA repair are more important. Nevertheless, immune senescence does influence various aspects of tumor growth and spread. In this article these aspects are reviewed.

Augmentation of influenza antibody response in elderly men by thymosin alpha one. A double-blind placebo-controlled clinical study

Influenza remains a major cause of illness and death in elderly people despite current vaccination programs. One factor is an immunization failure rate in the elderly that may be as high as 50%. To test whether administration of thymosin alpha 1 would result in greater antibody production, we administered it (900 micrograms/m2 subcutaneously twice weekly for eight doses) in conjunction with the 1986 trivalent influenza vaccine. Ninety men (65-99 years old, mean age 77.3 years) were randomized double-blind to receive thymosin alpha 1 or placebo by the same schedule; the sera from 85 of these men were acceptable for analysis. The two groups were similar with respect to underlying disease, medications, and age. No toxicity was observed in either group. Antibody response rate was defined as a four-fold rise in antibody titer over 3-6 weeks following vaccination and was measured by an enzyme-linked immunosorbent assay (ELISA). Analysis was performed on treatment groups and subgroups divided by the mean age: the older group consisted of subjects aged 77 years and older, and the younger group those aged from 65-76 years. Baseline and change in absolute antibody levels were compared by t test and using age as a continuous variable by multiple regression analysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Processing of human beta-globin mRNA precursor to mRNA is defective in three patients with beta+-thalassemia

Nucleated bone marrow cells from normal individuals and from three patients with homozygous beta+-thalassemia were pulse-labeled with tritiated nucleosides. The processing of the newly synthesized globin mRNA precursors was monitored by inhibiting additional transcription with actinomycin D for 30 min. Human beta-globin mRNA is derived from its precursor via a series of reactions that generate processing intermediates. In nonthalassemic cells the precursor is processed efficiently to mature mRNA during the chase. In contrast, in beta+-thalassemic cells the processing of beta-globin RNA is defective. In one patient the beta-globin mRNA precursor turns over during the chase, but some of the intermediate RNAs accumulate and are not processed to mRNA. In two other patients a large fraction of the precursor and intermediate RNAs is not processed to mRNA. The alpha-globin mRNA precursor and intermediates are processed efficiently to mRNA-sized molecules in thalassemic and normal cells. The reduction in the rate of beta-globin but not alpha-globin RNA processing accounts for the alpha/beta globin mRNA imbalance in thalassemic erythroid cells. We discuss the possibility that the genetic lesions in beta+-thalassemia are at splicing signal sites within intervening sequences of the beta-globin gene.

The androgen receptor: a mediator of diverse responses

Androgens mediate a number of diverse responses through the androgen receptor, a 110 kD ligand-activated nuclear receptor. Androgen receptor expression, which is found in a variety of tissues, changes throughout development, aging, and malignant transformation. The androgen receptor can be activated by two ligands, testosterone and dihydrotestosterone, which bind to the androgen receptor with different affinities. This difference in binding affinity results in different levels of activation of the androgen receptor by the two ligands. The androgen receptor acts as a transcriptional modifier of a variety of genes by binding to an androgen response element. The ability to confer androgen specific actions by the androgen response element may depend on other cell-specific transcription factors and cis-acting DNA elements in close proximity to it. Testosterone and dihydrotestosterone appear to act upon an identical nuclear receptor. However, in certain instances, they mediate different physiologic responses. For example, dihydrotestosterone, but not testosterone, is capable of mediating full sexual development of the male external genitalia. In some cases, the androgen receptor may induce opposite physiologic responses in similar tissue types depending on their location. For example, in male pattern baldness, activated androgen receptors may suppress the growth of distinct hair follicle populations through initiating stromal-epithelial actions, whereas other hair follicles continue to proliferate. In other cases, altered androgen receptor activity due to its mutation or altered expression may lead to pathology such as recurrence of prostate cancer due to development of androgen independence allowing tumor cell proliferation under androgen deprivation.