Geographic Relocation in Response to Parents' Health Shocks: Who Moves and How Close?

Objective

This article examines how parent-child geographic proximity changes around the onset of parental health shocks in the United States. Differences in the likelihood of moving closer across social groups are also investigated.

Background

Adult children often care for older parents with health problems, but this requires relatively close proximity. As families are becoming smaller and many adult children live away from their parents, it is unclear how responsive families will be to older adults' health problems.

Method

We estimate a series of fixed effects and event study models on data from the Health and Retirement Study (2004-2018) to assess changes in parent-child proximity after parents' first onset of cognitive impairment and functional limitations.

Results

We find robust evidence that parents and children tend to stay close or move closer to each other in response to parent's health declines. Moves occur immediately and in subsequent waves after the onset of health shocks. Reductions in parent-child distance are consistently larger among mother-daughter dyads, dyads without spouses or multiple children, and non-Hispanic white families.

Conclusion

The geographic availability of adult children to provide care is responsive to parents' needs. After the onset of a serious health condition, most older adults have a spouse or child living close enough to provide care. Parents' and children's lives are dynamically linked, and either or both may relocate to facilitate care.

The Changing Demography of Late-Life Family Caregiving: A Research Agenda to Understand Future Care Networks for an Aging U.S. Population

Repeated claims that a dwindling supply of potential caregivers is creating a crisis in care for the U.S. aging population have not been well-grounded in empirical research. Concerns about the supply of family care do not adequately recognize factors that may modify the availability and willingness of family and friends to provide care to older persons in need of assistance or the increasing heterogeneity of the older population. In this paper, we set forth a framework that places family caregiving in the context of older adults' care needs, the alternatives available to them, and the outcomes of that care. We focus on care networks, rather than individuals, and discuss the demographic and social changes that may alter the formation of care networks in the future. Last, we identify research areas to prioritize in order to better support planning efforts to care for the aging U.S. population.

Intergenerational support and retirement timing among older men and women by race/ethnicity

Retirement timing is associated with health and economic outcomes for older adults. However, it is unclear how the pressures of supporting older parents and young adult children are associated with retirement. This study uses a life course perspective to consider how the linked lives of working older adults and their support of adult children and parents are associated with retirement. Cox proportional hazard models are estimated using the Health and Retirement Study (1992-2014) to assess the relationship between intergenerational support exchanges and retirement timing by gender and race/ethnicity. Providing most types of intergenerational support and especially providing time support are associated with an increased risk of retirement. Unlike all other respondents, Hispanic women providing intergenerational time support have similar retirement risks as those not providing any intergenerational support. These differing patterns by race/ethnicity suggest that earlier life course trajectories may shape older adults' ability to respond to family needs.

Gender, Age of Migration, and Cognitive Life Expectancies Among Older Latinos: Evidence From the Health and Retirement Study

OBJECTIVES

Migration and gender are important factors that differentiate the Latino immigrant experience in the United States. We investigate the association between nativity status, age of migration, and cognitive life expectancies among a nationally representative sample of Latino adults aged 50 and older to explore whether age of migration and gender influence cognitive aging across the life course.

METHODS

This study used data from the Health and Retirement Study (1998-2016) to estimate Sullivan-based life tables of cognitive life expectancies by nativity, age of migration, and gender for older Latino adults. Cognitive status was based on the Langa-Weir algorithm. We test for both within-group (i.e., nativity and age of migration) and gender differences to explore the overall burden of disease among this rapidly growing population.

RESULTS

Foreign-born Latinos, regardless of age of migration or gender, spend a greater number of years after age 50 with cognitive impairment/no dementia than U.S.-born Latinos. However, the number of years spent with dementia varied by subgroup with midlife immigrant men and late-life immigrant men and women exhibiting a significant disadvantage relative to the U.S.-born. Furthermore, we document a gender disadvantage for all Latino women, regardless of immigrant status.

DISCUSSION

The robust relationship between nativity, age of migration, and cognitive aging suggests that older foreign-born Latinos experiencing cognitive decline may place serious burdens on families. Future research should target the needs of different subgroups of older Latinos who are entering their last decades of life to develop culturally appropriate long-term care programs.

Co-residence beliefs 1973-2018: Older adults feel differently than younger adults

Objective

This brief study examines support for co-residence (i.e. aging parents living with their adult children), and how age predicts support for this belief considering the rapidly aging U.S. population.

Background

Co-residence, a form of intergenerational transfer between family members, can help facilitate care for aging parents as well as help older adults age in the community. Support for this type of co-residence was on the rise in the 1970s and 1980s.

Method

Support for co-residence of older adults living with their adult children is estimated using 36,843 responses from the U.S. General Social Survey from 1973 to 2018. Descriptive analyses, logistic regression, and decomposition analyzes are used to test explanatory factors in trends, focusing on differences for older (age 65 and older) vs. younger (under 65) respondents.

Results

Older adults are less supportive than younger adults of co-residence even as support has generally increased across time. Decomposition results show that a little over half of the difference between younger and older adults is explained by cohort replacement, with two-fifths of the difference unexplained by social or demographic factors.

Conclusion

Findings suggest that although cohort replacement has contributed to an attitude shift over time, important age differences in attitudes remain. Older adults are less supportive of co-residence than younger adults.

Mitigating Poverty through the Formation of Extended Family Households: Race and Ethnic Differences

In times of hardship, moving in with family is one strategy for alleviating economic deprivation and uncertainty. The ability of the family to buffer against poverty may vary by the resources available to and the economic needs of individuals. I assess how the formation of extended-family households is associated with a move into or out of poverty and how this association varies by race and ethnicity, since economic resources and norms around extended-family households differ. Using longitudinal data that span four years, I estimate linear fixed effects regression models to assess how changes in living arrangements are related to changes in poverty. I find that moving into an extended-family household reduces poverty, especially for the joining family unit. Most of this poverty reduction occurs through a family safety net, with a non-poor family taking in poor family units.

Gender and Age of Migration Differences in Mortality Among Older Mexican Americans

OBJECTIVES

Using a gendered life course perspective, we examine whether the relationship between age of migration and mortality is moderated by gender among a cohort of older Mexican Americans.

METHODS

Data from the Hispanic Established Populations for the Epidemiological Study of the Elderly and recently matched mortality data are used to estimate Cox proportional hazard models.

RESULTS

Our findings indicate that the relationship between age of migration and mortality is moderated by gender, suggesting a more nuanced perspective of the immigrant mortality paradox. Among men, midlife migrants exhibit an 18% lower risk of mortality compared to their U.S.-born co-ethnics, possibly due to immigrant selectivity at the time of migration. Conversely, late-life migrant women exhibit a 17% lower risk of mortality relative to U.S.-born women, attributed in part to socio-cultural characteristics that influence lifestyle risk factors across the life course.

DISCUSSION

Selection mechanisms and acculturation processes associated with the immigrant experience are contingent on both age and gender, suggesting the utility of an integrated life course approach to contextualize the mortality profiles of older immigrants. These findings demonstrate the heterogeneity among immigrants and highlight the need to understand gender differences in the migration process when assessing the immigrant mortality paradox.

Nativity and Country of Origin Variations in Life Expectancy With Functional Limitations Among Older Hispanics in the United States

This study examined racial/ethnic, nativity, and country of origin differences in life expectancy with and without functional limitations among older adults in the United States. We used data from the National Health Interview Survey (1999-2015) to estimate Sullivan-based life tables of life expectancies with functional limitations and without functional limitations by sex for U.S.-born Mexicans, foreign-born Mexicans, U.S.-born Puerto Ricans, island-born Puerto Ricans, foreign-born Cubans, and U.S.-born Whites. We find that Latinos exhibit heterogeneous life expectancies with functional limitations. Among females, U.S.-born Mexicans, foreign-born Mexicans, and foreign-born Cubans spend significantly fewer years without functional limitations, whereas island-born Puerto Ricans spend more years with functional limitations. For men, U.S.-born Puerto Ricans were the only Latino subgroup disadvantaged in the number of years lived with functional limitations. Conversely, foreign-born Cubans spend significantly fewer years without functional limitations. To address disparities in functional limitations, we must consider variation in health among Latino subgroups.

Abstract 2700: CD8 T-cell infiltration into renal tumors requires a supportive antigen-presenting niche

Background: Tumor infiltrating immune cells have a clear prognostic benefit in many tumor types. Immune variables have independently improved prognostication in various cancer types, with tumor-infiltrating lymphocytes (TILs) more accurately predicting patient survival than currently employed methods. This has been shown using the Immuno-score, which predicts disease progression in colorectal cancer based on CD8 T cell infiltration. Many recent studies have also highlighted similar observations in other cancers, including breast cancer, lung cancer, and melanoma. These observations raise the question of whether the level of CD8 T cell infiltration into renal cell tumors may also predict patient survival, and more fundamentally, why some patients may mount a strong immune response to their tumors and others do not.

Methods: Tumor tissue was collected from 68 renal cell carcinoma (RCC) patients undergoing surgery at Emory University Hospital. Patients had a minimum follow up time of 24 months. Intraoperative tumor samples were processed and analyzed by flow cytometry and immunofluorescence.

Results: The proportion of CD8 TILs, measured by flow cytometry, was found to vary widely in RCC patients. This CD8 T cell response is independent of standard risk assessment tools, tumor size, pathology, and patient demographics. Significantly, an increasing percent of tumor CD8 T cells is associated with improved cancer-specific survival in these patients, and this association is particularly strong in a small cohort of stage III patients.

The phenotype and functional capacity of TILs were examined, and presence of a stem-like CD8 T cell—that can proliferate and differentiate—was required to generate a strong anti-tumor Tcell response. When this stem-like T cell is lost, there is a poor anti-tumor immune response and patients experience progressive disease. Flow cytometry analysis revealed that the number of dendritic cells in the tumor correlates with T cell infiltration, and immunofluorescence image analysis showed that stem-like T cells reside in areas of high antigen-presenting cell density. Tumors with poor T cell infiltration lack APC density, suggesting that an antigen presenting niche is required for a strong T cell response.

Conclusions: Measuring CD8 T cell infiltration in RCC predicts cancer-specific survival, particularly in patients with advanced disease. As this patient population is one for whom improved prognostication is a critical clinical goal, this study represents an opportunity to inform future prognostic measures and to direct reduction or intensification of therapy.

The T cell response was found to be maintained by a population of cells, which harbor both proliferative and differentiation capacity. These stem-like cells require a supportive niche inside the tumor in order to persist, and without this support, the T cell response collapses, resulting in disease progression.

Citation Format: Caroline S. Jansen, Nataliya Prokhnevska, Viraj A. Master, Jennifer W. Carlisle, Mehmet A. Bilen, Adriana M. Reyes, Haydn T. Kissick. CD8 T-cell infiltration into renal tumors requires a supportive antigen-presenting niche [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2700.

Physical Functioning and Disability Trajectories by Age of Migration Among Mexican Elders in the United States

Objectives

To address a gap in our understanding of the long-term consequences of nativity and age of migration for the health of the Mexican elderly population.

Method

We employ age graded latent growth curve models stratified by gender to examine the extent of physical functioning and disability, measured in terms of performance-oriented mobility assessments (POMAs) and activities of daily living (ADLs) after age 65 in a large longitudinal sample of Mexican-origin individuals.

Results

Self-care measured ADLs show no age of migration differences. However, physical capacity measured POMAs differ significantly for men and women by age of migration. Migrants who arrived in midlife have fewer functional limitations at age 65 but have steeper increase in POMAs with age.

Discussion

With a rapidly changing demographic profile that includes a large number of aging Mexican-origin immigrants, our society must implement social and health policies to ameliorate the negative health outcomes among immigrant and U.S.-born minority groups.

Physical Functioning and Disability Trajectories by Age of Migration Among Mexican Elders in the United States

Objectives

To address a gap in our understanding of the long-term consequences of nativity and age of migration for the health of the Mexican elderly population.

Method

We employ age graded latent growth curve models stratified by gender to examine the extent of physical functioning and disability, measured in terms of performance-oriented mobility assessments (POMAs) and activities of daily living (ADLs) after age 65 in a large longitudinal sample of Mexican-origin individuals.

Results

Self-care measured ADLs show no age of migration differences. However, physical capacity measured POMAs differ significantly for men and women by age of migration. Migrants who arrived in midlife have fewer functional limitations at age 65 but have steeper increase in POMAs with age.

Discussion

With a rapidly changing demographic profile that includes a large number of aging Mexican-origin immigrants, our society must implement social and health policies to ameliorate the negative health outcomes among immigrant and U.S.-born minority groups.

Race Differences in Linking Family Formation Transitions to Women's Mortality

We examine how the timing and sequencing of first marriage and childbirth are related to mortality for a cohort of 4,988 white and black women born between 1922 and 1937 from the National Longitudinal Survey of Mature Women. We use Cox proportional hazard models to estimate race differences in the association between family formation transitions and mortality. Although we find no relationships between marital histories and longevity, we do find that having children, the timing of first birth, and the sequencing of childbirth and marriage are associated with mortality. White women who had children lived longer than those who had none, but the opposite was found for black women. The effects of birth timing also differed by race; delaying first birth to older ages was protective for white women but not black women. These results underscore the importance of social context in the study of life course transitions.

Cumulative Advantage, Cumulative Disadvantage, and Evolving Patterns of Late-Life Inequality

Purpose of the Study

Earlier studies have identified a pattern of cumulative advantage leading to increased within-cohort economic inequality over the life course, but there is a need to better understand how levels of inequality by age have changed in the evolving economic environment of recent decades. We utilized Survey of Income and Program Participation (SIPP) data to compare economic inequality across age groups for 2010 versus 1983-1984.

Design and Methods

We examined changing age profiles of inequality using a summary measure of economic resources taking into account income, annuitized value of wealth, and household size. We adjusted for survey underreporting of some income and asset types, based on National Income Accounts and other independent estimates of national aggregates. We examined inequality by age with Gini coefficients.

Results

Late-life (65+) inequality increased between the 2 periods, with Gini coefficients remaining higher than during the working years, but with a less steep age difference in inequality in 2010 than in 1983-1984. Inequality increased sharply within each cohort, particularly steeply in Depression-era, war-baby, and leading-edge baby boom cohorts. The top quintile of elderly received increasing shares of most income sources.

Implications

Increasing inequality among older people, and especially in cohorts approaching late life, presages upcoming financial challenges for elderly persons in the lower part of the income distribution. Implications of this increasingly high-inequality late-life environment need to be carefully evaluated as changes are considered in Social Security and other safety-net institutions, which moderate impacts of economic forces that drive increasingly disparate late-life economic outcomes.

The Economic Organization of Extended Family Households by Race/Ethnicity and Socioeconomic Status

This study examines differences in the amount of economic support or mutual benefit derived from extended family living arrangements by studying differences in monetary contributions to essential household expenditures across family units in extended family households. Using the 2008 Survey of Income and Program Participation, multivariate regression and selection models are estimated to assess racial differences in family contributions toward household expenses in extended family households. Extended family households have very unequal monetary contributions towards household rent and utilities, although Hispanics have less unequal monetary contributions compared to other racial groups. Hispanic and Asians extended family households experience decreasing inequality in financial contributions as the income of each family increases whereas no relationship between financial contributions and income is found for whites or blacks. This suggests a different cultural orientation to extended family living arrangements for Asians and Hispanics compared to non-Hispanic whites.

Age of Migration and the Incidence of Cognitive Impairment: A Cohort Study of Elder Mexican-Americans

BACKGROUND AND OBJECTIVES

To explore nativity and age of migration differentials in the incidence of cognitive impairment among older Mexican-Americans.

RESEARCH DESIGN AND METHODS

We employ maximum-likelihood discrete time hazard models to estimate risk ratios of cognitive impairment in a sample of 2,708 Mexican-Americans 65 and older who were cognitively healthy at baseline over a follow-up period of up to 20 years.

RESULTS

Late-life immigrant women have a 46% higher risk of cognitive impairment compared to U.S.-born Mexican-American women. Conversely, midlife immigrant men have a 29% lower risk of cognitive impairment compared to U.S.-born Mexican-American men. The incidence of cognitive impairment did not differ for early-life and midlife immigrant women relative to U.S.-born women or for early- and late-life immigrant men compared to U.S.-born men.

DISCUSSION AND IMPLICATIONS

Differences in cognitive impairment risk between U.S.-born and foreign-born Mexican-American subgroups may be partly due to health selectivity. Cognitive impairment is more prevalent among immigrant groups which may result in a higher burden on family members and/or high dependency on public resources. Programs are needed that can detect decline at earlier stages and reduce the risk for cognitive impairment among older immigrants entering their last decades of life.

Prevalence and Trends in Morbidity and Disability Among Older Mexican Americans in the Southwestern United States, 1993-2013

This study examines the prevalence of morbidity and disability among older Mexican Americans using 5-year age groups. Twenty-year panel data from the Hispanic Established Populations for the Epidemiological Study of the Elderly are used to make detailed comparisons by nativity and gender. Results show that prevalence rates for most chronic conditions for both males and females do not vary by nativity. For disabilities, nativity is a significant predictor of increased instrumental activity of daily living disability for foreign-born females and reduced activity of daily living disability for U.S.-born males. Additionally, results show significant interactions between nativity and age cohorts, with the gap increasing with age for males and decreasing with age for females. These results have important implications for health services and health policy. Given the rapid aging of the Mexican American population, the prevention and treatment of medical conditions, particularly among the foreign-born, should be a major public health priority to reduce dependence from disabilities.

The Longevity Legacy of World War II: The Intersection of GI Status and Mortality

PURPOSE OF THE STUDY

We examine hypotheses involving the potential health advantages of selection into military service and the potential health disadvantages associated with the experience of military service by comparing later-life mortality rates for veterans and nonveterans as well as among veterans based on their cohort of reentry into civilian life.

DESIGN AND METHODS

We use data on 3,453 men, including 1,496 veterans from the older men cohort of the National Longitudinal Surveys to estimate Cox proportional hazard mortality models. We distinguish between veterans and nonveterans and further classify veterans by age at exit while incorporating measures associated with military selection, health behaviors, and socioeconomic status.

RESULTS

Veterans who were discharged from the military at older ages have a mortality advantage relative to veterans discharged at younger ages. For the 1914-1921 birth cohorts, the mortality advantage for veterans who exited around age 30 is apparent for deaths before age 65, but rates equalize across all groups when deaths at older ages are included. These findings are robust to the inclusion of background characteristics, education, occupation, body mass index, smoking, marital status, and proxies for service deferments.

IMPLICATIONS

Rather than focusing on a general health effect of military service, per se, future research should distinguish among individual traits; the nature, timing, and duration of exposures relative to life course stage; and the sociohistorical context of military service to expand our understanding of the differential health consequences of military service.

Trends in cancer screening by citizenship and health insurance, 2000-2010

While early detection through screenings for breast, cervical, and colorectal cancer is essential in improving cancer survival, it is not evenly utilized across class, race, ethnicity, or nativity. Given that utilization of early detection through screenings is not evenly distributed, immigrants who have much lower rates of health insurance coverage are at a disadvantage. We use National Health Interview Survey data linked with the Medical Expenditures Panel Survey to examine the trend in screening rates for breast, cervical, and colorectal cancer from 2000 to 2010, comparing U.S.-born natives, foreign-born citizens, and foreign-born non-citizens. We find that citizenship is clearly advantageous for the foreign-born, and that screening rates are higher among citizens compared to non-citizens overall, but uninsured non-citizens sometimes have higher screening rates that uninsured natives. Health insurance is pivotal for higher screening rates with clear differences among the insured and uninsured. Policies aimed at reducing disparities in cancer screening need to take into account nativity, citizenship, and access to health insurance.

Health insurance instability among older immigrants: region of origin disparities in coverage

OBJECTIVES

We provide a detailed analysis of how the dynamics of health insurance coverage (HIC) at older ages differs among Latino, Asian, and European immigrants in the United States.

METHOD

Using Survey of Income and Program Participation data from the 2004 and 2008 panels, we estimate discrete-time event history models to examine first and second transitions into and out of HIC, highlighting substantial differences in hazard rates among immigrants aged 50-64 from Asia, Latin America, and Europe.

RESULTS

We find that the likelihood of having HIC at first observation and the rates of gaining and losing coverage within a relatively short time frame are least favorable for older Latino immigrants, although immigrants from all three regions are at a disadvantage relative to native-born non-Hispanic Whites. This disparity among immigrant groups persists even when lower rates of citizenship, greater difficulty with English, and low-skill job placements are taken into account.

DISCUSSION

Factors that have contributed to the lower rates and shorter durations of HIC among older immigrants, particularly those from Latin America, may not be easily resolved by the Affordable Care Act. The importance of region of origin and assimilation characteristics for the risk of being uninsured in later life argues that immigration and health care policy should be jointly addressed.

How health conditions translate into self-ratings: a comparative study of older adults across Europe

Using data from the Survey of Health, Ageing and Retirement in Europe, we examine how respondents translate morbidity and disability into self-rated health (SRH), how national populations differ in SRH, and how normative and person-specific reporting styles shape SRH. We construct proxy variables that allow us to specify cultural differences in reporting styles and individual differences in relative rating behavior. Using generalized logistic regression, we find that both of these dimensions of subjectivity are related to SRH; however, their inclusion does not significantly alter the connection between SRH and the set of disease and disability indicators. Further, country differences in SRH persist after controlling for all these factors. Our findings suggest that observed country differences in SRH reflect compositional differences, cultural differences in reporting styles, and perceptions of how health restricts typical activities. SRH also seems to capture underlying but unmeasured health differences across populations.

The impact of local black residents' socioeconomic status on white residents' racial views

This paper extends the study of contextual influences on racial attitudes by asking how the SES of the local black community shapes the racial attitudes of local whites. Using responses to the 1998-2002 General Social Surveys merged with year 2000 census data, we compare the influences of black educational and economic composition on white residents' attitudes. Finally, the independence of these effects from the impact of white contextual SES is assessed. Across three dimensions of racial attitudes, white residents' views are more positive in localities where the black population contains more college graduates. However, such localities tend also to have highly educated white populations, as well as higher incomes among blacks and whites, and the multiple influences are inseparable. In contrast, many racial attitude measures show an independent effect of black economic composition, white residents reporting more negative views where the local African American community is poorer.

Another health insurance gap: gaining and losing coverage among natives and immigrants at older ages

As the immigrant population grows older and larger, limitations on access to health insurance may create a new subgroup of people who remain outside or on the margin of coverage. Using the Survey of Income and Program Participation (SIPP) data from the 2004 and 2008 panels, we address the health insurance gap between foreign-born and native-born adults among those aged 50-64 and the 65 and older, two sub-populations that have received relatively little attention in past research. We argue that current practices leave a significant minority of older foreign-born residents inconsistently covered or without any insurance. We find that health insurance coverage for older immigrants is both less likely and more episodic even when compositional differences in SES and assimilation are controlled.

Prediction of the elastic strain limit of tendons

The elastic strain limit (ESL) of tendons is the point where maximum elastic modulus is reached, after which micro-damage starts. Study of damage progression in tendons under repetitive (fatigue) loading requires a priori knowledge about ESL. In this study, we propose three different approaches for predicting ESL. First, one single value is assumed to represent the ESL of all tendon specimens. Second, different extrapolation curves are used for extrapolating the initial part of the stress-strain curve. Third, a method based on comparing the shape of the initial part of the stress-strain curve of specimens with a database of stress-strain curves is used. A large number of porcine tendon explants (97) were tested to examine the above-mentioned approaches. The variants of the third approach yielded significantly (p<0.05) smaller error values as compared to the other approaches. The mean absolute percentage error of the best-performing variant of the shape-based comparison was between 8.14±6.44% and 9.96±9.99% depending on the size of the initial part of the stress-strain curves. Interspecies generalizability of the best performing method was also studied by applying it for prediction of the ESL of horse tendons. The ESL of horse tendons was predicted with mean absolute percentage errors ranging between 10.53±7.6% and 19.16±14.31% depending on the size of the initial part of the stress-strain curves and the type of normalization. The results of this study suggest that both ESL and the shape of stress-strain curves may be highly different between different individuals and different anatomical locations.

Direct inhibition of the hexose transporter GLUT1 by tyrosine kinase inhibitors

The facilitative hexose transporter GLUT1 is a multifunctional protein that transports hexoses and dehydroascorbic acid, the oxidized form of vitamin C, and interacts with several molecules structurally unrelated to the transported substrates. Here we analyzed in detail the interaction of GLUT1 with a group of tyrosine kinase inhibitors that include natural products of the family of flavones and isoflavones and synthetic compounds such as the tyrphostins. These compounds inhibited, in a dose-dependent manner, the transport of hexoses and dehydroascorbic acid in human myeloid HL-60 cells, in transfected Chinese hamster ovary cells overexpressing GLUT1, and in normal human erythrocytes, and blocked the glucose-displaceable binding of cytochalasin B to GLUT1 in erythrocyte ghosts. Kinetic analysis of transport data indicated that only tyrosine kinase inhibitors with specificity for ATP binding sites inhibited the transport activity of GLUT1 in a competitive manner. In contrast, those inhibitors that are competitive with tyrosine but not with ATP failed to inhibit hexose uptake or did so in a noncompetitive manner. These results, together with recent evidence demonstrating that GLUT1 is a nucleotide binding protein, support the concept that the inhibitory effect on transport is related to the direct interaction of the inhibitors with GLUT1. We conclude that predicted nucleotide-binding motifs present in GLUT1 are important for the interaction of the tyrosine kinase inhibitors with the transporter and may participate directly in the binding transport of substrates by GLUT1.

The C1-C2 interface residue lysine 50 of pig kidney fructose-1, 6-bisphosphatase has a crucial role in the cooperative signal transmission of the AMP inhibition

To understand the mechanism of signal propagation involved in the cooperative AMP inhibition of the homotetrameric enzyme pig-kidney fructose-1,6-bisphosphatase, Arg49 and Lys50 residues located at the C1-C2 interface of this enzyme were replaced using site-directed mutagenesis. The mutant enzymes Lys50Ala, Lys50Gln, Arg49Ala and Arg49Gln were expressed in Escherichia coli, purified to homogeneity and the initial rate kinetics were compared with the wild-type recombinant enzyme. The mutants exhibited kcat, Km and I50 values for fructose-2,6-bisphosphate that were similar to those of the wild-type enzyme. The kinetic mechanism of AMP inhibition with respect to Mg2+ was changed from competitive (wild-type) to noncompetitive in the mutant enzymes. The Lys50Ala and Lys50Gln mutants showed a biphasic behavior towards AMP, with total loss of cooperativity. In addition, in these mutants the mechanism of AMP inhibition with respect to fructose-1,6-bisphosphate changed from noncompetitive (wild-type) to uncompetitive. In contrast, AMP inhibition was strongly altered in Arg49Ala and Arg49Gln enzymes; the mutants had > 1000-fold lower AMP affinity relative to the wild-type enzyme and exhibited no AMP cooperativity. These studies strongly indicate that the C1-C2 interface is critical for propagation of the cooperative signal between the AMP sites on the different subunits and also in the mechanism of allosteric inhibition of the enzyme by AMP.

Suppression of kinetic AMP cooperativity of fructose-1,6-bisphosphatase by carbamoylation of lysine 50

Selective treatment of pig kidney fructose 1,6-bisphosphatase with cyanate leads to the formation of an active carbamoylated derivative that shows no cooperative interaction between the AMP-binding sites, but completely retains the sensitivity to the inhibitor. By an exhaustive carbamoylation of the enzyme a derivative is formed that has a complete loss of cooperativity and a decrease of sensitivity to AMP. It was proposed that the observed changes of allosteric properties were due to the chemical modification of two lysine residues per enzyme subunit [Slebe et al. (1983), J. Protein Chem. 2, 437-443]. Studies of the temperature dependence of AMP sensitivity and the interaction with Cibacron Blue Sepharose of carbamoylated fructose 1,6-bisphosphatase derivatives indicate that the lysine residue involved in AMP sensitivity is located at the allosteric AMP site, while the lysine residue involved in AMP cooperativity is at a distinct location. Using [14C]cyanate, we identified both lysine residues in the primary structure of the enzyme; Lys50 is essential for AMP cooperativity and Lys112 appears to be the reactive residue involved in the AMP sensitivity. According to the fructose 1,6-bisphosphatase crystal structure, Lys50 is strategically positioned at the C1-C2 interface, near the molecular center of the tetramer, and Lys112 is in the AMP-binding site. The results reported here, combined with the structural data of the enzyme, strongly suggest that the C1-C2 interface is critical for the propagation of the allosteric signal among the AMP sites on different subunits.

Hexose transporter expression and function in mammalian spermatozoa: cellular localization and transport of hexoses and vitamin C

We analyzed the expression of hexose transporters in human testis and in human, rat, and bull spermatozoa and studied the uptake of hexoses and vitamin C in bull spermatozoa. Immunocytochemical and reverse transcription-polymerase chain reaction analyses demonstrated that adult human testis expressed the hexose transporters GLUT1, GLUT2, GLUT3, GLUT4, and GLUT5. Immunoblotting experiments demonstrated the presence of proteins of about 50-70 kD reactive with anti-GLUT1, GLUT2, GLUT3, and GLUT5 in membranes prepared from human spermatozoa, but no proteins reactive with GLUT4 antibodies were detected. Immunolocalization experiments confirmed the presence of GLUT1, GLUT2, GLUT3, GLUT5, and low levels of GLUT4 in human, rat, and bull spermatozoa. Each transporter isoform showed a typical subcellular localization in the head and the sperm tail. In the tail, GLUT3 and GLUT5 were present at the level of the middle piece in the three species examined, GLUT1 was present in the principal piece, and the localization of GLUT2 differed according of the species examined. Bull spermatozoa transported deoxyglucose, fructose, and the oxidized form of vitamin C, dehydroascorbic acid. Transport of deoxyglucose and dehydroascorbic acid was inhibited by cytochalasin B, indicating the direct participation of facilitative hexose transporters in the transport of both substrates by bull spermatozoa. Transport of fructose was not affected by cytochalasin B, which is consistent for an important role for GLUT5 in the transport of fructose in these cells. The data show that human, rat, and bull spermatozoa express several hexose transporter isoforms that allow for the efficient uptake of glucose, fructose, and dehydroascorbic acid by these cells.

Human erythrocytes express GLUT5 and transport fructose

Although erythrocytes readily metabolize fructose, it has not been known how this sugar gains entry to the red blood cell. We present evidence indicating that human erythrocytes express the fructose transporter GLUT5, which is the major means for transporting fructose into the cell. Immunoblotting and immunolocalization experiments identified the presence of GLUT1 and GLUT5 as the main facilitative hexose transporters expressed in human erythrocytes, with GLUT2 present in lower amounts. Functional studies allowed the identification of two transporters with different kinetic properties involved in the transport of fructose in human erythrocytes. The predominant transporter (GLUT5) showed an apparent Km for fructose of approximately 10 mmol/L. Transport of low concentrations of fructose was not affected by 2-deoxy-D-glucose, a glucose analog that is transported by GLUT1 and GLUT2. Similarly, cytochalasin B, a potent inhibitor of the functional activity of GLUT1 and GLUT2, did not affect the transport of fructose in human erythrocytes. The functional properties of the fructose transporter present in human erythrocytes are consistent with a central role for GLUT5 as the physiological transporter of fructose in these cells.

Genistein is a natural inhibitor of hexose and dehydroascorbic acid transport through the glucose transporter, GLUT1

Genistein is a dietary-derived plant product that inhibits the activity of protein-tyrosine kinases. We show here that it is a potent inhibitor of the mammalian facilitative hexose transporter GLUT1. In human HL-60 cells, which express GLUT1, genistein inhibited the transport of dehydroascorbic acid, deoxyglucose, and methylglucose in a dose-dependent manner. Transport was not affected by daidzein, an inactive genistein analog that does not inhibit protein-tyrosine kinase activity, or by the general protein kinase inhibitor staurosporine. Genistein inhibited the uptake of deoxyglucose and dehydroascorbic acid in Chinese hamster ovary (CHO) cells overexpressing GLUT1 in a similar dose-dependent manner. Genistein also inhibited the uptake of deoxyglucose in human erythrocytes indicating that its effect on glucose transporter function is cell-independent. The inhibitory action of genistein on transport was instantaneous, with no additional effect observed in cells preincubated with it for various periods of time. Genistein did not alter the uptake of leucine by HL-60 cells, indicating that its inhibitory effect was specific for the glucose transporters. The inhibitory effect of genistein was of the competitive type, with a Ki of approximately 12 microM for inhibition of the transport of both methylglucose and deoxyglucose. Binding studies showed that genistein inhibited glucose-displaceable binding of cytochalasin B to GLUT1 in erythrocyte ghosts in a competitive manner, with a Ki of 7 microM. These data indicate that genistein inhibits the transport of dehydroascorbic acid and hexoses by directly interacting with the hexose transporter GLUT1 and interfering with its transport activity, rather than as a consequence of its known ability to inhibit protein-tyrosine kinases. These observations indicate that some of the many effects of genistein on cellular physiology may be related to its ability to disrupt the normal cellular flux of substrates through GLUT1, a hexose transporter universally expressed in cells, and is responsible for the basal uptake of glucose.

Refolding of the precursor and mature forms of mitochondrial aspartate aminotransferase after guanidine hydrochloride denaturation

The mitochondrial isozyme of aspartate aminotransferase (mAspAT), a dimeric pyridoxal phosphate (PLP)-dependent enzyme, is encoded by the nuclear genome and synthesized in the cytoplasm as a precursor protein (pmAspAT) containing a 29-residue amino-terminal signal peptide which is essential for its targeting and import into mitochondria. In the cytosolic-like environment of rabbit reticulocyte lysate, newly synthesized rat liver pmAspAT has been found to slowly fold and bind PLP (Mattingly, J. R., Jr., Youssef, J., Iriarte, A. and Martinez-Carrion, M. (1993) J. Biol. Chem. 268, 3925-3937). On the other hand, isolated mammalian (pig) mAspAT, when denatured with guanidine hydrochloride, seems unable to refold to a catalytically active state (West, S. M., and Price, N. C. (1990) Biochem. J. 265, 45-50). With the availability of rat liver recombinant precursor and mature forms of mAspAT as homogeneous, stable preparations, an assessment of the influence of the signal peptide on the in vitro refolding of this protein can be made. Following unfolding induced by guanidine hydrochloride, we have investigated the refolding process of this complex, dimeric coenzyme-dependent protein system by activity, fluorescence, and circular dichroism. Both mAspAT and pmAspAT can be efficiently renatured after rapid dilution of the denaturing agent at low protein concentrations. The equilibrium unfolding/refolding transitions and the kinetics of folding are protein concentration-independent and identical for both protein forms. Binding of coenzyme into the active site pocket seems to occur at a late step in the folding process of both mAspAT and pmAspAT, suggesting that in these proteins the coenzyme does not direct the folding of the polypeptide chain. These results indicate that the in vitro refolding of mAspAT is not regulated or influenced by the presence of the amino-terminal signal peptide. On the other hand, in vitro refolding in buffer is significantly faster than the folding of newly synthesized precursor protein in reticulocyte lysate examined in our previous report (reference above), pointing at the likely influence of cytosolic factors in modulating folding in the cell.

Modification of Cys-128 of pig kidney fructose 1,6-bisphosphatase with different thiol reagents: size dependent effect on the substrate and fructose-2,6-bisphosphate interaction

Treatment of fructose 1,6-bisphosphatase with N-ethylmaleimide was shown to abolish the inhibition by fructose 2,6-bisphosphate, which also protected the enzyme against this chemical modification [Reyes, A., Burgos, M. E., Hubert, E., and Slebe, J. C. (1987), J. Biol. Chem. 262, 8451-8454]. On the basis of these results, it was suggested that a single reactive sulfhydryl group was essential for the inhibition. We have isolated a peptide bearing the N-ethylmaleimide target site and the modified residue has been identified as cysteine-128. We have further examined the reactivity of this group and demonstrated that when reagents with bulky groups are used to modify the protein at the reactive sulfhydryl [e.g., N-ethylmaleimide or 5,5'-dithiobis-(2-nitrobenzoate)], most of the fructose 2,6-bisphosphate inhibition potential is lost. However, there is only partial or no loss of inhibition when smaller groups (e.g., cyanate or cyanide) are introduced. Kinetic and ultraviolet difference spectroscopy-binding studies show that the treatment of fructose 1,6-bisphosphatase with N-ethylmaleimide causes a considerable reduction in the affinity of the enzyme for fructose 2,6-bisphosphate while affinity for fructose 1,6-bisphosphate does not change. We can conclude that modification of this reactive sulfhydryl affects the enzyme sensitivity to fructose 2,6-bisphosphate inhibition by sterically interfering with the binding of this sugar bisphosphate, although this residue does not seem to be essential for the inhibition to occur. The results also suggest that fructose 1,6-bisphosphate and fructose 2,6-bisphosphate may interact with the enzyme in a different way.