The idea of a receptor reserve in mediating cellular function is well known but direct biochemical evidence has not been easy to obtain. This study stems from our results showing that L15 of epidermal growth factor (EGF) is important in both EGF receptor (EGFR) binding and activation, and the L15A analog of human EGF (hEGF) partially uncouples EGFR binding from EGFR activation (Nandagopal et al., [1996] Protein Engng 9:781-788). We address the cellular mechanism of mitogenic signal amplification by EGFR tyrosine kinase in response to L15A hEGF. L15A is partially impaired in receptor dimerization, shown by chemical cross-linking and allosteric activation of EGFR in a substrate phosphorylation assay. Immunoprecipitation experiments reveal, however, that L15A can induce EGFR autophosphorylation in intact murine keratinocytes by utilizing spare receptors, the ratio of total phosphotyrosine content per receptor being significantly lower than that elicited by wild-type. This direct biochemical evidence, based on function, of utilization of a receptor reserve for kinase stimulation suggests that an EGF variant can activate varying receptor numbers to generate the same effective response. L15A-activated receptors can stimulate mitogen-activated protein kinase (MAPK) that is important for mitogenesis. The lack of linear correlation between levels of receptor dimerization, autophosphorylation, and MAPK activation suggests that signal amplification is mediated by cooperative effects. Flow cytometric analyses show that the percentages of cells which proliferate in response to 1 nM L15A and their rate of entry into S-phase are both decreased relative to 1 nM wild-type, indicating that MAPK activation alone is insufficient for maximal stimulation of mitogenesis. Higher concentrations of L15A reverse this effect, indicating that L15A and wild-type differ in the number of receptors each activates to induce the threshold response, which may be attained by cooperative activation of receptor dimers/oligomers by van der Waal's weak forces of attraction. The maintenance of a receptor reserve underscores an effective strategy in cell survival.

Alanine↗

PubMedDate not supplied

The effect of age on antigen-sensitive cells.

The primary humoral immune response to heterologous red cell antigens declines in mice after 12 months of age. The cellular response to histocompatibility antigens of lymph node lymphocytes from mice 3 months and 12 to 27 months old are evaluated in these studies. It was found that lymphocytes from aged mice are as able to transform into large pyroninophilic cells and proliferate after exposure to antigen as lymphocytes from 3- to 4-month-old mice. However, results of cellular responses of intermediate age groups suggest that the cells that respond to antigen in the old mice may be a different population of lymphocytes from the cells that respond in the young mice.

Aging↗

PubMedDate not supplied

Qualitative changes in immunocompetent cells with age: reduced sensitivity to cortisone acetate.

Recent studies suggest that immunocompetent cells that respond to primary antigenic stimulus in old mice are different cell types from those that respond in young mice. This hypothesis was tested by determining the cortisone acetate sensitivity of antigen-sensitive lymphocytes from young and old donors. It was found that antigen-reactive lymph node lymphocytes from young donors are cortisone-acetate sensitive whereas the antigen-reactive lymph node lymphocytes from old donors are cortisone-acetate resistant.

Animals↗

PubMedDate not supplied

Critical sample sizes for determining the statistical significance of mutation frequencies.

Based on the assumption that the numbers of mutations observed in an untreated and treated sample of individuals are binomial random variables, a method is presented to compute the probability of observing a specific number of mutations as a function of the sample sizes and the number of mutations in the untreated control sample. Knowledge of the true mutation frequencies is not required. The formalism is then used to compute critical sample sizes for testing hypotheses concerning mutation frequencies in the two populations.

Mutagenicity Tests↗

PubMedDate not supplied

Bacterial retention mechanisms of membrane filters.

Explore the source record for details and available documents.

Membranes, Artificial↗