Hormone receptors as regulators of hormone action

UVB radiation induces phosphorylation of the epidermal growth factor receptor, decreases EGF binding and blocks EGF induction of ornithine decarboxylase gene expression in SV40-transformed human keratinocytes

We previously demonstrated that epidermal growth factor (EGF) induces a several-fold increase in ornithine decarboxylase (ODC) activity and the steady-state level of ODC mRNA in cultured SV40-transformed human keratinocytes (1). Pretreatment of cell cultures with ultraviolet B (UVB) radiation resulted in a reduction of EGF-induced ODC activity. To determine whether UVB inhibits the accumulation of ODC mRNA by EGF, cells were pretreated with 20 mJ/cm2 UVB or sham-irradiated and then incubated with 100 ng/ml EGF. Northern blot analysis revealed that UVB irradiation entirely blocked the EGF induction of ODC mRNA. Since the binding of EGF to its plasma membrane receptor is the first step in initiating a biological response, the effect of UVB on EGF binding was evaluated. UVB treatment of cultured keratinocytes resulted in an immediate and dose-dependent reduction of EGF binding. Scatchard analysis revealed that the reduction of EGF binding was due to a 52% decrease in the number of available receptors, from 6.2 x 10(4)/cell to 3.0 x 10(4)/cell. However, UVB decreased the EGF-binding affinity very little (Kd = 0.60 nM in control and Kd = 0.75 nM in UVB-treated Z114 cells). In addition, UVB did not alter the rate of EGF internalization. These data suggest that UVB blocks the signal transduction pathway of EGF that is involved in regulation of ODC gene expression. Immunoblot analysis of extracts from irradiated cells showed that UVB induced tyrosine phosphorylation of EGFR and that the quantity of EGFR protein was unaffected by UVB treatment. Phosphorylation of EGFR may be responsible for decreased binding of EGF to its receptor.

Metabolism of sheep adipose tissue during pregnancy and lactation. Adaptation and regulation

1. Changes in the mean volume, the rate of fatty acid and acylglycerol glycerol synthesis, the activity of lipoprotein lipase and the numbers and affinities of insulin receptors of subcutaneous adipocytes are reported for sheep at different stages of pregnancy and lactation. In addition, the serum concentrations of insulin, progesterone, prolactin, choriomammotropin, somatotropin, glucose, acetate, L-lactate, glycerol and unesterified fatty acids are reported for these sheep. 2. A switch from lipid accumulation to net lipid mobilization accompanied by a decline in the capacity for lipid synthesis, occurred at the onset of the last third of pregnancy. Net lipid mobilization continued during lactation. 3. The changes that occurred in the serum concentrations of the various hormones listed above are discussed in relation to their possible roles in the modulation of adipose tissue metabolism in sheep during pregnancy and lactation. The observations are compared with those from previous studies on the hormonal control of adipose tissue metabolism in the rat during pregnancy and lactation.

Regeneration of neurites on long-term cultures of sympathetic neurons deprived of nerve growth factor

Sympathetic neurons from newborn rats, cultured for 1 month or longer in the virtual absence of nonneuronal cells, were capable of regenerating neurites after neuritotomy. Regeneration occurred even after nerve growth factor was withdrawn from the cultures, although it was much less extensive and appeared limited to a few days following neuritotomy. Even after 29 days of nerve growth factor deprivation, reintroduction of the protein prompted a resumption of neurite growth. Possible roles of both nerve growth factor-independent and -dependent components in adult nerve regeneration are discussed.

The somatomedin activity in plasma from patients with multiple mechanical injuries: with observations on plasma cortisol

The somatomedin activity (SMA) was assayed in 247 plasma samples from 30 patients with moderate or severe injuries. The injury severity score (ISS) was assessed for each patient and ranged from 9 to 50. The plasma SMA was reduced or eliminated within 4 days of injury. The activity then increased in the patients who recovered, normal levels being attained within 1 or 2 weeks of the injury. Five patients had at least one further fall in plasma SMA during treatment. Seven patients died and 4 of these had little or no SMA before death. The magnitude and duration of the depression of SMA after injury correlated both with the ISS and with the probability of mortality. The plasma cortisol concentration was elevated in all but 4 patients within 48 hours of injury. There was no relationship between the plasma SMA and the cortisol concentration. When plasma samples from 10 patients taken soon after injury were mixed with normal human serum, samples from only 1 patient were found to contain an inhibitor(s) of SMA. The reduced plasma SMAs in the remaining patients may have resulted from the impaired production of somatomedins (SM), the degradation of SM, the dilution of circulating SM with transfusion fluids or the binding of SM to damaged tissues.

Epidermal growth factor-induced down-regulation of receptor does not occur in HeLa cells grown in defined medium

125I-Labeled epidermal growth factor (EGF) binds to HeLa cells in culture under completely defined conditions in a specific and saturable manner. When the cultures are maintained in serum, all the reported phenomena of down-regulation occur. In contrast, in cultures maintained in serum-free hormone-supplemented medium, binding is maximal after 50 min and remains constant for at least 18 hr. EGF retains its mitogenic activity in this defined system; as little as 4 pM EGF elicits a 2-fold increase in cell number in 5 days. EGF appears to be in reversible equilibrium with a constant number of surface receptors, and gel filtration analysis indicates a decrease in the rate of degradation and release of bound 125I-labeled EGF. These results indicate that EGF can retain its mitogenic activity in the absence of concomitant down-regulation.