Regulation of DNA synthesis in fat cells and stromal elements from rat adipose tissue.

The incorporation of tritiated thymidine into the deoxyribonucleic acid (DNA) of adipose fat and stromal cells was followed under a variety of conditions. After in vitro incubation of adipose slices or up to 2 days after in vivo injection of the isotope, all DNA radioactivity was in the stromal cell fraction. From 2 to 15 days after thymidine injection total tissue DNA radioactivity was constant, while between 2 and 5 days after injection label in fat cell DNA increased markedly. Thus new labeled fat cells, initially collected in the stromal pool, required 2-5 days after completion of DNA synthesis to accumulate sufficient lipid to be harvested in the fat cell fraction. Fasting before thymidine injection practically abolished DNA synthesis in primordial fat cells and reduced less drastically formation of stromal elements. However fasting sufficient to deplete lipid stores by 50% neither destroyed mature fat cells nor impaired their capacity to reaccumulate fat with refeeding. Other studies evaluated the role of new fat cell formation in the process of lipid accretion accompanying refeeding. These experiments indicated that at least during the early phase of rapid weight gain, accumulation of fat was due to deposition of triglyceride in existing cells rather than to accelerated formation of new fat cells. Studies with hypophysectomized rats demonstrated that pituitary ablation variably affected stromal DNA synthesis and nearly abolished the formation and (or) maturation of primordial fat cells. In these animals growth hormone markedly enhanced thymidine incorporation into stromal DNA but had no effect on fat cell precursors. In intact animals the predominant effect of growth hormone was also on the stromal fraction, although an action of the hormone of lesser magnitude on fat cell precursors was also evident.

Varying capacities for replication of rat adipocyte precursor clones and adipose tissue growth.

Rat adipocyte precursor populations contain clones varying in capacity for replication. In this study we explored factors controlling the frequency of clones of varying replicative capacities (clonal composition). We also explored the relationship between this frequency and fat depot growth. In perirenal and epididymal depots clonal composition was identical bilaterally; perirenal depots contained more extensively replicating clones. Although there were large interanimal differences in clonal composition, variation between animals was always in the same direction for both depots. Clonal composition was unaffected by undernutrition while with animal growth the frequency of the most extensively replicating clones was reduced. Differentiation of precursors occurred in all clones, while differentiation did not occur in skin fibroblasts cloned under identical conditions. Clonal composition and mature fat cell number were related in that fat cell numbers were identical bilaterally in both depots and increased more extensively with growth in perirenal than epididymal tissue. We conclude (a) that clonal composition of adipocyte precursor populations is regulated genetically and by age, (b) that this composition determines, at least in part, the capacity for adipose depot growth.

Release of mitogenic factors by cultured preadipocytes from massively obese human subjects.

In this study, possible paracrine factors in adipose tissue from lean and obese subjects were sought. Conditioned media were prepared by incubation in alpha minimum essential medium of adipocyte precursors derived from lean and massively obese subjects. Adipocyte-precursor-derived conditioned media from the obese stimulated replication of cultured rat perirenal adipocyte precursors by about fourfold over control. The effect of media conditioned by precursors derived from lean subjects was much less evident. The mitogenicity of conditioned media was abolished by trypsin, indicating the protein nature of the mitogenic factor(s). Sephacryl S-200 chromatography of adipocyte-precursor-derived conditioned media from obese subjects revealed one major active fraction with molecular masses in the range of 25,000-40,000. Our results demonstrate that adipocyte precursors derived from massively obese subjects release factors mitogenic on cultured rat adipocyte precursors. These principles may act as paracrine factors contributing to the development of the adipocyte hyperplasia characteristic of massive obesity.

Influence of anatomic site and age on the replication and differentiation of rat adipocyte precursors in culture.

Using a propagating cell culture system of adipocyte precursors from 70-400-g rats, we explored the possibility that regional variations in properties of adipose tissue may reflect site-specific characteristics intrinsic to the cells, rather than extracellular influences. Initially, studies were made of the nature of the fibroblastlike cells from perirenal adipose tissue stroma. Using colony-forming techniques, it was shown that these cells were adipocyte precursors; each confluent colony that was derived from a single cell displayed differentiated adipocytes. This characteristic was evident in cells from rats of all ages and persisted during secondary culture. At all ages of rats studied, perirenal cells replicated more rapidly than epididymal precursors, e.g., for 179-g rats the population-doubling times were 19.3 +/- 0.7 vs. 25.5 +/- 1.2 h (means +/- SEM, P less than 0.03). With aging of the rats, the replication rate of their perirenal cells decreased progressively. Under clonal conditions, the colony size distribution of both perirenal and epididymal precursors revealed heterogeneity in their capacity for replication, perirenal cells showing greater proliferation. These also differentiated more extensively by morphologic and enzymatic criteria. Age and site had effects that persisted through many cell generations; however, high-fat feeding had no perpetuating influence. The dissimilar properties of perirenal as compared with epididymal precursors may reflect differences in regulation of gene expression. The data are also compatible with a later development in embryological life of perirenal tissue. We suggest that the composition of the adipocyte precursor pool is an important determinant of the growth of adipose tissue that occurs in response to a nutrient load. Interregional or interindividual variation in composition may explain regional and individual differences in fat accumulation.

Very low density lipoprotein binding to adipocytes.

Human very low density lipoprotein (VLDL) has been found to interact with both isolated adipocytes and adipocyte membranes in a manner which is saturable, reversible and dependent on time and temperature. Except for a difference in maximum binding capacity, a similar pattern is seen with both rat epididymal adipocytes and human omental adipocytes. The capacity of rat cells is 0.04 micrograms VLDL per 2 x 10(5) cells. For human cells the capacity is 0.321 micrograms VLDL per 2 x 10(5) cells. Scatchard plots of the competition data are linear. This, and dissociation studies conducted in the presence or absence of unlabelled VLDL suggest that there is no cooperative interaction between the binding sites. Unlabelled VLDL and HDL each compete equally with 125I-labelled VLDL for binding sites. LDL is 25 times weaker as a competitor. Intralipid and unrelated peptides have no effect. These data suggest that the ligand is not apolipoprotein B and not apolipoprotein A. The competitive effect of HDL is not dependent on its apolipoprotein E content. A preparation of C apolipoproteins (75% C-11) is as potent as unlabelled VLDL in competing with 125I-labelled VLDL for binding sites. These data indicate that VLDL can bind to adipocytes. The receptor can interact with other lipoproteins. If differs from the LDL receptor as it does not interact with apolipoprotein B or apolipoprotein E, but binds to a C apolipoprotein.

Effect of hypophysectomy on rat preadipocyte replication and differentiation.

The effects of hypophysectomy, which results in decreased fat pad weight, fat cell number, and new fat cell formation, on preadipocyte replication were examined. Perirenal and epididymal preadipocytes were cultured from hypophysectomized, sham-operated and unoperated 3-month-old rats. In cloned preadipocytes, hypophysectomy resulted in a 19% decrease in cloning efficiency and a 60% reduction in cell number after 3 weeks in culture. Perirenal cells underwent more extensive replication than epididymal cells. The mechanism of reduced replication following hypophysectomy differed from that of donor site: hypophysectomy resulted in an increased percentage of cells which underwent 2 or fewer population doublings at the expense of cells capable of more than 2 doublings. However, donor site had little effect specifically on these slowly replicating preadipocytes; rather, perirenal preadipocytes underwent more extensive replication than epididymal cells because of the higher percentage of preadipocytes capable of 13 or more doublings in perirenal fat pads. Hypophysectomy did not result in decreased differentiation of preadipocytes. These observations are in accord with the hypothesis that hypophysectomy reduces fat cell number in maturing rats partly through an effect on preadipocyte replicative capacity. Additionally, it seems that more than one form of preadipocyte exists, the various forms having differing susceptibilities to factors such as pituitary function and anatomic site.

Effects of aging on ribosomal protein L7 messenger RNA levels in cultured rat preadipocytes.

Ribosomal protein L7 mRNA is a cell cycle-independent message whose levels are lower in late passage "senescent" fibroblasts than early passage cultures. To determine whether decreases in L7 mRNA levels also occur during aging in tissues in vivo and whether reduced L7 mRNA is caused by terminal differentiation, we measured L7 and adipsin (a differentiation-dependent serine protease) mRNA levels in undifferentiated and differentiated preadipocytes and glyceraldehyde-3-phosphate dehydrogenase mRNA in differentiated preadipocytes cultured from perirenal fat depots of 3-, 17-, and 24-month-old male rats. L7 mRNA levels decreased with increasing age and were not affected by differentiation. In the same cultures, adipsin mRNA levels did not increase with age but did increase with differentiation, confirming that the preadipocytes exposed to enriched medium had, in fact, differentiated. Glyceraldehyde-3-phosphate dehydrogenase mRNA levels did not change with age indicating that the decrease in L7 mRNA was not a result of a general decrease in mRNA with age. These observations are consistent with the hypotheses that decreasing L7 mRNA levels are associated with aging and that late passage fibroblasts have features in common with senescence. The observations are not consistent with the hypothesis that senescent changes in cellular function are caused by terminal differentiation.

Adipocyte precursor clones vary in capacity for differentiation.

Adipocyte precursor populations derived from rat perirenal or epididymal fat were found to be composed of clones varying in capacity for differentiation. Perirenal tissue contained a greater proportion of those clones that differentiated extensively. It is hypothesized that differences between regions and individuals in growth of adipose tissue may be due to differences in the frequency within adipocyte precursor pools of clones with unusual capacities for replication and differentiation.

Excessive proliferation in culture of reverted adipocytes from massively obese persons.

Differentiated omental adipocytes from lean and massively obese persons lost their spherical shape in culture and regained the ability to replicate. In propagating culture, reverted cells from each group multiplied to a greater extent than corresponding stromal adipocyte precursors. Reverted adipocytes from the massively obese proliferated at significantly higher rates than those from lean subjects. Reverted cells derived from 1-2 adipocytes also revealed these differences.

Long-chain fatty acids decrease lipoprotein lipase activity of cultured rat adipocyte precursors.

The effect of fatty acids on rat adipocyte precursor lipoprotein lipase (LPL) activity was examined. Cellular LPL activity in cultured perirenal precursors reached a maximum after 6 days. At day 6, addition of 10(-8) mol/L oleic acid to the culture medium for 6 hours resulted in a significant reduction of LPL activity. Exposing cultured precursors to 10(-4) mol/L oleic acid caused more than a 50% decrease of intracellular LPL activity measured in either acetone-ether or detergent extracts and more than a 60% decrease of heparin-releasable LPL activity. These reductions were evident within 2.5 hours of exposure to oleic acid, and exposure to oleic acid for as little as 15 minutes caused a subsequent decrease in LPL activity. LPL activity recovered 48 hours after removal of oleic acid from culture medium. Decreased LPL activity after oleic acid exposure was also noted in epididymal cells and in differentiated adipocyte precursors. The extent of decrease of LPL activity upon fatty acid exposure was dependent on the presence of the carboxyl group and was affected by acyl chain length. Although oleic acid did not affect protein synthesis estimated by [3H]-leucine incorporation, LPL mRNA levels were decreased following exposure of cells to oleic acid. Glycerol-3-phosphate dehydrogenase (G3PD) activity and mRNA levels were not affected by oleic acid exposure. Hence, fatty acids cause a dose-, acyl chain-, and carboxyl group-dependent specific decrease of heparin-releasable and intracellular LPL activities in cultured rat adipocyte precursors; this effect is associated with and is likely caused at least in part by a decrease in LPL mRNA levels.

Medullary carcinoma of the thyroid.

Medullary cancer of the thyroid is rare but of unusual biologic interest. It originates in the thyroid parafollicular or C cells that are of neural crest origin and that secrete calcitonin. Calcitonin measurements, particularly after pentagastrim administration, are useful in detecting the tumor and following its progression. Ninety percent of medullary cancers are sporadic and 10% are familial; the latter may be associated with pheochromocytoma and parathyroid hyperplasia-adenoma. Initial symptoms of both the sporadic and familial varieties include thyroid mass, diarrhea, and less often, flushing. Uninvolved members of kindreds with the disease should be followed up by repeated measurements of calcitonin after pentagastrim and calcium infusion and should be treated when a positive test result is obtained. Therapy involves total thyroidectomy plus node dissection if indicated. In addition, postoperative radiation may reduce the recurrence rate.

The uncertain future of Canadian academic medicine.

During the 1960's and 70's, academic medicine in Canada grew rapidly in size and scope and a number of research and clinical programs of the highest quality emerged. During the 1980's not only was this impetus not sustained, in some disciplines and regions there was likely a reversal of previous success. These adverse effects were produced by continued uncertainty and insufficiency of federal funding of research, underfunding of Canadian universities and of teaching hospitals, and by a decline within Canadian society of the images of both the physician and the teaching hospital with its technologically-based clinical and research programs. These negative influences were mitigated somewhat by the development during the 80's of more sources of research support from certain provincial governments and of a number of new biomedical research institutes and networks. The adverse influences of the 1980's will likely be perpetuated into the 1990's. Indeed an impending economic downturn and the return of constitutional disarray will worsen the climate for longterm growth in science and related health care research and technology. For Canadian academic medicine to survive these adverse influences, it must seek relationships and sources of support external to government. Most importantly however, it must strike new arrangements with provincial governments such that the imperatives of the academic health center and government become recognized in the planning process of the other party. For the academic health center this will mean involvement in government approaches to cost containment and health promotion; for provincial governments it will mean a commitment to health research and faculty renewal.(ABSTRACT TRUNCATED AT 250 WORDS)

Nurturing the young academician.

The young academic physician requires a well prepared and supportive environment if the trials and uncertainties associated with the early years of an academic appointment are to be successfully overcome. It is the responsibility of the appropriate university and hospital authorities to create this environment and to ensure that the appointed academician is appropriately supported within it. Of particular importance is the development by all parties of an understanding of the job description of the new faculty member and of the time and income arrangements that must be made if this description is to be fulfilled.

The effect of health care reform on academic medicine in Canada. Editorial Committee of the Canadian Institute for Academic Medicine.

Although Canadian health care reform has constrained costs and improved efficiency, it has had a profound and mixed effect on Canadian academic medicine. Teaching hospitals have been reduced in number and size, and in patient programs have shifted to ambulatory and community settings. Specialized care programs are now multi-institutional and multidisciplinary. Furthermore, the influence of regional planning bodies has grown markedly. Although these changes have likely improved clinical service, their impact on the quality of clinical education is uncertain. Within the academic clinical department, recruitment of young faculty has been greatly complicated by constraints on licensing, billing numbers, fee-for-service income and research funding. The departmental practice plan based on university funds and fee-for-service income is being replaced by less favourable funding arrangements. However, emphasis on multidisciplinary programs has rendered these departments more flexible in structure. The future of Canadian academic medicine depends on an effective alliance with government. Academia and government must agree, particularly on human-resource requirements, research objectives and the delivery of clinical and academic programs in regional and community settings. The establishment of focal points for academic health sciences planning within academic health sciences centres and within governments would assist in these developments. Finally, government and the academic health sciences sector must work together to remove the current impediments to the recruitment of highly qualified young faculty.