Autoradiographic studies of pyridine nucleotide metabolism in human culture cells

Degradation of microinjected proteins: effects of lysosomotropic agents and inhibitors of autophagy

HeLa cells, injected with radioiodinated proteins by fusion with RBC ghosts, were exposed to inhibitors of lysosomal proteolysis and autophagy. The degradation of injected [125I]bovine serum albumin (BSA) was unaffected by chloroquine, NH4Cl, nocodazole, colcemid, puromycin, cycloheximide, or enucleation. Although degradation of [125I]lactate dehydrogenase (LDH) and [125I]pyruvate kinase (PK) was inhibited one-third by chloroquine or ammonia, their degradation was unaffected by the other compounds. In contrast, enhanced degradation of 125I-PK resulting from depriving injected HeLa cells of amino acids and serum was inhibited 70% by colcemid and abolished by chloroquine or ammonia. Similarly, degradation of [14C]sucrose-labeled BSA-polylysine conjugates that entered HeLa cells by endocytosis was inhibited as much as 80% by chloroquine and ammonia. Sensitivity of both enhanced proteolysis and degradation of exogenous proteins to ammonia or chloroquine indicates they are effective inhibitors of lysosomal proteolysis in HeLa cells. Failure of ammonia or chloroquine to inhibit degradation of injected 125I-BSA and the modest inhibition of degradation of injected 125I-LDH or 125I-PK indicates that virtually all BSA molecules and most PK or LDH molecules are degraded by a nonlysosomal proteolytic system. Components of this degradative system are present in vast excess or are long lived, since inhibition of protein synthesis for 20 hr had no effect on the degradation of injected proteins.

Comparative studies on microinjected high-mobility-group chromosomal proteins, HMG1 and HMG2

The nonhistone chromosomal proteins, HMG1 and HMG2, were iodinated and introduced into HeLa cells, bovine fibroblasts, or mouse 3T3 cells by erythrocyte-mediated microinjection. Autoradiographic analysis of injected cells fixed with glutaraldehyde consistently showed both molecules concentrated within nuclei. Fixation with methanol, on the other hand, resulted in some leakage of the microinjected proteins from the nuclei so that more autoradiographic grains appeared over the cytoplasm or outside the cells. Both injected and endogenous HMG1 and HMG2 partitioned unexpectedly upon fractionation of bovine fibroblasts, HeLa, or 3T3 cells, appearing in the cytoplasmic fractions. However, in calf thymus, HMG1 and HMG2 molecules appeared in the 0.35 M NaCl extract of isolated nuclei, as expected. These observations show that the binding of HMG1 and HMG2 to chromatin differs among cell types or that other tissue-specific components can influence their binding. Coinjection of [125I]HMG1 and [131I]HMG2 into HeLa cells revealed that the two molecules display virtually equivalent distributions upon cell fractionation, identical stability, identical intracellular distributions, and equal rates of equilibration between nuclei. In addition, HMG1 and HMG2 did not differ in their partitioning upon fractionation nor in their stability in growing vs. nongrowing 3T3 cells. Thus, we have not detected any significant differences in the intracellular behavior of HMG1 and HMG2 after microinjection into human, bovine, or murine cells.

Degradation of proteins microinjected into cultured mammalian cells

Iodinated proteins were degraded after injection into HeLa cells at first-order rates with half-lives varying from three hours for the trout monhistone chromosomal protein, HMG-T, -to 60 hours for whale myoglobin. Fluoresceinated-bovine serum albumin (fl-BSA) was degraded almost twice as fast as unmodified BSA. The rate of degradation of 125I-BSA was very similar in eight cell lines of mouse, human, monkey and rat origin. Microinjected proteins were analyzed on SDS-acrylamide gels after injection, and for BSA and immunoglobin G, all remaining intracellular 125I migrated at the molecular weight of the injected proteins. By contrasting, more than 80% of the extracellular 125I chromatographed as iodotyrosine. With the exception of fl-BSA, which exhibited perinuclear accumulation in approximately one-half of the injected cells, autoradiography showed that throughout the period of study the injected proteins remained dispersed in the cytoplasm.

Microinjection of the nonhistone chromosomal protein HMG1 into bovine fibroblasts and HeLa cells

The nonhistone chromosomal protein HMG1 associated rapidly with the nuclei of HeLa cells and bovine fibroblasts following its introduction into the cytoplasm by red cell-mediated microinjection. A number of non-nuclear proteins, on the other hand, failed to concentrate in HeLa or bovine fibroblast nuclei. Autoradiography of thin sections showed that 125I-labeled HMG1 localized within nuclei, and further established that it remained associated with metaphase chromosomes at mitosis. When uninjected HeLa cells were fused with 125I-HMG1-injected HeLa cells, the labeled molecules equilibrated between nuclei within 12 hr. Similar results were obtained with bovine fibroblasts, indicating that a dynamic equilibrium exists between HMG1 and chromatin within living cells. Electrophoresis of 125I-HMG1 retrieved from HeLa cells or bovine fibroblasts up to 48 hr after injection showed that more than 80% of the molecules were intact. Autoradiographic analysis of cells fixed over a period of several days after injection produced apparent half-lives for 125I-HMG1 of 80 hr in HeLa cells and 100 hr in bovine fibroblasts.

Studies on the intranuclear distribution of human and mouse genomes and formation of human-mouse hybrid cells

Autoradiographic examination of early hybrid cells formed by the fusion of 3H-thymidine labeled D98/AH2 cells and 3T3-4E cells revealed that human and mouse chromosomes are often separated within metaphase and interphase nuclei. Although the marked separation of human and mouse chromosomes progressively disappeared with succeeding cell division, the occurrence of sectored nuclei in 16-cell hybrid colonies and the labeling pattern of human chromosomes within separated metaphases indicate that separation of human and mouse chromosomes may persist through several mitoses. Fusion of 3H-thymidine labeled D98/AH2 cells and 3T3-4E cells was coupled with aminopterin selection to study other aspects of hybrid cell formation. Hybrid cells arose from heterokaryons by cell division. Nuclear fusion may have occurred in two out of several thousand heterokaryons examined. However, these could reflect the close apposition of adjacent nuclei. A large fraction (greater than 0.5) of D98/3T3 heterokaryons underwent at least one cell division. However, the number of hybrid colonies containing more than eight cells at ten days following fusion was about 0.03 of the total number of heterokaryons. Many hybrid colonies arrested growth before the 8-cell stage, and the cells in such colonies exhibited nuclear abnormalities.

Pyridine nucleotide metabolism in mitotic cells

The biosynthesis and turnover of nicotinamide adenine dinucleotide (NAD) have been examined in mitotic cells of the human culture line, D98/AH2. No significant difference in the incorporation of nicotinic acid or nicotinamide could be detected between mitotic and interphase cells. The distribution of newly-incorporated nicotinic acid among the various pyridine nucleotides was also identical in mitotic and interphase cells. Whereas previous results have shown that the nucleus is necessary for NAD biosynthesis, the present results show that an intact nucleus is not required. In contrast to the equivalent rates of biosynthesis in mitotic and interphase cells, the pyridine ring of NAD was lost twice as fast from mitotic as from interphase cells. Loss of the pyridine ring to the medium is not necessarily an accurate measure of turnover, and the difference between mitotic and interphase cells may reflect differential reutilization of the pyridine ring within the cell. However, it is clear that NAD turnover is substantial in mitotic cells and possibly greater in mitotic cells than interphase cells.

Autoradiographic studies of nicotinic acid utilization in human-mouse heterokaryons and inhibition of utilization in newly-formed hybrid cells

Although most mammalian cell lines can utilize either nicotinic acid or nicotinamide for the biosynthesis of nicotinamide adenine dinucleotide (NAD), thymidine kinase-deficient, mouse 3T3-4F cells are unable to utilize nicotinic acid. When 3T3-4E cells were fused with human D98/AH2 cells, autoradiography showed that the resultant heterokaryons synthesized NAD from nicotinic acid at rates comparable to the human parental cell. The rate of nicotinic acid utilization in heterokaryons remained unchanged over the four-day period of study following cell fusion. In contrast to the results observed with heterokaryons, nicotinic acid utilization was markedly reduced in hybrid cells. Of 100 hybrid clones examined at four or five days following cell fusion, 60 utilized nicotinic acid at rates less than one tenth that of the parental human cell. Similar results were observed in hybrid clones at nine or ten days following fusion. Uniformly high rates of NAD biosynthesis were observed in hybrid clones with nicotinamide as the precursor. This excludes the possibility that the reduction in nicotinic acid utilization in hybrid cells is due to a general metabolic dysfunction. The biochemical mechanism by which nicotinic acid utilization is markedly reduced has not been determined with certainty, however, several observations suggest genetic suppression.