In human rabdomiosarcoma cells (TE671/RD) chronic exposure to 500 nM thapsigargin (a powerful inhibitor of the endoplasmic reticulum Ca2+-ATPases) resulted in the induction of the stress protein GRP78/BIP. Making use of the surface biotinylation method, followed by the isolation of the GRP78 using ATP-agarose affinity chromatography, it was found that a fraction of the thapsigargin-induced GRP78 is expressed on the cell surface. The presence of GRP78 on the membrane of thapsigargin-treated cells was confirmed by fractionation of cell lysates into a soluble and a membrane fraction, followed by Western blot analysis with an anti-GRP78 antibody. It was also found that conspicuous amounts of GRP78 are present in the culture medium collected from thapsigargin-treated cultures. This extracellular GRP78 originates mostly by an active release from intact cells and does not result solely from the leakage of proteins from dead cells. Moreover, small amounts of circulating, free GRP78 and naturally-occurring anti-GRP78 autoantibodies were detected in the peripheral circulation of healthy human individuals.

In the present study it was found that the synthesis of the 78 kD glucose-regulated protein (GRP 78 or BIP) is vigorously induced in human rabdomiosarcoma cells (TE 671/RD) following both short-term (1 h) and prolonged (18 h) exposure to 100 nM thapsigargin (Tg). Flow cytometric analysis with a specific anti-GRP 78 polyclonal antibody showed that Tg-treated cells express the GRP 78 on the plasma membrane. Cell surface localization of the Tg-induced GRP 78 was confirmed by biotinylation of membrane-exposed proteins and subsequent isolation of the biotin-labelled proteins by streptavidin/agarose affinity chromatography. It was found that a fraction of the Tg-induced GRP 78 is present among the biotin-labelled, surface-exposed, proteins. Conversely, the GRP 78 immunoprecipitated from unfractionated lysates of Tg-treated and biotin-reacted cells was found to be biotinylated. This is the first report demonstrating surface expression of GRP 78 in cells exposed to a specific GRP 78-inducing stimulus.

Human immunodeficiency virus infection is associated with a wide spectrum of anorectal disease, of which the most common lesions are anal condylomata and painful ulcers. The majority of these anal ulcers gave negative culture and biopsy results. In addition, there seems to be a high incidence of anorectal neoplasia in this patient population.

In Daudi cells, a fraction of the 60 kDa heat shock protein (Hsp60), which is typically a mitochondrial protein, is located on cell membrane. This was demonstrated by the recovery of biotinylated Hsp60 in the anti-Hsp60 immunoprecipitate obtained from cells in which surface-exposed proteins were selectively labeled with biotin. In further experiments, isolated membrane proteins (obtained by two different biochemical methods) were probed in Western blot with two antibodies (N-20 and K-19) directed against different epitopes located, respectively, at the amino- and at the carboxyl-terminus of the Hsp60. Both these antibodies recognized, among the isolated membrane proteins, a unique band with an electrophoretic mobility identical to that of the cytoplasmic Hsp60, thus demonstrating that Hsp60 is present on cell surface as an intact, full-length, protein. FACS analysis, performed with the same two highly specific anti-Hsp60 antibodies, confirmed that both the N-terminus and the C-terminus of the Hsp60 are exposed outside the cell and are accessible for recognition by the corresponding antibody. Moreover, quantitative analysis of the data showed that constitutive cell surface expression of the Hsp60 is limited to a small fraction (about 10%) of the whole cell population.

The relationship between cell pigmentation and radiosensitivity was investigated in a cell model in which melanogenesis was suppressed by a glycosylation inhibitor. It was found that X-irradiation of melanotic B-16 melanoma cells and their amelanotic counterparts, obtained by glucosamine treatment, showed an inverse correlation between radiosensitivity and melanin contents. Since melanogenesis interruption by glucosamine does not affect the DNA repair capacity of nonpigmented cells, it is likely that intracellular melanins play a role in the relative resistance of pigmented cells to X-irradiation.