B700, a melanoma-specific antigen, catalyzes metabolism of prostaglandin E2

Albumin-like proteins are critical regulators of vascular redox signaling

This laboratory previously identified an albumin-like protein (denoted as p70) as a component of the macromolecular complex assembled within the 5'-regulatory region of redox-sensitive genes in vascular smooth muscle cells (vSMCs). Here we show that p70 is present in the cytosolic and nuclear compartments of vSMCs and dynamically responsive to redox status. Intense cytoplasmic and perinuclear staining, coupled with enhanced nuclear localization, was observed in vSMCs, but not HepG2 cells, treated with benzo(a)pyrene (BaP), H(2)O(2), or N-acetylcysteine, agents known to modulate redox status. 3' RACE indicated that p70 is not generated as a product of endogenous gene expression, but rather taken up from the extracellular compartment. While p70 was undetectable in cells grown for 24 hours under serum-free conditions, cell-associated, acid-resistant albumin was detected 30 min after the addition of exogenous albumin. vSMCs incubated at 4°C with 100  μ g/mL unlabeled BSA and 10  μ g/mL FITC-BSA for 60 minutes and switched to 37°C to examine temperature-sensitive label uptake showed punctate structures throughout the cell consistent with albumin internalization at the higher temperature. Albumin was found to influence redox-signaling, as evidenced by modulation of cyp1a1 gsta1 and Ha-ras gene inducibility. Together, these results implicate albumin and albumin-like proteins as critical regulators of vascular redox signaling.

Proteinuria of B700, a 67 kD albumin-like melanoma-specific antigen

B700 is a murine melanoma antigen that is closely related to, but distinct from, serum albumin. The present study examined the metabolic fate and anatomic distribution of radioiodinated B700 and mouse serum albumin (MSA) administered s.c. to mice. In blood, both proteins were associated with the plasma fraction where the halflife of B700, a glycoprotein, was 0.5 days, compared to 2.7 days for MSA. Of particular interest was the observation that B700, a 67 kD anionic protein, was excreted primarily in urine. The selective B700-proteinuria did not alter urinary volumes or produce hematuria or edema. SDS-polyacrylamide gel electrophoresis and western blot analysis using the H-2-3-3 B700-specific monoclonal antibody revealed that B700 proteinuria occurred in B-16 murine melanoma bearing animals but not in control mice. These studies demonstrate that the tumor-bearing host readily distinguishes between very similar normal protein (MSA) and tumor-associated antigen (B700) molecules and processes them differently.

B700, an albumin-like melanoma-specific antigen, is a vitamin D binding protein

B700, a murine melanoma-specific antigen, is a member of the serum albumin protein family. Other members include serum albumin and vitamin D binding protein. The primary structure and biochemical functions of B700, as well as its in vivo metabolic fate, are largely unknown. We compared murine albumin, vitamin D binding protein and B700 for their ability to specifically bind [3H]-1,25-dihydroxy-vitamin D3. Scatchard analysis revealed a single binding site for B700 with a Ka of 51,000 mol/l and a Bmax of 4.51 x 10(-7) mol/l. There was no significant difference in the Ka and Bmax among the albuminoid proteins. However, differences in the binding sites could be distinguished by competition experiments where vitamin D3, vitamin D2 or 7-dehydrocholesterol competed for the specific binding of 1.25-dihydroxyvitamin D3 to a greater extent by B700 than by vitamin D binding protein. The albumin binding site more closely resembles vitamin D binding protein than B700, but the data indicate that the binding function of the albuminoid proteins is conserved in B700.