Immunochemical and biological studies on two molecular variants of pregnancy specific beta 1 glycoprotein (SP1) SP1 alpha and SP1 beta

Short-term exposure to pregnancy levels of estrogen prevents mammary carcinogenesis

It is well established that pregnancy early in life reduces the risk of breast cancer in women and that this effect is universal. This phenomenon of parity protection against mammary cancer is also observed in rodents. Earlier studies have demonstrated that short-term administration of estradiol (E) in combination with progesterone mimics the protective effect of parity in rats. In this study, the lowest effective E dosage for preventing mammary cancer was determined. Rats were injected with N-methyl-N-nitrosourea at 7 weeks of age; 2 weeks later, the rats were subjected to sustained treatment with 20 microg, 100 microg, 200 microg, or 30 mg of E in silastic capsules for 3 weeks. Treatments with 100 microg, 200 microg, and 30 mg of E resulted in serum levels of E equivalent to those of pregnancy and were highly effective in preventing mammary cancer. E treatment (20 microg) did not result in pregnancy levels of E and was not effective in reducing the mammary cancer incidence. In another set of experiments, we determined the effect of different durations of E with or without progesterone treatments on mammary carcinogenesis. These experiments indicate that a period as short as one-third the period of gestation is sufficient to induce protection against mammary carcinogenesis. The pioneering aspect of our study in contrast to long-term estrogen exposure, which is thought to increase the risk of breast cancer, is that short-term sustained treatments with pregnancy levels of E can induce protection against frank mammary cancer.

Pregnancy specific beta 1-glycoprotein (SP1) conversion of SP1 beta to SP1 alpha by "acidification": an artefact

Late pregnancy serum and plasma were examined in rocket immunoelectrophoresis (RIE) and crossed immunoelectrophoresis (XIE) before and after "acid treatment" (pH 2.5, 10 min). The analysis revealed changes in the precipitation morphology in the majority of circulating proteins extending from the albumin to the gamma-region. In RIE, the precipitation pattern of SP1 was altered, this change being independent of polyethylene glycol. In addition, XIE analysis of SP1 revealed a change in the precipitation morphology with no change in electrophoretic mobility. The results can be explained by non-specific aggregation and denaturation due to "acid treatment", and are therefore unlikely to be of any biological or immunochemical importance.

Observations on the separation of SP1 alpha and SP1 beta by affinity chromatography

SP1 antigens in late pregnancy blood, in placental extracts and in acidified plasma have been purified on positive affinity chromatography columns with immobilized SP1 antiserum and on negative affinity columns with immobilized antihuman non-pregnancy serum. Both SP1 alpha and SP1 beta are bound by the positive column but only SP1 alpha is bound by the negative column. This is adduced as further support for the view that SP1 alpha is formed by the combination of SP1 beta with a protein present in non-pregnancy serum.

Studies on the subunit and polypeptide structure of a polymeric form of pregnancy-specific beta 1-glycoprotein

The structure of a high-molecular-weight form of Pregnancy-Specific beta 1-Glycoprotein designated PSB1G-I that had been previously isolated from maternal serum was studied by gel chromatography in guanidine hydrochloride dissociating solvent. Evidence was obtained that subunits of identical size (bonded together non-covalently) but of differing polypeptide compositions constitute the PSB1G-I molecule. The subunits of PSB1G-I have been designated alpha and beta subunits. Reduction of PSB1G-I results in an intact alpha chain and two beta chains (beta 1 and beta 2) that are linked in the parent beta subunit by covalent bonds. The collective molecular weights of the alpha, beta 1 and beta 2 chains are equal to 97 800 which is assumed to represent the 'true' molecular weight of 'native' serum PSB1G.