Localization of activin and inhibin subunits, receptors and SMADs in the mouse mammary gland

Activin and inhibin, two closely related protein hormones, are members of the transforming growth factor beta (TGF beta) superfamily of growth factors. Activin and TGF beta have been associated with mouse mammary gland development and human breast carcinogenesis. TGF beta expression in the mammary gland has been previously described, and was found to be expressed in nonparous tissue and during pregnancy, down-regulated during lactation, and then up-regulated during involution. The expression pattern of activin subunits, receptors and cytoplasmic signaling molecules has not been thoroughly described in post-natal mammary gland development. We hypothesize that activin signaling components are dynamically regulated during mammary gland development, thereby permitting activin to have distinct temporal growth regulatory actions on this tissue. To examine the activin signal transduction system in the mammary gland, tissue from CD1 female mice was dissected from nonparous, lactating day 1, 10, and 20 and post-weaning day 4 animals. The expression of the activin receptors (ActRIIA, ActRIIB and ActRIB), the inhibin co-receptor (betaglycan), and ligand subunit (alpha, beta A and beta B), mRNA was measured by semi-quantitative RT-PCR in these tissues. In addition, the cellular compartmentalization of the activin signaling proteins, including the cytoplasmic signaling co-activators, Smads 2, 3 and 4, were examined by immunohistochemistry. Generally, mRNA abundance of activin signaling components was greatest in the nonparous tissue, and then decreased, whereas protein immunoreactivity for activin signaling components increased during lactation and decreased during involution. The alpha-subunit protein was detected in nonparous and lactating day 1 tissue only. Importantly, Smad 3, but not Smad 2, was detected in epithelial cell nuclei during all time points examined, indicating that activin signaling is mediated by Smad 3 at these times. These findings suggest that activin's growth regulatory role during lactation may be distinguished from that of TGF beta during post-natal mammary development. Future studies will focus on determining the exact role this ligand plays in mammary tissue differentiation and neoplasia.

Effect of phospholipase A2 digestion on the conformation and lysine/fibrinogen binding properties of human lipoprotein[a]

In vitro hydrolysis of human lipoprotein[a] (Lp[a]) by phospholipase A2 (PLA2) decreased the phosphatidylcholine (PC) content by 85%, but increased nonesterified fatty acids 3.2-fold and lysoPC 12.9-fold. PLA2-treated Lp[a] had a decreased molecular weight, increased density, and greater electronegativity on agarose gels. In solution, PLA2-Lp[a] was a monomer, and when assessed by sedimentation velocity it behaved like untreated Lp[a], in that it remained compact in NaCl solutions but assumed the extended form in the presence of 6-amino hexanoic acid, which was shown previously to have an affinity for the apo[a] lysine binding site II (LBS II) comprising kringles IV5-8. We interpreted our findings to indicate that PLA2 digestion had no effect on the reactivity of this site. This conclusion was supported by the results obtained from lysine Sepharose and fibrinogen binding experiments, in the presence and absence of Tween 20, showing that phospholipolysis had no effect on the reactivity of the LBS-II domain. A comparable binding behavior was also exhibited by the free apo[a] derived from each of the two forms of Lp[a]. We did observe a small increase in affinity of PLA2-Lp[a] to lysine Sepharose and attributed it to changes in reactivity of the LBS I domain (kringle IV10) induced by phospholipolysis. In conclusion, the extensive modification of Lp[a] caused by PLA2 digestion had no significant influence on the reactivity of LBS II, which is the domain involved in the binding of apo[a] to fibrinogen and apoB-100. These results also suggest that phospholipids do not play an important role in these interactions.

Specificity of ligand-induced conformational change of lipoprotein(a)

The conformation of Lp(a) was probed with a set of omega-aminocarboxylic acids and other analogs of 6-aminohexanoic acid (6-AHA). Using the viscosity-corrected sedimentation coefficient, six additional ligands were shown to induce a major conformational change in Lp(a), from a compact form to an extended form. These were trans-4-(aminomethyl)cyclohexanecarboxylic acid (t-AMCHA), proline, 4-aminobutyric acid, 8-aminooctanoic acid, Nalpha-acetyllysine, and glycine. Lysine, Nepsilon-acetyllysine, glutamic acid, and adipic acid were determined not to cause a conformational change. Urea and guanidine hydrochloride were ineffective at inducing this conformational change at concentrations at which the above ligands did unfold Lp(a). The conformational change was inhibited by 100 mM NaCl and to a lesser extent by 20 mM sodium glutamate. Despite the fact that these two salts have nearly the same ionic strengths, the greater inhibition of the unfolding by NaCl is consistent with a proposed stabilization of interkringle interactions by chloride ions. In 100 mM NaCl, which most closely resembles physiological conditions, only proline, 4-aminobutyric acid, 6-AHA, and t-AMCHA were effective ligands. By analyzing the dimensions of the conformation altering ligands, we propose that a critical variable in determining the effectiveness of a ligand in disrupting Lp(a) is the distance between the carboxyl and amine functions of the ligand. The optimal distance is approximately 6 A, which agrees with the observed 6.6-6.8 A separation of the cationic and anionic centers of known plasminogen and apo(a) lysine binding sites. These studies have implications for the mechanism of Lp(a) particle assembly.

Molecular weight determination of lipoprotein(a) [Lp(a)] in solutions containing either NaBr or D2O: relevance to the number of apolipoprotein(a) subunits in Lp(a)

Molecular weight determination of low-density lipoprotein (LDL) is usually performed in solutions containing high concentrations of salt (up to 13.4 M NaBr) by sedimentation velocity and diffusion experiments, because it does not preferentially bind salt or water. Considering that lipoprotein(a) [Lp(a)] is structurally similar to LDL, differing only by the presence of Apo(a), the molecular weight, M, of Lp(a) has also been measured in solutions containing high concentrations of NaBr. We questioned the suitability of this practice by comparing the apparent molecular weight, Mapp, and partial volume, phi', of Lp(a) determined by sedimentation and flotation equilibrium in a three-component system containing NaBr with the analogous parameters, M and partial specific volume, v, determined in a two-component system containing D2O. LDL served as a control. In agreement with previous findings obtained with different methods, our results indicate no significant differences in M and v of four different LDL samples and apparently no significant preferential binding of solvent components. In contrast, values of Mapp and phi' of Lp(a) evaluated in NaBr are significantly greater than M and v. Preferential binding of solvent components appeared to be a function of Apo(a) mass or the number of kringle IV domains, as expressed by increasing percentage differences between the two sets of parameters, ranging from 4 to 13% in M and 0.2 to 0.5% in v of Lp(a) species having Apo(a) with 15-27 kringle IV domains. Furthermore, our results indicate that the variable Apo(a) kringle IV domains are more involved in this process than the constant domain of Apo(a). These findings indicate that the Lp(a) molecular weight should be determined in D2O and that high concentrations of NaBr should be avoided as their use would lead to overestimated molecular weights and partial specific volumes. Application of this method to the question of how much Apo(a) is released upon the reduction of Lp(a) led to the conclusion that Lp(a) contains only one Apo(a) molecule.