Sigma-1 and sigma-2 receptors are expressed in a wide variety of human and rodent tumor cell lines.

Thirteen tumor-derived cell lines of human and nonhuman origin and from various tissues were examined for the presence and density of sigma-1 and sigma-2 receptors. Sigma-1 receptors of a crude membrane fraction were labeled using 3H-pentazocine, and sigma-2 receptors were labeled with [3H]1,3-di-o-tolylguanidine ([3H]DTG); in the presence or absence of dextrallorphan. 3H-Pentazocine-binding sites were heterogeneous. In rodent cell lines (e.g., C6 glioma, N1E-115 neuroblastoma, and NG108-15 neuroblastoma x glioma hybrid), human T47D breast ductal carcinoma, human NCI-H727 lung carcinoid, and human A375 melanoma, 3H-pentazocine bound to high- and low-affinity sites with Kd1 = 0.67-7.0 nM, Bmax1 = 25.5-108 fmol/mg protein, Kd2 = 127-600 nM, and Bmax2 = 942-5431 fmol/mg protein. However, 3H-pentazocine bound to a single site in other cell lines. In human U-138MG glioblastoma, SK-N-SH neuroblastoma, and LNCaP.FGC prostate, Kd = 28-61 nM and Bmax = 975-1196 fmol/mg protein, whereas in ThP-1 leukemia Kd = 146 nM and Bmax = 1411 fmol/mg protein. The sigma-1-like nature of 3H-pentazocine-binding sites was confirmed by competition studies which revealed high affinity for haloperidol and enantioselectivity for (+)-pentazocine over (-)-pentazocine. Interestingly, human MCF-7 breast adenocarcinoma showed little or no specific binding of 3H-pentazocine, suggesting the absence of sigma-1 receptors in this cell line. All cell lines examined expressed a high density of sigma-2 receptors with Kd values for [3H]DTG ranging from 20 to 101 nM and Bmax values of 491 to 7324 fmol/mg protein. Competition studies indicated possible heterogeneity of sigma-2 receptors. While sites labeled by [3H]DTG in all cell lines tested exhibited affinity for haloperidol and preference for (-)-pentazocine over the (+)-enantiomer, human cell lines generally showed 4- to 7-fold lower affinity for haloperidol and approximately 10-fold lower affinity for (-)-pentazocine compared with the rodent cell lines. The high density of sigma-1 and sigma 2-binding sites in these cell lines suggests important cellular functions in cancer, as well as potential diagnostic utility for tumor-imaging agents which target sigma sites. These cell lines may be useful as model systems in which to study the functions of sigma sites in normal tissues, as well as their possible role in tumor biology.