Expression of ERK in cultured and primary HD cells. Total cell lysates were examined for the expression of ERK and phosphorylated ERK (p-ERK) by Western blot. (A) Expression of ERK in 4 HD cell lines (HD-MYZ, HD-LM2, L-428, and KMH-2) and in 4 other CD30⁺ lymphoid cell lines. All HD-derived cell lines expressed p-ERK, but HD-LM2 expressed the highest level. The Jurkat cell line also expressed high levels of p-ERK. The multiple myeloma cell line 8226 and the anaplastic large-cell lymphoma cell line SUP-M2 did not express p-ERK. The p-ERK protein was used as a positive control. (B) Expression of p-ERK in peripheral blood lymphocyte (PBL) samples from 3 healthy individuals was either weak or undetectable. (C) Histochemical staining of a normal tonsil for total ERK (upper panel) and p-ERK (lower panel) (× 40). Normal tonsillar cells expressed total ERK, but they did not express p-ERK. (D) A section of lymph node involved with nodular sclerosis HD was stained for total ERK (upper panel) or p-ERK (lower panel). Both benign and malignant cells expressed total ERK. However, only the malignant Reed-Sternberg cells expressed p-ERK (lower panel, × 10). Inset in the lower panel shows a Reed-Sternberg cell with intense nuclear staining of p-ERK (× 40).