The HGF/c-Met signaling pathway regulates a variety of biological processes, including proliferation, survival and migration. Aberrant c-Met activation has been implicated in the pathogenesis of many human cancers. Furthermore, c-Met is of prognostic significance in several types of cancer such as diffuse large B cell lymphoma, bladder cancer, breast cancer, colorectal cancer and ovarian cancer. Expression of c-Met in Hodgkin lymphoma (HL) patients, as well as increased levels of HGF in the urine of HL patients has been reported, but no prognostic studies or functional data have been reported.
We studied the prognostic significance of c-Met expression by immunohistochemistry on paraffin sections of classical HL (cHL) patients from two independent patient cohorts. Functional studies were performed on HL cell line L428 that showed high c-Met levels, constitutive phosphorylated c-Met and no HGF expression. The effect of stimulation by HGF and inhibition by c-Met kinase inhibitor SU11274 was investigated by Western blot, cell proliferation and cell cycle progression analysis.
Expression of c-Met was detected in Hodgkin Reed-Sternberg (HRS) cells in 52% (79/152) of primary cHL tissue samples and expression of its ligand, hepatocyte growth factor (HGF), was detected in HRS cells in 8% (10/120) of the cHL patients. Co-expression of HGF and c-Met in HRS cells was observed in only 3% (4/120) of cHL patients. A variable percentage of infiltrating cells stained positive for HGF, supporting a predominant paracrine activation route. In contrast to its adverse prognostic impact in other cancers, high c-Met expression significantly correlated with favorable 5 year progression free survival in both patient cohorts. To explain these unexpected findings we studied the c-Met/HGF signaling pathway in the L428 cHL cell line. The levels of phosphorylated c-Met, Akt, and Erk1/2 were upregulated upon HGF stimulation and this induction could be blocked by inhibiting c-Met activation with SU11274. Activation with HGF did not effect cell growth, while SU11274 alone suppressed cell growth. SU11274, as well as inhibitors of PI3K, MEK1/2 and Erk1/2 (downstream targets of the HGF/c-Met signaling pathway) induced G2/M cycle arrest.
Expression of c-Met in tumor cells was observed in 52% of cHL patients. In contrast to its prognostic value in other cancers, expression of c-Met in HRS cells of cHL patients correlated with favorable prognosis in two independent cohorts. Although triggering of c-Met in L428 cells with HGF induced activation of its downstream factors, no effect was observed on proliferation. Remarkably, the c-Met inhibitor did suppress cell growth of L428 cells.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.