Absence of suppressor of cytokine signaling 2 turns cardiomyocytes unresponsive to LIF-dependent increases in Ca(2+) levels.

Little is known regarding the role of suppressor of cytokine signaling (SOCS) in the control of cytokine signaling in cardiomyocytes. We investigated the consequences of SOCS2 ablation for leukemia inhibitory factor (LIF)-induced enhancement of intracellular Ca(2+) ([Ca(2+)]i) transient by performing experiments with cardiomyocytes from SOCS2-knockout (ko) mice. Similar levels of SOCS3 transcripts were seen in cardiomyocytes from wild-type and SOCS2-ko mice, while SOCS1 mRNA was reduced in SOCS2-ko. Immunoprecipitation experiments showed increased SOCS3 association with gp130 receptor in SOCS2-ko myocytes. Measurements of Ca(2+) in wild-type myocytes exposed to LIF showed a significant increase in the magnitude of the Ca(2+) transient. This change was absent in LIF-treated SOCS2-ko cells. LIF activation of ERK and STAT3 was observed in both wild-type and SOCS2-ko cells, indicating that in SOCS2-ko, LIF receptors were functional, despite the lack of effect in the Ca(2+) transient. In wild-type cells, LIF-induced increase in [Ca(2+)]i and phospholamban Thr17 [PLN(Thr17)] phosphorylation was inhibited by KN-93, indicating a role for CaMKII in LIF-induced Ca(2+) raise. LIF-induced phosphorylation of PLN(Thr17) was abrogated in SOCS2-ko myocytes. In wild-type cardiomyocytes, LIF treatment increased L-type Ca(2+) current (ICa,L), a key activator of CaMKII in response to LIF. Conversely, SOCS2-ko myocytes failed to activate ICa,L in response to LIF, providing a rationale for the lack of LIF effect on Ca(2+) transient. Our data show that absence of SOCS2 turns cardiomyocytes unresponsive to LIF-induced [Ca(2+)] raise, indicating that endogenous levels of SOCS2 are crucial for full activation of LIF signaling in the heart.