Lactate administration activates the ERK1/2, mTORC1, and AMPK pathways differentially according to skeletal muscle type in mouse

Abstract

Skeletal muscle is described as an endocrine organ, constitutively or intermittently secreting bioactive molecules. The signaling pathways by which these molecules mediate changes in skeletal muscle and regulate interorgan crosstalk are only partly understood. Lactate is widely described as a signaling molecule in different cells, but the role of lactate as a signaling molecule in mature skeletal muscle has not been fully unveiled. The aim of this study was to determine the role of lactate on activation of signaling pathways in adult mouse skeletal muscle. Male mice were injected intraperitoneally with lactate or saline, and tissues were dissected after 40 min. Phosphorylation levels of relevant proteins in muscle were assessed by Western blotting. After lactate administration, we found an increase in p-ERK1/2^{Thr202/Tyr204} (3.5-fold; P = 0.004) and p-p70S6K^Thr389 (1.9-fold; P = 0.01) in quadriceps; and an increase in p-rpS6^Ser235/236 in both quadriceps (6.3-fold; P = 0.01) and EDL (2.3-fold; P = 0.01), without changes in soleus. There was a tendency toward an increase in p-AMPK^Thr172 (1.7-fold; P = 0.08), with a significant increase in p-ACC^Ser79 (1.5-fold; P = 0.04) in soleus, without changes in quadriceps and EDL. These results support the hypothesis that lactate plays a role in the molecular signaling related to hypertrophy and to oxidative metabolism on adult skeletal muscle and suggest that this activation depends on the skeletal muscle type. The mechanisms that underlie the effect of lactate in mature skeletal muscles remain to be established.

Keywords: metabolism; molecular signaling; skeletal muscle.

Figure 1

Lactate administration increases blood lactate levels without changes in blood glucose and insulin levels. Blood lactate (A), glucose (B), and insulin (C) levels after intraperitoneal injection of vehicle (VEH, n = 3–5, gray line) or lactate (LAC, n = 5–7, black line). The data are presented as the median and interquartile range.

Figure 2

Lactate administration increases intracellular signaling related to ERK1/2 and Akt/mTORC1 pathways. Phosphorylation of ERK1/2^{Thr202/Tyr204} (A), IGF‐1R^{T yr1135/1136} (B), Akt^Thr308 (C), Akt^Ser473 (D), p70S6K^{T hr389} (E), and rpS6^Ser235/236 (F) in quadriceps (Quad), extensor digitorum longus (EDL), and soleus (Sol) skeletal muscles after vehicle (VEH, n = 5) or lactate (LAC, n = 7) administration. Bars represent the median in the scatterplot. Representative immunoblots are shown in G. VEH, vehicle; LAC, lactate. Values in graphs are arbitrary units (A.U.). Statistical significance compared with vehicle is indicated by *P < 0.05 and **P < 0.01. d: effect size with 95% confidence interval.

Figure 3

Lactate administration increases intracellular signaling related to AMPK pathway. Phosphorylation of AMPK^{T hr172} (A), ACC^{S er79} (B), TBC1D1^Ser237 (C), TBC1D4^Thr642 (D), PDH‐E1α ^Ser293 (E), and PDH‐E1α ^Ser300 (F) in quadriceps (Quad), extensor digitorum longus (EDL), and soleus (Sol) skeletal muscles after vehicle (VEH, n = 5) or lactate (LAC, n = 7) administration. Bars represent the median in the scatterplot. Representative immunoblots are shown in G. VEH, vehicle; LAC, lactate. Values in graphs are arbitrary units (A.U.). Statistical significance compared with vehicle is indicated by *P < 0.05 and **P < 0.01. d: effect size with 95% confidence interval.

Figure 4

Lactate‐induced hypertrophic and oxidative signaling pathways activation in skeletal muscle. The working hypothesis for signaling pathways activated by in vivo lactate administration in mixed/fast (A) and slow (B) skeletal muscles. ERK1/2: extracellular signal‐regulated protein kinases 1 and 2; Akt: protein kinase B; TSC2: tuberous sclerosis complex 2; mTORC1: mammalian target of rapamycin complex 1; p70S6K: ribosomal S6 kinase; rpS6: ribosomal protein S6; AMPK: 5’‐AMP‐activated protein kinase; TBC1D4: TBC1 domain family member 4; ACC: Acetyl‐CoA carboxylase; PDH: pyruvate dehydrogenase. → represents activation; ···I represents inhibition. +P: increased phosphorylation; ‐P: decreased phosphorylation. ? represents unknown mechanisms.