Effect of epidermal growth factor/transforming growth factor α on lactate production in porcine Sertoli cells: Glucose transport and lactate dehydrogenase isozymes as potential sites of action

Abstract

Germ cell development is dependent upon the delivery of essential nutriments such as lactate originating from Sertoli cells. Lactate production is under the systemic control but probably also under a local control exerted via certain growth factors. By using a model of porcine cultured Sertoli cells, we have characterized the action of epidermal growth factor (EGF) on lactate production and further delineated the potential biochemical mechanisms involved in the EGF action. EGF stimulated lactate production in a time and dose dependent manner with a half-maximal (ED 50) and maximal effects, respectively with 3.8 (0.6 × 10 −9 M) and 22 ng/ml of EGF. Lactate formation involves several biochemical steps among which the glucose substrate uptake and transport system as well as the lactate dehydrogenase (LDH) activity appear to play key roles. We report here that EGF increased the uptake of glucose evaluated through that of 2-deoxy-D-glucose (2-DOG), a non-metabolizable glucose analog. Such an increase in glucose substrate uptake occurs both after a long term (48 h) and a short term treatment (ED 50 = 6.4 ng/ml, 1.1 × 10 −9 M EGF). Moreover, EGF was also able to enhance the activity of the Sertoli cell LDH. The maximal effect of the growth factor on LDH activity was observed after a long term (24 h) treatment with an ED 50 of 7 ng/ml (1.2 × 10 −9 M). By using an agarose gel electrophoresis system, we further show that the increase in total LDH activity after EGF treatment selectively occurred on LDH-5 (A4) (as well as its related isozymes containing A subunit) but not on LDH-1 (B4). The maximal effect of EGF on LDH-5 (A4) activity was observed after 24 h treatment with an ED 50 of 8 ng/ml (1.3 × 10 −9 M). Since LDH-5 (A4) is known to favor the conversion of pyruvate into lactate, we suggest that the increase in the activity of this LDH isozyme (and its related isozymes) coupled to the enhancement of glucose substrate uptake may probably represent two key steps involved in the stimulatory effect of EGF on lactate production. Close if not similar stimulatory effects to those of EGF on lactate production were observed with transforming growth factor α (TGFα). Since (i) TGFa but not EGF is produced in the gonad and (ii) EGF and TGFα bind to the same receptor, the stimulatory effects on lactate production reported here are probably related to TGFα action in in vivo situations.