This is from Einstein's Paper and Einstein's Equation E=mc^2. Einstein in his paper proved that "inertia of a mass is dependent on its energy content". So, we can write m=E/c^2. Gravitational Force F=G*m1*m2/r^2 can be re-written as F=G(E1/c^2)(E2/c^2)/r^2 or F=(G/c^4)(E1*E2)/r^2. "Strong Nuclear Force" keep proton and neutron together. This nucleus keep the whole atom together. So, "strong nuclear force" keep the "energy content(E)" of a mass(m) together. So, inertia of a mass(m) is dependent on "Strong Nuclear Force". As "Gravitational Force" is dependent on mass(m), so it is also dependent on "Strong Nuclear Force".